Fisherbrand Accumet Electrodes Handbook

Fisherbrand Accumet Electrodes Handbook
Fisher Scientific accumet® Electrodes
Fisher Scientific accumet® Electrodes
pH Electrodes For Virtually Any Sample Type
When selecting a pH system, choose your meter based on what features
you need, i.e. resolution, output, memory, etc. Choose the corresponding pH
electrode for your meter based primarily on your sample type and conditions,
i.e. wastewater with sulfides, room temperature, student use, 5 days/week, etc.
Take a glance in the Fisher Scientific catalog and you will find 100’s of pH
electrodes to choose from. While many electrodes might be work adequately
for a particular application, not all will perform equally or last as long as
others. Usually in situations in which a pH electrode “didn’t last long”,
the electrode is not matched well for the application resulting in poor
performance, and ultimately failure. Understanding the different electrode
options that are available and knowing how to use them to your advantages is
a critical step to getting the most out of your pH measurement system.
The following guide is designed to help you understand the ABC’s of electrode
selection. If you are still undecided or have questions regarding any product,
your Fisher Scientific representative and our electrochemistry experts
(888-358-4706 or [email protected]) are there to help guide you!
The ABC’s Of Electrode Selection
Combination Or Half-Cell
There are two components within a pH electrode system. A pH indicating
(or sensing) electrode develops a potential dependent on the pH, and the
reference electrode which provides a constant potential to completes
the electrical circuit. Combination electrodes have both the indicating
and reference electrodes “combined” into one electrode. Alternatively,
separate half-cell electrodes can be used. Since reference electrodes often
outlast sensing electrodes, replacing indicating electrodes can mean lower
replacement costs than replacing an equivalent combination electrode.
In practical terms, nearly all electrodes used today are combination
electrodes. There are many reasons including; many pH meters require
an adapter to accept half-cell electrodes, handling multiple electrodes is
un-desirable (or impossible with small samples!), half cells don’t have built in
ATC and therefore require a third electrode for temperature compensation, the
complexity of diagnosing electrode problems and most of all, the reduced cost
and performance of today’s combination electrode designs.
Combination electrode (13-620-299)
Reference half cell (13-620-658)
Indicating half cell (13-620-187)
Verdict: Use a combination electrode unless the method you must follow calls
for half-cells. You’ll have many more choices available to you. Combination
Glass Or Plastic Body
It probably goes without saying, but if an electrode literally breaks into pieces,
it is useless and can not be repaired. Combination glass and combination
plastic electrodes use an indicating electrode with a glass sensing bulb on
the end. This is important for several reasons. First, plastic electrodes are
not immune from breakage. Second, if an electrode breaks it will likely be at
the tip, not the body itself. A plastic electrode with little to no bulb protection
defeats the purpose of a plastic electrode in the first place.
To decide on which to use, let us look at the advantages and disadvantages
of each, starting with the glass body electrode. Glass electrodes are easier to
clean and maintain since they can tolerate just about any solvent and inorganic
material (with the exception of HF!) and can handle higher temperatures quite
nicely – typically to 100 °C. The fact that glass electrodes also have a glass
sensing bulb is also an advantage. Since the seal that combines the bulb
to the body is similar material, it is one less thing that can go wrong during
measurement and doesn’t become the source of junction potential as it does in
plastic electrodes. This is especially important consideration for applications
that have repeated and extreme heating and cooling – the expansion and
contraction that occurs is handled much better by glass electrodes. The
downside of glass electrodes is fairly easy – they are generally more expensive
then plastic, and they have a greater potential for breakage.
Plastic electrodes are less expensive than glass equivalents and can usually
take much abuse in the lab and in the field. Most electrodes with built-in
temperature compensation elements are plastic due to the complexity in
manufacturing them. As a result, they are most popular with field and portable
meters, but can also be used in laboratory environments (such as 13-620-631).
To protect the glass sensing bulb, many plastic electrodes use an integral
housing that limit the bulb exposure, but often can be difficult to clean.
Verdict: Glass electrodes are definitely worth the upgrade if you have
Single Or Double-Junction (Tris Compatible)
Open
The single-junction electrode on the left has a black, clogged junction
and is no longer responsive.
Close
This decision is extremely important and should not be overlooked. If you will
be measuring samples that have sulfides, proteins, heavy metals, TRIS, or
anything that might react with silver, or if you will be testing samples that are
unknown, use a double-junction electrode. Calomel electrodes would also be
suitable but have fallen out of favor due to mercury content and regulations
that ban shipments of them in specific states in the US. Single-junction
electrodes are less expensive, but offer no other advantages. If you use a
single-junction electrode in a solution with TRIS, it’s just a matter of time
before it fails.
Refillable electrodes use
our patented twist open and
close mechanism.
Color coded electrode bands simplify
electrode selection:
S
Purple= TRIS Compatible
S
Blue = General Purpose
Verdict: If you will only measure drinking water, you can save money by
using a single-junction pH electrode. If you have TRIS, sulfides, proteins,
heavy metals or are measuring samples that are unknown, look for a
Fisher Scientific accumet® electrode with a purple ring – indicating that
is it compatible.
significant temperature fluctuations. If bulb breakage is a concern, consider
Fisher Scientific accumet® accuTupH electrodes with thick glass bulbs! If you
want ATC built-in to your electrode, expect to settle for plastic.
Refillable Or Non-Refillable (Gel)
All pH electrodes use/leak solution. Refillable electrodes do so more quickly,
and can be replenished when they require more filling solution. Gel filled
electrodes do so very slowly and when they run out or the gel is no longer
flowing, can not be replenished and must be replaced.
Refillable electrodes are generally more expensive than gel-filled equivalent
electrodes but respond much faster. They also last longer, because the filling
solution can be replaced indefinitely; however the periodic addition of filling
solution that is required also happens to be the main disadvantage. Another
downside is that when the filling hole is left open for an extended period, dried
salt may be left behind which often involves cleaning. The act of refilling and
opening and closing the fill hole with Fisher Scientific accumet® electrodes is
extremely easy due to the patented filling mechanism. It takes just seconds to
open the hole and a few seconds more to fill the probe.
Gel-filled electrodes are less expensive, require less maintenance, and are
usually plastic. High quality gel formulations have also extended the once
limited shelf-life in recent years.
cells may or may not have a temperature sensor built-in.
Verdict: Refillable electrodes are usually worth the extra maintenance –
40
pH, pH/ATC, ORP, and Ion Selective Electrodes
especially if it’s a Fisher Scientific accumet® electrode.
Over 30 years of experience in the design, development,
and manufacture of electrodes go into each Fisher Scientific
accumet® electrode.
We offer electrodes that provide fast, accurate measurements in
hundreds of different applications – including yours!
A complete line for every application: made with care and precision.
All Fisher Scientific accumet® electrodes feature continuous
electrical shielding and insulation of the internal elements, cable
and connectors for extremely stable, reproducible readings with a
minimum of electrical noise. Each electrode is individually tested,
serialized to meet GLP requirements, and backed by a knowledgeable
support staff (888-358-4706 or [email protected]) and
1 year warranty.
41
Fisher Scientific accumet® pH Electrodes
Fisher Scientific accumet® pH Electrodes
High Performance Models For Critical Research
State-of-the-art design for fast, accurate measurements despite sample
temperature differences – plus extra durability. Feature innovative reference
system that controls chemical equilibria, prevents precipitation of solution
components at reference element from 0 to 100 °C; plus internal electrolyte
with minimal temperature coefficient. Result: highly predictable, super
reliable electrodes that respond quickly at any temperature. Cycle between
25 and 80 °C samples, reach reproducible pH in 30 seconds (vs. 1 to 3 minutes
Top Selling Rugged Glass And Capillary Junction Electrodes
for other electrodes). Drift and accuracy problems are virtually eliminated.
Read sample pH in <20 seconds, correct to ±0.02 pH; pH value stays constant
at any temperature. Best of all, these electrodes read pH consistently at
elevated temperatures – and without premature loss in performance.
Five times thicker than conventional glass pH electrodes. For applications
where glass bulbs break frequently and epoxy body electrodes aren’t
practical. Up to 40 times tougher than conventional glass pH electrodes,
without sacrificing response times.
Choice of standard-size glass body, epoxy body with flushable junction,
and glass body with flushable junction.
13-620-185 also utilizes accupHast temp reference for top performance from
a rugged glass electrode.
40-mil accuTupH
rugged bulb
accuTupH+ &
accuTupH
pH Electrodes
accu•pHast R
pH Electrodes
®
Catalog No.
Special
Parameter
Combination Or Half Cell
ATC Connection
Refillable Or Gel (Sealed)
Refill Solution
Junction Type
Glass Or Plastic Body
Max Temp
Length x Diameter (mm)
Note:
8-mil standard
electrode
13-620-195
accu•pHast® R
pH
Combination
n/a
Refillable
SP138-500
Double S
Glass
100 °C
102 x 12
High performance,
ideal for samples temp variation
13-620-196
accu•pHast® R accuFlow
pH
Combination
n/a
Refillable
SP138-500
Double S
Plastic
80 °C
102 x 12
High performance,
flushable junction for tough samples
Fast and accurate for samples at widely varying temperatures. Patented
design: dual ceramic junctions, sealed reference, and special internal
electrolyte to eliminate slow response when measuring samples at different
temperatures in quick succession. Accurate to ±0.01 pH at 25 °C and ±0.05 pH
from -5 to 100 °C. Response times of 20 seconds or less. Negligible drift.
13-620-197
accu•pHast® R accuFlow
pH
Combination
n/a
Refillable
SP138-500
Double S
Glass
100 °C
102 x 12
High performance,
flushable junction for tough samples
Isolated reference and outer KCl fill solution prevent clogging from silvercompound precipitates. Unique pH bulb is filled with special crystals to
speed thermal equilibrium. Choice of four styles: standard-size glass body,
MicroProbe™ extra-long glass body, extra-long epoxy body, and pH/ATC
epoxy body.
Catalog No.
Special
Parameter
Combination Or Half Cell
ATC Connection
Refillable Or Gel (Sealed)
Refill Solution
Junction Type
Glass Or Plastic Body
Max Temp
Length x Diameter (mm)
Note:
13-620-185
accuTupH+,
accu•pHast®
pH
Combination
n/a
Refillable
SP138-500
accupHast
(Tris compatible) S
Glass
100 °C
102 x 10
Rugged & fast temp
response, high
performance, includes
bulb protector
13-620-183A
accuTupH
pH
Combination
n/a
Refillable
SP138-500
Double S
13-620-182A
accuTupH, US
standard connector
pH
Combination
n/a
Refillable
SP138-500
Double S
Glass
100 °C
102 x 12
Rugged, Tris compatible,
includes bulb protector.
Included with AB15+ &
AB15 Bio kits
Glass
100 °C
102 x 10
Rugged, Tris compatible,
includes bulb protector.
US std connector
(not BNC)
Single-pore capillary junction provides a flow channel about 200 times larger
than typical reference junctions. Combined with a specially formulated
13-620-181
accuTupH
13-620-187
accuTupH
pH
Combination
n/a
Refillable
SP135-500
Single S
pH indicating
Half Cell, BNC
n/a
n/a
n/a
n/a
Glass
100 °C
102 x 10
Rugged, general purpose.
includes bulb protector
Glass
100 °C
106 x 12
Rugged, use with
reference half cell
flowing gel reference electrolyte (13-636-430), provides a fast, virtually
clog-free reference. The result is a faster, more stable pH measurement.
Electrolyte & syringe
(13-636-430)
accuCap® Capillary
Junction Electrodes
accu•pHast®
pH Electrodes
Catalog No.
Special
13-620-296
accu•pHast®
Parameter
Combination Or Half Cell
ATC Connection
Refillable Or Gel (Sealed)
Refill Solution
Junction Type
pH
Combination
n/a
Refillable
SP138-500
accupHast
(Tris compatible) S
Glass
100 °C
102 x 10
High performance,
ideal for samples
temp variation
Glass Or Plastic Body
Max Temp
Length x Diameter (mm)
Note:
42
13-620-297
accu•pHast®
long & narrow
pH
Combination
n/a
Refillable
SP138-500
accupHast
(Tris compatible) S
Glass
80 °C
165(L) x 75 x 5
High performance
13-620-298
accu•pHast®
long
pH
Combination
n/a
Refillable
SP138-500
accupHast
(Tris compatible) S
Plastic
80 °C
140 x 10
High performance
13-620-113
accu•pHast®
13-620-114
accu•pHast®
pH/ATC
Combination
13-620-16
Refillable
SP138-500
accupHast
(Tris compatible) S
Plastic
80 °C
143 x 10
See page 53 for list
of discontinued meters
using 13-620-16 ATC
pH/ATC
Combination
13-620-19
Refillable
SP138-500
accupHast
(Tris compatible)
Plastic
80 °C
143 x 10
ATC fits XL, AB,
and AR meters
Catalog No.
Special
Parameter
Combination Or Half Cell
ATC Connection
Refillable Or Gel (Sealed)
Refill Solution
Junction Type
S
Glass Or Plastic Body
Max Temp
Length x Diameter (mm)
Note:
13-620-130
accuCap
pH
Combination
n/a
Refillable
13-636-430
Capillary open pore
(Tris compatible) S
Glass
80 °C
160 x 12
Research quality,
included with XL series
pH kits
13-620-131
accuCap
pH
Combination
n/a
Gel
n/a
Capillary open pore
(Tris compatible) S
Glass
80 °C
130 x 12
Non-refillable glass
electrodes like this are
hard to find
13-620-132
accuCap
pH
Combination
n/a
Gel
n/a
Capillary open pore
(Tris compatible) S
Plastic
60 °C
120 x 12
General purpose
13-620-133
accuCap, spear tip
pH
Combination
n/a
Gel
n/a
Capillary open pore
(Tris compatible) S
Glass
50 °C
80(L) x 25 x 6
Spear tip and 6 mm diameter
useful for semi solids &
small samples
43
Fisher Scientific accumet® pH Electrodes
pH Electrodes To Match Your Application Type
Refillable Glass
pH Electrodes
Catalog No.
Special
Parameter
Combination Or Half Cell
ATC Type
Refillable Or Gel (Sealed)
Refill Solution
Junction Type
Glass Or Plastic Body
Max Temp
Length x Diameter (mm)
Note:
Refillable Plastic
pH Electrodes
13-620-285
n/a
pH
Combination
n/a
Refillable
SP135-500
Single S
Glass
100 °C
102 x 12
General purpose.
Included with AB15+
and AB15 kits.
Includes bulb protector
13-620-223A
n/a
pH
Combination
n/a
Refillable
SP138-500
Double S
Glass
100 °C
102 x 12
Tris compatible,
includes bulb protector
13-620-291
Semi-micro
pH
Combination
n/a
Refillable
SP135-500
Single S
Glass
100 °C
100 x 6
6 mm diameter for small
samples, test tubes
13-620-292
Semi-micro
pH
Combination
n/a
Refillable
SP135-500
Single S
Glass
100 °C
150 x 6
Same as 13-620-291
but longer
13-620-293
Semi-micro
pH
Combination
n/a
Refillable
SP138-500
Calomel S
Glass
80 °C
160(L) x 120 x 6
Tris compatible
Micro pH And
pH/ATC Refillable
Electrodes
Catalog No.
Special
Parameter
Combination Or Half Cell
ATC Type
Refillable Or Gel (Sealed)
Refill Solution
Junction Type
Glass Or Plastic Body
Max Temp
Length x Diameter (mm)
Note:
44
Catalog No.
Special
Parameter
Combination Or Half Cell
ATC Type
Refillable Or Gel (Sealed)
Refill Solution
Junction Type
Glass Or Plastic Body
Max Temp
Length x Diameter (mm)
Note:
13-620-287A
n/a
pH
Combination
n/a
Refillable
SP135-500
Single S
Plastic
80 °C
106 x 12
Same as 13-620-530A
without ATC
13-620-221
n/a
pH
Combination
n/a
Refillable
SP138-500
Double S
Plastic
80 °C
102 x 10
Tris compatible,
includes bulb protector
13-620-300
n/a
pH
Combination
n/a
Refillable
SP138-500
Calomel S
Plastic
80 °C
106 x 12
Tris compatible,
with integral bulb guard
13-620-288
n/a
pH
Combination
n/a
Refillable
SP138-500
Calomel S
Plastic
80 °C
106 x 12
Tris compatible
13-620-289
Flat surface
pH
Combination
n/a
Refillable
SP135-500
Single S
Plastic
80 °C
114 x 13
Flat surface for agar,
cheese, food, and more
13-620-108A
n/a
pH
Combination
n/a
Gel
n/a
Single S
Plastic
80 °C
106 x 12
Ecomomical,
general purpose
13-620-290
Long & thin
pH
Combination
n/a
Gel
n/a
Single S
Plastic
80 °C
178 x 6
Tall flasks, bottles
13-620-299A
n/a
pH
Combination
n/a
Gel
n/a
Double S
Plastic
80 °C
106 x 12
Ecomomical,
Tris compatible
13-620-111
n/a
pH/ATC
Combination
13-620-19
Gel
n/a
Double S
Plastic
80 °C
106 x 12
ATC for AB, AR,
and XL meters
13-620-112
n/a
pH/ATC
Combination
13-620-16
Gel
n/a
Double S
Plastic
80 °C
106 x 12
See page 53 for list of
discontinued meters
using 13-620-16 ATC
Gel-Filled Plastic
pH Electrodes
13-620-95
MicroProbe
pH
Combination
n/a
Refillable
SP138-500
accupHast
(Tris compatible) S
Glass body / Teflon stem
80 °C
254(L) x 150 x 3
Tris compatible,
small samples,
long test tubes / NMR
13-620-96
MicroProbe
pH
Combination
n/a
Refillable
SP138-500
accupHast
(Tris compatible) S
Glass body / Teflon stem
80 °C
127(L) x 38 x 3
Tris compatible, small samples
13-620-530A
n/a
pH/ATC
Combination
13-620-19
Refillable
SP135-500
Single S
13-620-631
n/a
pH/ATC
Combination
13-620-19
Refillable
SP138-500
Double S
Plastic
80 °C
106 x 12
ATC for AB, AR, and XL meters
Plastic
80 °C
106 x 12
Tris compatible, ATC for AB,
AR, and XL meters, included
with AB15E kits
Catalog No.
Special
Parameter
Combination Or Half Cell
ATC Type
Refillable Or Gel (Sealed)
Refill Solution
Junction Type
Glass Or Plastic Body
Max Temp
Length x Diameter (mm)
Note:
45
Fisher Scientific accumet® pH Electrodes
Fisher Scientific accumet® pH And Half Cell Electrodes
New pH/ATC Electrodes For Non-accumet® Meters
• Combination pH mercury-free electrodes with built-in
temperature compensation
pH/ATC Electrodes For Portable Fisher Scientific accumet® Meters
• Epoxy body is impact resistant and ideal for rough handling
• Fast, accurate response from 5 to 80 °C
• Double-junction pH/ATC electrodes compatible with Tris, proteins
and sulfides
All electrodes have a BNC connector and ATC connector; ATC will differ with
meter type. 3-ft cable and electrode storage bottle are included, refillable
models also include a 30 mL bottle of filling solution.
Universal pH/ATC
Electrodes
Catalog No.
Parameter
Combination Or Half Cell
ATC Connection
Refillable Or Gel (Sealed)
Refill Solution
Junction Type
Glass Or Plastic Body
Max Temp
Length x Diameter (mm)
Note:
13-620-31C
pH/ATC
Combination
RCA (Cinch) plug
Refillable
SP135-500
Single S
Plastic
80 °C
106 x 12
ATC for Mettler®, Pinnacle® and
Corning® meters
13-621-701
pH/ATC
Combination
RCA (Cinch) plug
Gel
n/a
Double S
Plastic
80 °C
106 x 12
ATC for Mettler®, Pinnacle® and
Corning® meters
13-621-702
pH/ATC
Combination
3.5 audio plug
Gel
n/a
Double S
Plastic
80 °C
106 x 12
ATC for Beckman® meters
13-621-703
pH/ATC
Combination
Banana plug
Gel
n/a
Double S
Plastic
80 °C
106 x 12
ATC for WTW® and Pinnacle®
(part numbers ending with “P”)
pH/ATC Electrodes
For Field Use,
accuTupH Half Cell
Catalog No.
Special
Parameter
Combination Or Half Cell
ATC Type
Refillable Or Gel (Sealed)
Refill Solution
Junction Type
Glass Or Plastic Body
Max Temp
Length x Diameter (mm)
Note:
13-620-AP52
n/a
pH/ATC
Combination
13-620-16
Gel
n/a
Double S
Plastic
80 °C
102 x 12
ATC for AP60 & AP100
series meters
13-620-AP61
n/a
pH/ATC
Combination
13-620-AP53
Refillable
SP138-500
Double S
Plastic
80 °C
102 x 12
ATC for AP60 & AP100
series meters. Replaces
13-620-AP51
13-620-AP50A
n/a
pH/ATC
Combination
13-620-AP53
Refillable
SP135-500
Single S
Plastic
80 °C
102 x 12
ATC for AP60 & AP100
series meters
13-620-AP55
n/a
pH/ATC
Combination
13-620-20
Refillable
SP135-500
Single S
Plastic
80 °C
102 x 12
ATC for AP70 & AP80
series meters
13-620-187
accuTupH
pH indicating
Half cell, BNC
n/a
n/a
n/a
n/a
Glass
100 °C
106 x 12
Rugged, use with
reference half cell
13-620-284
n/a
pH indicating
Half cell, BNC
n/a
n/a
n/a
n/a
Glass
100 °C
106 x 12
Use with reference
half cell
13-620-294
n/a
pH indicating
Half cell, BNC
n/a
n/a
n/a
n/a
Plastic
80 °C
106 x 12
Use with reference
half cell
13-620-295
Low na error
pH indicating
Half cell, BNC
n/a
n/a
n/a
n/a
Plastic
80 °C
106 x 12
Ideal for samples
>pH 11, use with
reference half cell
13-620-51
Ships dry
Reference
Half cell, pin
n/a
Refillable
SP138-500
Calomel S
Glass
80 °C
106 x 12
Pin connector, use with
indicating half cell
13-620-52
Ships filled
Reference
Half cell, pin
n/a
Refillable
SP138-500
Calomel S
Glass
80 °C
106 x 12
Pin connector, use with
indicating half cell
Half-Cell Electrodes
Catalog No.
Parameter
Combination Or Half Cell
ATC Connection
Refillable Or Gel (Sealed)
Refill Solution
Junction Type
Glass Or Plastic Body
Max Temp
Length x Diameter (mm)
Note:
46
13-620-111T
pH/ATC
Combination
Mini DIN
Gel
n/a
Double S
Plastic
80 °C
106 x 12
ATC for Thermo Scientific Star meters
13-620-111
pH/ATC
Combination
Mini phone
Gel
n/a
Double S
Plastic
80 °C
106 x 12
ATC for Denver meters
13-620-631
pH/ATC
Combination
Mini phone
Refillable
SP138-500
Double S
Plastic
80 °C
106 x 12
ATC for Denver meters
Catalog No.
Special
Parameter
Combination Or Half Cell
ATC Type
Refillable Or Gel (Sealed)
Refill Solution
Junction Type
Glass Or Plastic Body
Max Temp
Length x Diameter (mm)
Note:
47
Fisher Scientific accumet® Electrodes
Half Cell, ORP, And ISFET Electrodes
Reference Electrodes Use Common Pin Connector Type
Electrodes For Titrations, Redox, And Specialty Applications
Reference,
Metallic, And
ORP Electrodes
Half Cell
Reference
Electrodes
Catalog No.
Special
Parameter
Combination Or Half Cell
Refillable Or Gel (Sealed)
Refill Solution
Junction Type
Glass Or Plastic Body
Max Temp
Length x Diameter (mm)
Note:
13-620-79
Minature
Reference
Half cell, pin
Refillable
SP138-500
Calomel S
Glass
80 °C
41 x 12
Pin connector, use with
indicating half cell
13-620-57
–
Reference
Half cell, pin
Refillable
SP138-500
Calomel S
Glass
80 °C
106 x 12
Pin connector, nonaqueous samples, use
with indicating half cell
13-620-62
Sleeve junction
Reference
Half cell, pin
Refillable
SP138-500
Calomel S
Glass
80 °C
106 x 12, 16 (with sleeve)
Pin connector, for
viscous samples, use
with indicating half cell
13-620-61
Reverse sleeve
Reference
Half cell, pin
Refillable
SP138-500
Calomel S
Glass
80 °C
106 x 12, 16 (with sleeve)
Pin connector, for
viscous samples, use
with indicating half cell
13-620-258
–
Reference
Half cell, pin
Refillable
SP138-500
Calomel S
Plastic
80 °C
106 x 12
Pin connector, use with
indicating half cell
With cover
Two pins
connector
With cover
Catalog No.
Special
Parameter
Combination Or Half Cell
13-620-45
Side-arm
Reference
Half cell, pin
13-620-115
Metallic
Indicating
Half cell, pin
13-620-122
Metallic
Indicating
Half cell, pin
13-620-149
Metallic
Indicating
Dual platinum
13-620-123
Metallic
Indicating
Dual platinum
Refillable Or Gel (Sealed)
Refill Solution
Junction Type
Glass Or Plastic Body
Max Temp
Length x Diameter (mm)
Note:
Refillable
SP135-500
Single
Glass
100 °C
106 x 12
Pin connector,
remote fill with
side arm, use with
indicating half cell
n/a
n/a
Redox
Glass
100 °C
140 x 12
ORP and titrations
n/a
n/a
Redox
Glass
100 °C
140 x 12
Silver and
halide titrations
n/a
n/a
Redox
Plastic
80 °C
114 x 13
Chlorine titrations
with Cl titrimeter.
Connect to BNC
using 13-620-488
(page 53)
n/a
n/a
Redox
Glass
100 °C
140 x 13
KF, dead-stop
titrations, sulfur
13-620-81
Metallic
Redox (ORP)
Combination,
BNC
Refillable
SP135-500
Redox
Glass
100 °C
140 x 12
BNC, most ORP
measurements,
includes
bulb protector
See page 53 for adapters.
Combination pH/ATC probe in a completly glass-free design. Uses solid-state
lon-Selective Field Effect Transistor (ISFET) sensing design. Special models
for XL meters include cup style for small volume measurements and a flat
Catalog No.
Special
Parameter
Combination Or Half Cell
Refillable Or Gel (Sealed)
Refill Solution
13-620-259
–
Reference
Half cell, pin
Gel
n/a
13-620-53
–
Reference
Half cell, pin
Refillable
SP135-500
13-620-273
–
Reference
Half cell, pin
Refillable
SP138-500
13-620-46
–
Reference
Half cell, pin
Refillable
SP135-500
Junction Type
Glass Or Plastic Body
Max Temp
Length x Diameter (mm)
Note:
Calomel S
Plastic
80 °C
106 x 12
Pin connector, use with
indicating half cell
Single S
Glass
100 °C
106 x 12
Pin connector, use with
indicating half cell
Double S
Glass
100 °C
106 x 12
Pin connector, use with
indicating half cell
Single S
Plastic
100 °C
108 x 13
Pin connector, use with
indicating half cell
48
13-620-658
–
Reference
Half cell, pin
Refillable
Inner 13-620-433
Outer 13-620-434
Double S
Plastic
100 °C
108 x 13
Pin connector, bromide,
chloride, copper, iodide,
lead, nitrate, silver/
sulfide, redox, and pH
applications requiring
sample-compatible
electrolyte, use with
indicating half cell
surface design. Low maintenence, gel-filled electrode with no refill solution
or bulb breakage to worry about. Ideal for food testing applications where
traditional glass sensors cannot be used.
accuFet
pH Electrodes
Catalog No.
Special
Parameter
Combination Or Half Cell
ATC Connection
Refillable Or Gel (Sealed)
Refill Solution
Junction Type
Glass Or Plastic Body
Max Temp
Length x Diameter (mm)
Note:
13-620-700
accuFet, cone tip
pH/ATC
Combination
accuFet
Gel
n/a
Single S
Plastic
80 °C
140 x 12
For XL series pH meters
13-620-710
accuFet, cup style
pH/ATC
Combination
accuFet
Gel
n/a
Single S
Plastic
80 °C
140 x 12
For XL series pH meters
13-620-755
accuFet
pH/ATC
Combination
AB15 Plus, AB15
Gel
n/a
Single S
Plastic
80 °C
140 x 12
For AB15+, AB15,
and AR series
pH meters
13-620-758
accuFet
pH/ATC
Combination
Basic
Gel
n/a
Single S
Plastic
80 °C
140 x 12
For discontinued
Basic meters
49
Fisher Scientific accumet® Ion-Selective Electrodes (ISE)
Combination And Half Cell ISE’s Have BNC For Universal Meter Use
Polymer Membrane
And Solid State ISE
Catalog No.
Parameter
Type
Combination Or Half Cell
Refillable Or Gel (Sealed)
Refill Solution
Ionic Strength Adjuster
Glass Or Plastic Body
Max Temp
Length x Diameter (mm)
Connection
Interferences
Range (ppm)
Calibration Solutions
0.1 M
Typical Applications
& Notes:
Solid State ISE
13-620-536
Calcium
ISE polymer membrane
Combination
Refillable
SP138-500
13-620-534
Nitrate
ISE polymer membrane
Combination
Refillable
ACCU0807-500
13-641-851
Plastic
40 °C
102 x 13
BNC
Mg++, Zn++, Ba++, K+, Na++,
Ni++, Cu++, Fe++, Sr++, H++,
Hg++, PbV
0.2 to 40,000
100 ppm
13-641-862
CaCl2 (13-641-811)
Foods, beverages,
soil, pharmaceuticals,
explosives, fertilizers,
plants, EDTA
titration endpoint
13-641-850
Plastic
40 °C
102 x 13
BNC
Cl-, NO2-, Br-, CN-,
ClO3-, I-, ClO4
0.5 to 14,000
1000 ppm = 13-641-910
100 ppm = 13-641-924
NaNO3 (13-641-888)
Pollution testing, foods,
pharmaceuticals,
fertilizers, plants,
meants, pickling baths
13-620-532
Potassium
ISE polymer membrane
Combination
Refillable
Fill solution (Dilute
ISA from 5M to 0.1M)
13-641-927
Plastic
40 °C
102 x 13
BNC
Cs+, NH4+, TI+, H+, Ag+,
Tris, Li+, Na+
0.04 to 39,000
1000 ppm = 650016
100 ppm = 649716
KCl (13-641-917)
Body fluids, soils,
sewage, fertilizers,
foods, beverages
13-620-521
Bromide
ISE solid-state
Sensing half cell
n/a
n/a
13-620-525
Bromide
ISE solid-state
Combination
Refillable
ACCU0834-500
Catalog No.
Parameter
Type
Combination Or Half Cell
Refillable Or Gel (Sealed)
Refill Solution
Ionic Strength Adjuster
13-620-549
Lead
ISE - solid-state
Combination
Refillable
ACCU0834-500
–
13-620-543
Lead
ISE - solid-state
Sensing half cell
n/a
n/a
–
ACCU0820-500
Plastic
80 °C
102 x 13
BNC
I-, CN-, S--
ACCU0820-500
Glass
80 °C
108 x 13
BNC
S--, I-, CN-,
High Cl- levels, NH3
Glass Or Plastic Body
Max Temp
Length x Diameter (mm)
Connection
Interferences
Range (ppm)
Glass
80 °C
108 x 13
BNC
Ag+, Hg++, Cu++, CD++, Fe++
0.2 to 20,700
Plastic
80 °C
102 x 13
BNC
Ag+, Hg++, Cu++, CD++, Fe++
0.2 to 20,700
0.4 to 79,000
1000 ppm
ACCU0822-500
NaBr (ACCU0821-500)
Biological fluids, soil,
plants, foots, effluents,
Method ASTM
approved. Requires
reference half cell
0.4 to 79,000
1000 ppm
ACCU0822-500
NaBr (ACCU0821-500)
Biological fluids, soil,
plants, foots, effluents,
Method ASTM approved
Calibration Solutions
0.1 M
Typical Applications
& Notes:
–
Pb(ClO4)2 (13-641-773)
Organic compounds,
water/wastewater,
plating baths
–
Pb(ClO4)2 (13-641-773)
Organic compounds,
water/wastewater,
plating baths. Requires
reference half cell
13-620-545
Silver/sulfide
ISE - solid-state
Sensing half cell
n/a
n/a
ACCU0820-500
SAOB
13-641-882
Plastic
80 °C
108 x 13
BNC
Hg++
0.01 to 107,900 (Ag+)
0.003 to 32,100 (S--)
–
–
Sewage effluent, soils,
sediments, plating
baths, pulping liquors,
photographic fixing
solution. Requires
reference half cell
13-620-538
Cyanide
ISE - solid-state
Combination
Refillable
ACCU0808-500
ACCU0802-500
Plastic
80 °C
102 x 13
BNC
Cl-, Br-, I-, S-- absent
0.1 to 260
–
–
Silicon petrochemical,
plating water, wastes
Gas-Sensing And
Glass Membrane ISE
Solid State ISE
Catalog No.
Parameter
Type
Combination Or Half Cell
Refillable Or Gel (Sealed)
Refill Solution
Ionic Strength Adjuster
13-620-627
Chloride
ISE - solid-state
Combination
Refillable
13-620-432
ACCU0820-500
13-620-519
Chloride
ISE - solid-state
Sensing half cell
Sealed
n/a
ACCU0820-500
13-620-547
Cupric
ISE - solid-state
Combination
Refillable
ACCU0834-500
13-641-852
Glass Or Plastic Body
Max Temp
Length x Diameter (mm)
Connection
Interferences
Range (ppm)
Calibration Solutions
0.1 M
Typical Applications
& Notes:
Plastic
80 °C
110 x 13
BNC
S--, I-, CN-, OH-, Br1.8 to 35,500
1000 ppm (ACCU0819-500)
NaCl (ACCU0818-500)
Water/wastewater,
soil, dairy, meats,
tomato/vegetable
products, Method
ASTM/AOAC approved
Plastic
80 °C
102 x 13
BNC
Br, I-, CN-, S--, OH1.8 to 35,500
1000 ppm (ACCU0819-500)
NaCl (ACCU0818-500)
Water/wastewater, soil,
dairy, meats, tomato/
vegetable products,
Method ASTM/AOAC
approved. Requires
reference half cell
Glass
80 °C
108 x 13
BNC
Ag+, Hg++, Cl-, Br-, Fe+, Cd++
0.00064 to 6,350
1000 ppm (23004)
Cu(NO3)2 (13-641-835)
Plating baths, water
50
13-620-551
Silver/sulfide
ISE - solid-state
Combination
Refillable
ACCU0834-500
ACCU0820-500
SAOB
13-641-882
Glass
80 °C
108 x 13
BNC
Hg++
0.01 to 107,900 (Ag+)
0.003 to 32,100 (S--)
–
–
Sewage effluent,
soils, sediments,
plating baths, pulping
liquors, photographic
fixing solution
13-620-629
Fluoride
ISE - solid-state
Combination
Refillable
13-620-431
ACCU0831-500
ACCU0835-500
13-641-874
Plastic
80 °C
110 x 13
BNC
OH0.02 to saturated
100 ppm (ACCU0825-500)
NaF (ACCU0824-500)
Drinking water,
wastwater and natural
waters, stack gases,
sea water, minerals,
soils, foods, biological
fluids, toothpaste,
ASTM/EPA Method
13-620-523
Fluoride
ISE - solid-state
Sensing half cell
Sealed
n/a
ACCU0831-500
ACCU0835-500
13-641-874
Plastic
80 °C
102 x 13
BNC
OH0.02 to saturated
100 ppm (ACCU0825-500)
NaF (ACCU0824-500)
Drinking water, wastwater
and natural waters,
air and stack gases,
sea water, minerals,
soils, foods, biological
fluids, toothpaste
Catalog No.
Parameter
Type
Combination Or Half Cell
Refillable Or Gel (Sealed)
Refill Solution
Ionic Strength Adjuster
Glass Or Plastic Body
Max Temp
Length x Diameter (mm)
Connection
Interferences
Range (ppm)
Calibration Solutions
0.1 M
Typical Applications
& Notes:
13-620-509
Ammonia
ISE - gas sensing
Combination
Refillable
ACCU0800-500
ACCU0802-500
Plastic
50 °C
108 x 12
BNC
Volatile amines, metal cations
that complex ammonia
0.009 to 1,700
1000 ppm
ACCU0801-500
NH4Cl (ACCU0800-500)
Sewage effluent, boiler water,
fish tanks, stack gases, sea
water, biological samples,
fertilizers. Uses 13-620-512
replacement membranes
13-620-511
Carbon Dioxide
ISE - gas sensing
Combination
Refillable
ACCU0806-500
ACCU0805-500
Plastic
50 °C
108 x 12
BNC
Volatile organic acids
0.44 to 1,320
–
13-620-503A
Sodium
ISE - glass membrane
Combination
Refillable
SP138-500
ACCU0832-500
Plastic
60 °C
106 x 12
BNC
Ag+, Li+, K+, NH4+
0.023 to 23,000
1000 ppm = ACCU0828-500
100 ppm = ACCU0827-500
10 % NaCl (ACCU0826-500)
NaHCO3 (ACCU0804-500)
Measures carbon dioxide,
Steam condensate, meats,
carbonate, bicarbonate in soft fish, dairy, fruit juices,
drinks, wines, ground/sea water, fermentation, ground/sea water,
fermentation. Uses 13-620-513 soils, body fluids
replacement membranes
13-620-501
Sodium
ISE - glass membrane
Sensing half cell
Refillable
SP138-500
ACCU0832-500
Glass
80 °C
108 x 13
BNC
Ag+, Li+, K+, NH4+
0.023 to 23,000
1000 ppm = ACCU0828-500
100 ppm = ACCU0827-500
10 % NaCl (ACCU0826-500)
Steam condensate, meats,
fish, dairy, fruit juices,
fermentation, ground/sea water,
soils, body fluids.
Requires reference half cell
51
Fisher Scientific accumet®
Conductivity, Dissolved Oxygen And Temperature Probes
Accessories/Cables/Printers
Keep Your Instruments Running Smoothly
Get The Most Out Of Your System
Conductivity Cells: For Fisher Scientific accumet® And Oakton® Meters
13-620-156
Catalog No.
13-637-670
13-637-669
13-637-668
13-637-667
13-302-222
13-637-670
15-500-012
Dissolved Oxygen Probes: For Fisher Scientific accumet® And Oakton® Meters
Dissolved Oxygen Probes And Accessories For Fisher Scientific accumet® AR60, AR40, AB40,
XL40 And XL60 Benchtop Meters
Description
Catalog No.
Self-Stirring DO/BOD/Temp Probe
13-620-SSP
13-637-DOADPT
Adapter To Connect YSI® Self Stirring DO Probes To AR, AB, XL Series Meters
Membrane Kit For 13-620-SSP. Includes (6) Membrane Caps, Polishing Disk,
13-637-DOM
And Electrolyte Filling Solution
Dissolved Oxygen Probes And Accessories For Fisher Scientific accumet® AP74, AP84, And
Oakton® DO 110 Portable Meters
Description
Catalog No.
DO/Temperature Probe With 10-Ft Submersible Cable
15-500-034
DO/Temperature Probe With 25-Ft Submersible Cable
15-500-035
Electrolyte Solution For Dissolved Oxygen Probes, 500 mL
15-500-036
DO Replacement Membrane Assembly, Preassembled Membrane, Membrane 15-500-037
Lock, O-ring, Cap
Replacement DO Probe Membranes, Pack Of 25, Requires Membrane
15-500-038
Installation Tool
Membrane Installation Tool, Required To Replace Membranes
15-500-039
Replacement Probe For DO 6, Includes 50 mL Electrolyte And Spare Cap
13-620-SSP
15-500-034
13-200-271
13-200-271
ATC/Temperature Probes: For Fisher Scientific accumet® And Oakton® Meters
All temperature probes are stainless steel and have a 3-ft cable.
Description
ATC For XL, AR, And AB Series Benchtop Meters
ATC For AP72, AP71, And PC 510 Meters
ATC For AP100 And AP60 Series Portable Meters
ATC For Discontinued Fisher Scientific accumet® Models 10, 15, 20, 25, 30, 50,
150, 750, 800MP Series, And 900 Series
ATC For Oakton pH 6, 11, 110, 510, 1100, 2100 Meters
52
Catalog No.
13-620-19
13-620-20
13-620-AP53
13-620-16
BNC Adapter Cables
Description
U.S. Standard/Pin To BNC Input Jack
BNC Plug To U.S. Standard/Pin Input Jack
Pin Plug To BNC Input Jack. Use With Metallic Electrodes Having
Pin Connectors
BNC/Pin Plugs To Single BNC Input Jack
Dual-Pin Plugs To BNC Input Jack
Catalog No.
13-620-498
13-620-491
13-620-488
13-620-AP54
Compact Printer With Date/Time Stamp
Description
Dot Matrix Portable Printer For AB15P. Includes Cable, Ribbon, Paper, 115 V
Printer Paper
Printer Replacement Ribbon
Printer Cable
13-620-489
13-620-162
13-620-490
Cell For Oakton® 600 Series Meters
1.0
Ultem Body
13-620-163
13-620-491
Cell For Oakton® CON 11, CON 110, pH/CON 510 And AP75, AP85 Meters
1.0
Ultem Body
13-620-AP54
13-620-160
13-620-498
Nominal Cell Constant
Glass Body Cat. No.
Epoxy Body Cat. No.
2-Cell Conductivity Cells Without ATC For AR And AB Series Meters
0.1
13-620-156
13-620-161
1.0
13-620-155
13-620-160
10.0
13-620-157
13-620-162
2-Cell Conductivity Cells With ATC For XL Series Meters
0.1
–
13-620-101
1.0
–
13-620-100
10.0
–
13-620-102
4-Cell Conductivity Cells With ATC For AR, AB, And XL Series Meters
1.0
13-620-163
13-620-165
10.0
13-620-164
13-620-166
Precision Calibration Resistor Kit
Check meter compliance to USP standards. Includes:
• Six precision resistors (nominal values 10 ohm, 100 ohm, 1 kilohm, 10 kilohm,
100 kilohm, and 1 megohm)
• Connectors for Fisher Scientific accumet® XL, AR, and AB conductivity meters
• Foam padded case
Description
Catalog No.
Fisher Scientific accumet® Conductivity Calibration Kit
13-637-674
13-620-490
13-620-489
13-620-488
Other Replacement Parts And Accessories
Description
1.5 V Batteries For Oakton Testrs, Pack Of (6)
Electrode Arm And Bracket For AR, AB, XL Series Benchtop Meters
Free-Standing Benchtop Electrode Support Arm
110/220 V Power Supply For XL Series Meters
110/220 V Power Supply For AB Series, AP60 Series, AP100 Series
100/240 V Power Supply For Oakton 11, 110, 1100, 2100 Series, US/UK/EUR Plug
100/240 V Power Supply For Oakton 600 Series, US/UK/EUR/JPN Plug
Meter To PC RS-232/Serial Cable, (M)DB9 To (F)DB9
Catalog No.
S60037BAT
13-637-671
50-633-374
13-636-XL60E
13-636-100
13-300-126
15-500-058
13-637-680
13-636-XL60E
13-620-499
50-633-374
13-637-671
13-637-680
13-620-19
15-500-003
13-637-DOM
13-637-DOADPT
53
Fisher Chemical Certified Buffer Solutions
Fisher Scientific Isotemp® Magnetic And Hotplate Stirrers
Standardized Against NIST Standard Reference Material
Fisher Chemical Certified Color-Coded pH Buffers
Accurate Measurement Requires Adequate Stirring
Fisher Chemical Certified Buffer Solutions
• Stated values certified accurate to 0.02 pH or 0.01 pH at time
buffer is bottled
• Label includes temperature correction chart
• 500 mL and 1 L sizes use rugged polyethylene bottles with leak proof
screw caps and plastic seals
• 10 L & 20 L quantities come in handy PolyPac* store-and
dispense containers
• Color-coded solutions with matching colored labels provides
instant recognition
• Tight tolerances:
- 0.01 for pH 4.0 and 7.0
- 0.02 for pH 10.0
• Standardized at 25 °C against NIST standard reference material
• Label includes temperature correction chart
• MSDS’s available from www.fishersci.com
pH
4.00
4.00
4.00
7.00
7.00
7.00
10.00
10.00
10.00
Color
Red
Red
Red
Yellow
Yellow
Yellow
Blue
Blue
Blue
Size
500 mL
4L
20 L
500 mL
4L
20 L
500 mL
4L
20 L
Catalog No.
SB101-500
SB101-4
SB101-20
SB107-500
SB107-4
SB107-20
SB115-500
SB115-4
SB115-20
Color
Red
Yellow
Blue
Size
500 mL
500 mL
500 mL
Catalog No.
SB105-500
Refilling And Storage Solutions
Description
Saturated KCL. Use With Double-Junction
And Calomel pH Electrodes (Purple Ring), Or
Fluoride (13-620-529), Sodium, And Calcium
Combination Ion Selective Electrodes
4 M KCL Saturated With AgCl. Use With
Single-Junction pH Electrodes (Blue Ring).
Skylite Electrolyte With Syringe For accuCap
13-620-130 pH Electrode
pH Electrode Storage Solution
Electrode Storage Bottle
54
Tolerance
±0.02
±0.02
±0.01
±0.01
±0.01
±0.01
±0.01
±0.02
±0.01
±0.01
±0.01
±0.01
±0.01
±0.01
±0.02
±0.02
±0.02
±0.02
±0.02
Size
500 mL
500 mL
500 mL
500 mL
1L
10 L
20 L
1L
500 mL
500 mL
500 mL
1L
10 L
500 mL
500 mL
500 mL
500 mL
1L
10 L
lab space
»Speed range from 60 to 1200 rpm
Catalog No.
SB140-500
SB96-500
SB97-500
SB98-500
SB98-1
SB98-10
SB98-20
SB100-1
SB102-500
SB104-500
SB108-500
SB108-1
SB108-10
SB110-500
SB112-500
SB114-500
SB116-500
SB116-1
SB116-10
Size
Catalog No.
500 mL
SP138-500
500 mL
SP135-500
60 mL
13-636-430
1L
–
SE40-1
13-620-499
Specifications and Ordering Information
Top Plate
Area
Nominal (L x W)
Load Capacity
Speed
Range
Stability
Temperature
(For Hotplate Stirrers)
Range
Overall
Operating Temp
Dimensions (L x W x H)
Power
Approvals
16 sq. in.
4 x 4” (10.2 x 10.2 cm)
15 lb (6.8 kg)
60 to 1200 rpm
±20 rpm
Ambient to 540 ºC
4 to 40 °C (39 to 104 °F)
5.5 x 3.8 x 9.7“ (14 x 9.3 x 24.7 cm)
120 V, 50/60 Hz
CSA/IEC 1010
Magnetic Stirrer: Includes manual, and a two-year parts, labor, and
travel warranty.
Description
4 x 4 in Magnetic Stirrer
4 x 4 in Hotplate Stirrer
Catalog No.
11-100-16S
11-100-16SH
Fisher Chemical Dry pH Buffer Salts
For the items in this catalog and thousands
of others, visit www.fishersci.com
• Easier to store, more stable than solutions
• Supplied in polyethylene-lined GramPac* envelopes
• Each makes 1 L of buffer
pH
4.00
6.86
7.41
9.18
10.4
You’ll Find:
• Our Entire Catalog On-Line
• Current Prices
• Search Engine For Thousands Of Products
• Material Safety Data Sheets
• Faster Order Processing
Catalog No.
B79
B78
B82
B80
B77
Oakton Conductivity Solutions
Buffer-Pac Includes 4, 7, 10 Buffers.
pH
4.00
7.00
10.00
pH
1.00
2.00
3.00
4.00
4.00
4.00
4.00
4.63
5.00
6.00
7.00
7.00
7.00
7.40
8.00
9.00
10.00
10.00
10.00
»Small footprint saves valuable
• Choose 1 pint (475 mL) plastic bottles or (20) single-use pouches
Value
10 µS
23 µS
84 µS
447 µS
447 µS
1413 µS
1413 µS
1500 µS
2070 µS
2764 µS
2764 µS
8974 µS
12880 µS
15000 µS
15000 µS
80000 µS
Quantity
20 pouches
475 mL
475 mL
475 mL
20 pouches
475 mL
20 pouches
475 mL
475 mL
475 mL
20 pouches
475 mL
475 mL
475 mL
20 pouches
475 mL
Catalog No.
50-632-479
13-300-114
13-300-111
13-300-117
13-300-129
13-300-112
13-300-130
13-300-110
13-300-115
13-300-113
13-300-131
13-300-119
13-300-109
13-300-118
13-300-132
13-300-116
Accurate pH measurement requires adequate
stirring. To ensure the best results from your
Fisher Scientific accumet® or Oakton® meter,
be sure you use a Fisher Isotemp® magnetic
stirrer. This sleek stirrer combines top-of-the-line
performance with a low-profile design and one
of the smallest footprints available. Ceramic top
plate provides a reflective white finish for better
viewing of color contrasts in titrations and other
applications. Plus, it’s acid- and alkali-resistant
and easy to clean.
Direct-drive motor delivers quiet, dependable
operation and has a speed range of 60 to 1200
rpm with a stirring stability of ±20 rpm. Instant-on
operation allows you to start stirring at 60 rpm
at the first click – with no splashing. Adjustable
control knob has graduations to provide speed
references and is easy to grip – even while
wearing heavy gloves. Illuminated LED lets you
know when stirrer is on.
The hotplate stirrer model provides the option
for a sample to be maintained at a constant
temperature for consistent pH measurements.
Built-in support rod mount at the rear of stirrer
will accommodate a rod up to 13 mm in diameter;
includes thumbscrew to secure rod. Unit is
designed to prevent liquid spills from damaging
the stirrer and can be used in environments from
4 to 40 °C (39 to 104 °F). Operates on 120 V,
50/60 Hz; CSA/IEC 1010 approved. Unit includes a
two-year parts, labor, and travel warranty.
55
About pH, ORP, And ISE Measurement
pH
Next to temperature, pH is quite possibly the most common laboratory
measurement today. It crosses over many disciplines from water/wastewater,
R&D, environmental, chemical & life sciences, and an endless number of
industrial applications.
The pH or power of Hydrogen, is the degree of acidity or alkalinity of a solution
based on the hydrogen ion activity, represented by the equation:
pH = -log [H+]
Stated mathematically, pH is the negative logarithmic value of the Hydrogen
ion. As it is based on a log scale, each pH unit represents a factor of 10, so a
solution with a pH of 5 is 100 times more acidic than pH of 7.
Here are examples of pH in a few common industrial and household products.
14
Household dye [13.6]
Copper plating [12.8]
13
[13.1] Bottle washing
[12.6] Bleach
12
Ammonia [11.4]
Milk of magnesia [10.3]
Borax [9.3]
11
[11.3] Brass plating
10
9
[9.4] Lime-soda softening
8
[8.4] Baking soda
[8.0] Seawater
Fresh water aquarium [7.1]
Milk [6.7]
7
[7.4] Swimming pool water
[7.0] Distilled water
Corn [6.2]
6
Saltwater aquarium [8.0]
Blood [7.5]
Boric acid [5.0]
Pickle processing [3.5]
Vinegar [2.9]
5
[6.3] Brewing process
[5.8] Nickel plating
[5.0] Food processing
Temperature Compensation
In a perfect pH electrode – one that measures zero mV at exactly pH 7 – there
is no temperature effect on the electrode sensitivity at pH 7 regardless of
temperature change. While pH electrodes are not perfect, errors related
to temperature are negligible near pH 7, and can be disregarded. As a rule
however, the further from pH 7 the solution is and the greater the temperature
changes, the greater the expected measurement error due to changes in the
electrode’s sensitivity. For most electrodes, the error is approximately 0.003
pH/ºC/pH away from pH 7.
Consider a pH meter that was calibrated at room temperature (25 ºC) and
measures a sample at pH 4 at 5 ºC,
Temperature difference
:25 ºC - 5 ºC = 20 ºC
Number of pH units away from neutral:7 pH - 4 pH = 3 pH units
Total error
:0.003 x 20 x 3 = 0.18 pH
pH meters that incorporate Automatic Temperature Compensation (ATC) are
able to overcome this error.
Q: So why does my pH 10.00 buffer read as 10.06?
A: ATC or not, temperature influences the pH of all solutions. While pH 10.00
buffer is certified to 10.00 at 25 ºC / 77 ºF, at 20 ºC / 68 ºF which is often room
temperature, it is actually 10.06.
[4.3] Orange juice
3
[3.2] Photo engraving
To understand the pH/temperature relationship, we need to look at two main
processes; Calibration & Measurement.
2
[2.3] Lemon juice
Calibration: Meters have a buffer/temperature look-up table stored into
memory for every pH buffer that the meter can calibrate to. For example, when
calibrating with a pH 10 buffer that is 20 ºC, the meter knows to calibrate to a
value of 10.06 when calibration is performed. Had the temperature of the buffer
been 25 ºC, the meter would calibrate instead to 10.00. Using a temperature
probe during calibration allows best accuracy as the meter is able to calibrate
to the most appropriate pH value.
0
Litmus paper can be used for rough pH measurements, but for highest
accuracy a potentiometric system using a pH meter and pH electrode is used.
The electrode is sensitive to H+ ions, which create a small voltage potential,
which is converted to a pH value by the pH meter. Electrode behavior is
described by the Nernst equation:
E = Eo + (2.3 RT/nF) log aH+
E is the measured potential from the sensing electrode, Eo is related to the
potential of the reference electrode, (2.3 RT/nF) is the Nernst factor and log
aH+ is the pH. The Nernst factor, 2.3 RT/nF, includes the Gas Law constant (R),
Faraday’s constant (F), the temperature in degrees Kelvin (T) and the charge
56
The pH electrode system consists of two half cells: a pH indicating electrode,
which develops a potential dependent on the pH of a solution; and a reference
electrode, which provides a constant potential and completes the electrical
circuit. Using separate pH indicating and reference half cells allows you
to select each cell without compromise, tailor the system precisely to the
sample’s nature, and achieve good accuracy. It can mean lower replacement
costs too, since usually only one of a pair is broken. For ATC, a third electrode
is required. Nevertheless, the combination electrode – an indicating half cell
and a reference half cell joined co-axially – is by far the most common choice
for the convenience and compactness it offers. Some combined electrodes
also offer built-in ATC for added convenience.
4
1
Battery acid [0.3]
of the ion (n). For pH, where n = 1, the Nernst factor is 2.3 RT/F. Since R
and F are constants, the factor and therefore electrode behavior is dependent
on temperature.
Measurement: As temperature changes, the pH electrode responds slightly
differently. Since this change is predictable, the meter is able to make the
appropriate adjustment. The meter senses the temperature and applies
the appropriate electrode slope (mV per pH unit) for that temperature. For
example, if the temp is 25 ºC, the instrument uses 59.16 mV for each pH unit.
If the temperature is 20 ºC, the meter uses 58.16 mV for each pH unit. Using a
temperature probe during measurement allows best accuracy as the meter is
able to correct for changes in electrode response due to temperature. Note:
Temperature compensation only corrects for the change in the output of the
electrode, not for the change in the actual solution pH. As such, the AB15 will
always display the actual pH of the solution at the current temperature.
pH Standardization Buffers
Buffers – solutions of known pH value – allow adjustments to the meter/
electrode system to reflect accurate measurements. Certified accurate
buffers are available as ready-to-use color coded solutions, concentrated
solutions, capsules and prepackaged salts. All have the special characteristic
of resisting pH change upon dilution or acid/base contamination. For best
accuracy, use a minimum of two-point standardization; first with a buffer value
close to the electrode system’s zero potential (typically pH 7); and next with an
acid or base buffer whose value brackets the expected pH value of the sample.
Microprocessor-based meters may permit additional calibrations – up to five
points in some models. For best accuracy, perform calibration with ATC at the
same temperature as the expected samples.
ORP Measurement
Oxidation-Reduction Potential (ORP) or “Redox” measurements are used
to monitor chemical reactions, to quantify ion activity, or to determine the
oxidizing or reducing properties of a solution. ORP is a measurement
of the total electrical potential of a solution – a sum of all oxidation and
reduction reactions.
ORP electrodes measure the voltage across a circuit formed by the measuring
metal half cell and the reference half cell. When the ORP electrode is placed
in the presence of oxidizing or reducing agents, electrons are constantly
transferred back and forth on its measuring surface, generating a tiny voltage.
ORP is expressed as +/- mV, and can be measured by using the millivolt mode
of any pH meter with an ORP electrode.
Like pH, an ORP electrode system consists of a sensing and reference
electrodes. For ORP, the indicating electrode is metallic – usually platinum,
gold, or silver.
ORP varies with temperature, but unlike pH, it is not completely predictable and
therefore, ATC is not used. Constant temperature is best for monitoring ORP.
In principle, ORP measurements should not require standardization; in practice
however, it may be necessary to check the system against standards of known
potential, as described in ASTM Method D 1498.
ORP is also useful in pool water treatment as an indication of sanitation in
relation to free chlorine. ORP technology has gained recognition worldwide
and is found to be a reliable indicator of bacteriological water quality. The
table below illustrates the Kill Time of E.Coli bacteria as a function of ORP
value. With a value of 600 mV, the life of the bacteria is almost 2 minutes; at
650 mV it reduces to 30 seconds. Above 700 mV the bacteria is killed within a
few seconds. It is therefore necessary for the water to have an ORP value of at
least 700 mV to ensure good water quality.
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ORP value also depends on the pH of pool water. The pH of the pool water
has to be maintained at the optimum level between 7.2 and 7.6 pH by dosing
appropriate chemicals. If the pH of the swimming pool water is acceptable
and the ORP value is below 700 mV, hypochlorite or other oxidizing chemicals
should be added.
Common ORP applications include the treatment of industrial wastes, study
of biological systems, oxidation of cyanide, bleaching of pulp, manufacture of
bleach and reduction of chromate wastes.
ISE Measurement
Ion-Selective Electrodes (ISE) respond to a particular ion in solution because
of the characteristics of the electrode’s sensing membrane. Ideally, the ISE
develops an electrical potential which is proportional to the concentration of
the ion for which the membrane is selective. The most widely-used ISE is the
glass-membrane pH electrode (selective for hydrogen ion).
When an ISE – the indicator electrode – and a reference electrode are placed
in a solution and connected to a pH/ion meter, they form a potentiometric cell.
At equilibrium, the meter measures the potential difference between the ISE
and the reference electrode. This potential is proportional to the activity of the
ion of interest.
The table below gives theoretical slope values at 25 ºC:
Species
Monovalent Cation
Monovalent Anion
Divalent Cation
Divalent Anion
Slope (mV/Decade)
+59.16
-59.16
+29.58
-29.58
Although ISE’s are most often measured by direct analysis, some sample
preparation is required. Normally, an ionic strength adjuster must be added
equally to samples and standards. Then, standards must be prepared which
are used to calibrate the meter, or to construct a calibration curve (by
plotting the electrode’s output in mV vs. the log of concentration). Sample
concentration can then be read directly from the meter or calibration curve.
Incremental methods can reduce errors caused by temperature variations,
complex matrices, and complexation. They’re also useful for applications
where only occasional samples are analyzed. Incremental methods include:
1) Known Addition; 2) Known Subtraction; 3) Analate Addition; and
4) Analate Subtraction.
Ion-selective electrodes can also be used to detect the endpoint of a titration.
The ISE can be selected to monitor either the addition of titrant or the depletion
of analate. The electrode potential is plotted vs. the volume of titrant added.
The volume corresponding to the equivalence point is determined from the
graph, and used to calculate sample concentration.
A number of metallic electrodes are also used in titrations. Dual platinum
wire and plate electrodes, are used with pH meters and titration instruments
in dead-stop and amperometric titrations; and silver billet electrodes are the
choice for silver and halide titrations.
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The ion meter/electrode system must be standardized by immersing the
electrodes into solutions having a known concentration of the ion of interest.
Stock ISE standards are available in a variety of molar, ppm, and percent
concentrations. Calibration is typically done using at least two points that are
10 fold apart, such as 1 and 10 ppm. An ionic strength adjuster is required to
eliminate interference from other ions. Since some ISE’s have a restricted pH
range, a pH adjustment solution may also be necessary. Other special reagents
and solutions are available for specific applications.
57
pH
A Practical Approach To Achieve Best Results
Testing/Care/Troubleshooting
Prevent Problems From Happening And Recognize When It’s Too Late
6. Also take notice of the electrode response time – the faster, the better. Don’t
expect an economy gel electrode to respond as fast as a high performance
glass refillable electrode, but note that a great slope alone is not meaningful
if the electrode takes 2 hours to stabilize.
Slope
pH Electrode Slope
A perfect pH electrode under ideal conditions at 25 ºC, produces a slope
of 59.16 mV per pH unit. For example, an electrode with 0 mV offset should
read mV value of 177.48 mV when placed in a pH 4.01 solution. The slope is
calculated as (177.48 mV – 0 mV) / 3 pH = 59.16 mV/pH. The difference between
this perfect slope reading and the electrode’s actual reading is called the slope
error. These theoretical values are not always achieved, even with brand new
electrodes. The slope of most new pH electrodes should fall between 92 % and
102 % of 59.16 mV.
mV
In theory under perfect
conditions, an ideal electrode
has the following properties:
Offset = 0 mV
Slope = 59.16 mV/pH @ 25 °C
ISO Potential Point
7
pH
Normal Aging
As electrodes are used or stored for long periods they can undergo changes
in performance. The electrode offset and slope can be monitored to evaluate
these changes. Periodic calibration is usually adequate to correct for these
changes. If an electrode is able to be calibrated and reads accurately in
certified standards, is stable, is responsive and repeatable, it is still considered
functional and may be used indefinitely although it no longer meets “new”
electrode specifications with regards to offset or slope.
How to check your pH electrode – including slope and offset:
1. Clear/reset any existing calibration stored in your meter then select the
millivolt (mV) mode of your pH meter
2. Using a pH electrode, obtain mV readings of two fresh calibration buffers
(i.e. pH 4.0 and pH 7.0 are best)
3. Determine the net mV change
4. Determine the net mV change per pH unit change and compare using the
chart below. For example, at 25 ºC: pH 4 = 170.5 mV, pH 7 = -3.4 mV: Net
change = 170.5 mV - (-3.4 mV) = 173.9 mV. Since pH 4 and pH 7 are 3 pH units
apart, using the chart below, 98 % = Very Good
5. The offset can be determined by observing the mV reading with your pH
electrode in pH 7 buffer. The closer to 0 mV the better. A mV more than
30 mV from 0 may be problematic.
58
At 25 ºC
Status
>102
>59.3
>178.0
>60.3
>181.0
Poor
102 %
59.3
178.0
60.3
181.0
Ok
Very Good
101 %
58.8
176.3
59.8
179.3
100 %
58.2
174.5
59.2
177.5
Ideal
99 %
57.6
172.8
58.6
175.7
Very Good
98 %
57.0
171.0
58.0
173.9
Very Good
97 %
56.4
169.3
57.4
172.2
Very Good
96 %
55.8
167.5
56.8
170.4
Very Good
95 %
55.3
165.8
56.2
168.6
Very Good
94 %
57.0
171.0
58.0
173.9
Ok
93 %
54.1
162.3
55.0
165.1
Ok
92 %
53.5
160.5
54.4
163.3
Ok
<92
<53.5
<160.5
<54.4
<163.3
Poor
Common Symptoms/Cause (Remedy)
▼
0
At 20 ºC
1 pH Unit 3 pH Units 1 pH Unit 3 pH Units
• No Response, All Buffers Or Samples Read The Same pH –
Usually pH 7.00 Or 0 mV
Broken sensing bulb or wiring problem (replace electrode), probe not
connected to input (verify correct channel selection when using
multiple-channel meters), probe is not in contact with sample (remove
electrode storage bottle or rubber bulb guard), meter automatically has
frozen reading (verify that the hold feature or auto read feature is set to off
when using meters with this feature, usually by pressing measure or read.)
• Slow Response With Excessive Crystallization Inside Probe Of
Refillable Electrodes
Electrolyte flow clogged from supersaturated electrolyte (“flush & fill” by
remove the filling solution through the fill hole with a syringe or by shaking
it upside down. Repeatedly flush and rinse the reference cavity with clean,
60-80 ºC water to dissolve crystals until removed. Replace filling solution
and apply gentle pressure to the filling hole. Re-hydrate electrode in storage
solution or pH 4 buffer), (ensure fill hole is in open position). To prevent this in
the future, ensure that the re-fill hole is closed when electrode is not in use.
• Slow Response Due To Clogged Junction-Usually With
Single-Junction Electrodes
Reaction with silver such as silver sulfide formation or protein deposits
which causes a dark spot on the ceramic reference junction (For protein
layers prepare a 1 % pepsin solution in 0.1 M of HCl and soak the reference
junction for one hour in this solution. Rinse the electrode with distilled water.
Alternatively, heat a diluted KCl solution to 60 to 80 ºC. Place the sensing
part of the electrode into the heated solution for about 10 minutes. Allow the
electrode to cool in some unheated KCl solution.
• Dried Salt Deposits Present
Electrolyte residue deposited on electrode surface – often with new
electrodes or periods of non-use. (simply dissolve the deposits in warm tap
water followed by a brief soak in pH 4 buffer).
• Slow Response, Noisy, Unstable, Or Erratic Readings
Sensing bulb dry or dirty (clean electrode with mild detergent & warm water
and re-hydrate electrode), temperature may be changing rapidly or electrode
may be thermally shocked (allow electrode to reach sample temperature),
sample may be non-aqueous (take 30 second readings and soak in pH buffer
for one minute between measurements).
pH
A perfect pH electrode under ideal conditions, when placed in pH 7.00
buffer at 25 ºC, will produce 0 mV. The difference between 0 mV and the
electrode’s actual mV reading is called the offset error, which usually becomes
problematic when it is more than ±30 mV or about ½ of a pH unit.
▼
pH
pH Electrode Offset
For additional support contact your Fisher Scientific sales or customer
service representative, call a electrochemistry specialist at 1-888-358-4706,
or send an email to: [email protected]
General Rules And Tips
• When using refillable electrodes, open the fill hole during calibration and
measurement – but remember to close it afterwards when finished!
• The level of electrolyte in the outer cavity of refillable electrodes should
be kept above the level of the solution being measured to prevent reverse
electrolyte flow.
• The electrode need only be immersed far enough to cover both the glass pH
sensing bulb and reference junction to obtain accurate readings.
• Electrodes perform best when they are hydrated. However, if they dry out
they can be reconditioned to normal performance again. Soaking in electrode
storage solution helps to optimize and re-establish the thin hydration layer on
the sensing bulb that is critical to pH measurement.
• Rinsing the electrode with deionized or distilled water between samples is
fine, but storage in deionized or distilled can be detrimental as it will rob
critical ions from the sensing bulb. Also, avoid wiping or touching the sensing
bulb to maintain the hydration layer and producing any electrical charge.
• Moving or touching the electrode cable may result in unstable readings
due to the high impedance (resistance) of the pH glass membrane and
introduce noise.
• To eliminate temperature errors associated with the electrode, manual
or automatic temperature compensation (ATC) should be used for best
accuracy. Since temperature changes pH, the sample temperature should
always be noted with pH readings. i.e.) Record results as “pH 8.43 @ 23.2 ºC”,
instead of “pH 8.43”.
• Always use fresh pH buffers for calibration. Excessive air exposure and
sunlight an alter the buffers value – especially pH 10.00 buffers which is
particularly susceptible to drift.
The common BNC
connector is an
abbreviation for Bayonet
Neill-Concelman, named
after its inventors.
59
About Conductivity Measurement
A Practical Approach To Achieve Best Results
Solution
Conductivity
Absolute Pure Water
0.055 µS
Power Plant Boiler Water
1.0 µS
Good City Water
50 µS
Ocean Water
53 mS
Distilled Water
0.5 µS
Deionized Water
0.1 - 10 µS
Drinking Water
0.1 - 1 mS
Wastewater
0.9 - 9 mS
Seawater
53 mS
10 % NaOH
355 mS
10 % H2SO4
432 mS
31 % HNO3
865 mS
Resistivity, Conductivity, and Salinity Units
The basic unit of conductivity is the Siemens (S), formerly called the mho.
Since cell geometry affects conductivity values, standardized measurements
are expressed in specific conductivity units (S/cm) to compensate for
variations in electrode dimensions. Specific conductivity (C) is simply the
product of measured conductivity (G) and the electrode cell constant (L/A),
where L is the length of the column of liquid between the electrode and A is
the area of the electrodes.
Figure 1
Conductivity Measurement
The principle by which instruments measure conductivity is simple – two
plates are placed in the sample, a potential is applied across the plates, and
the current is measured. Conductivity (G), the inverse of Resistivity (R) is
determined from the voltage and current values according to Ohm’s law.
G = I/R = I (amps) / E (volts)
Since the charge on ions in solution facilitates the conductance of electrical
current, the conductivity of a solution is proportional to its ion concentration.
However, conductivity may not correlate directly to concentration. The graphs
below illustrate the relationship between conductivity and ion concentration
for two common solutions. Notice that the graph is linear for sodium chloride
solution, but not for highly concentrated sulfuric acid.
C = G x (L/A)
Resistivity is simply the reciprocal of conductivity: conductivity = 1/resistivity.
In practice, resistivity units are used when describing ultra pure water such
as deionized or reverse-osmosis water. Conductivity units are used for water
ranging from drinking water to brackish water. Salinity units are reserved for
the highest concentrations expressed as parts per thousand (ppt) or more
commonly % concentrations of salt, such as 3.5 % sea water.
Conductivity
Resistivity
0.01 µS
100 MΩ
0.055 µS
18.0 MΩ
0.1 µS
10 MΩ
1 µS
1 MΩ
10 µS
0.1 MΩ
100 µS
0.01 MΩ
1 mS
1 kΩ
Total Suspended Solids (TSS) and Total Dissolved Solids (TDS)
Suspended Solids measurements are traditionally determined by filtering a
measured volume of water – usually 1 liter. Solids captured by a filter are
deemed “suspended solids”. The water that passes thru (filtrate) is oven dried
and the residue that remains is called “dissolved solids”. Both are usually
measured as mg/L or ppm. Unfortunately, drying of the filtrate to complete
evaporation is a long process and requires heating. TDS (conductivity
multiplied by a TDS factor) can be used as a quick, inexpensive alternative
method. TDS as measured with a conductivity meter will correspond to the
dissolved ionic content that is often a useful approximation for the total
dissolved material.
There are limitations when using TDS. First, a TDS factor used is salt specific
so if there are multiple or unknown salts in solution, it’s nearly impossible to
determine the ideal factor to use. Second, since ionic concentrations are not
linear, the TDS factor changes with concentration. Also, TDS is not preferred
for low measurements.
60
Temperature Compensation and Coefficient
Advanced Meter Features To Consider
Conductivity is greatly influenced by temperature. Most fluids increase in
conductivity as temperature increases. Most ionic solutions will increase
about 2 % for each 1 ºC increase. Unfortunately, this temperature coefficient
(TC) is not always. In the case of high resistance water it can be closer to 5 %
or so per ºC.
Many instruments adjust the conductivity value based on a TC and display
a value that is said to be corrected or normalized to 25 ºC. The meter will
automatically make corrections to the reading and display a value as if the
sample was 25 ºC, no matter what the actual temperature is. Some instruments
use a fixed TC of 2.0 % per ºC. Let us consider a meter that uses 2.0 % TC to
measure a 1413 µS standard at 25 ºC (77 ºF). If the standard is warmed to
30 ºC (86 ºF), the meter applies a correction of 5 degrees x 0.02 % x 1413 µS =
141.3. Without correction (0.0 % TC) the actual value of a 1413 µS standard of
KCl at 30 ºC (86 ºF) is 1548 µS. As the meter corrects for temperature, it displays
a value of 1548 µS minus 141.3 µS = 1407 µS. When the sample cools to 25 ºC,
it will again read 1413 µS as no correction is applied. Although conductivity
cell response is immediate, temperature corrected values will fluctuate as the
temperature measurement stabilizes. Advanced meters offer adjustable TC’s,
usually from 0.0 % to as much as 10 % per ºC.
•Adjustable TDS Conversion Factor
When a solution does not have a similar ionic content to natural water, the
ability to adjust the TDS conversion factor will enable improved accuracy.
•Adjustable Temperature Coefficients
Allows for the precise manipulation of the temperature compensation levels
for improved results at various temperatures.
•Multiple Cell Constants
Allows use of additional electrodes for best results – usually low or
high ranges.
Conductivity
Conductivity
Electrical Conductivity (EC) meters measure the capacity of ions in an aqueous
solution to carry electrical current. As the ranges in aqueous solutions are
usually small, the basic units of measurements are milliSiemens/cm (mS) and
microSiemens/cm (μS).
Calibration and Maintenance
Conductivity systems should be calibrated to certified solution standards
before using. When selecting a standard, choose one that has the
approximate conductivity of the solution to be measured. Unlike pH,
conductivity cells generally change less over time and therefore do not
require as frequent calibration.
A polarized or fouled electrode must be cleaned to renew the active surface of
the cell. In most situations, hot water with a mild liquid detergent is an effective
cleanser. Alcohol easily cleans most organic matter, and chlorine solutions will
remove algae, bacteria or molds. To prevent cell damage, abrasives or sharp
objects should not be used to clean an electrode. A cotton swab works well for
cleaning but care must be taken not to widen the distance of cell.
2-Cell Vs. 4-Cell
Most conductivity meters use a 2-cell electrode. The electrode surface is
usually platinum, titanium, gold-plated nickel, or graphite. The 4-cell electrode
is more expensive and uses a reference voltage to compensate for any
polarization or fouling of the electrode plates. The reference voltage ensures
that measurements indicate actual conductivity independent of electrode
condition, resulting in higher accuracy.
2-cell electrode
Both table salt and sugar dissolve
easily in water. However, since
sugar molecules don’t dissociate
there is no charge and they can’t
be detected with an electrical
conductivity/TDS meter.
4-cell electrode
61
About Dissolved Oxygen Measurement
A Practical Approach To Achieve Best Results
BOD & COD
Dissolved Oxygen (DO) is a measure of the amount of dissolved gaseous
oxygen in a solution. Some gases, such as ammonia, carbon dioxide and
hydrogen chloride, react chemically with water to form new compounds.
However, gases such as nitrogen and oxygen merely dissolve in water without
chemically reacting with it, and exist as microscopic bubbles between
water molecules.
The BOD test measures the molecular oxygen utilized in the biodegradation
of organic material and the oxidation of inorganic material. By measuring the
amount of oxygen dissolved in samples at the beginning and end of a specified
incubation period, the relative oxygen requirements of wastewaters, effluents,
and polluted waters can be determined.
There are two main ways in which dissolved oxygen occurs naturally in
water: From the surrounding atmosphere, where oxygen in the surrounding
air dissolves readily when mixed into water, up to saturation, during water
movements; Via photosynthesis when oxygen is produced by aquatic plants
and algae as a by-product of photosynthesis. The amount of oxygen dissolved
in water is usually measured in percent saturation, or expressed as a
concentration in milligrams per litre water. Accurate measurement of dissolved
oxygen is essential in processes where oxygen content affects reaction rates,
process efficiency or environmental conditions, such as biological wastewater
treatment, wine production, bio-reactions, environmental water testing.
BODt (mg/L) = D1 - D2
P
Basic Principle in DO Measurement
Dissolved Oxygen
DO Electrodes
In theory, the amount of DO in a solution is dependent on three factors, namely
temperature, salinity and atmospheric pressure.
1. Water Temperature
Solubility of oxygen reduces as temperature increases. Hence, the colder
the water, the more dissolved oxygen it contains. Since temperature affects
both the solubility and diffusion rate of oxygen, temperature compensation is
necessary for any standardized DO measurements.
2. Salinity
The amount of dissolved oxygen increases as salinity level decreases.
In other words, freshwater holds more oxygen than saltwater. Since the
presence of dissolved salts limits the amount of oxygen that can dissolve in
water, the relationship between the partial pressure and concentration of
oxygen varies with the salinity of the sample.
3. Atmospheric Pressure
There is a direct proportional relationship between the solubility of
dissolved oxygen and the surrounding atmospheric pressure. As pressure
decreases with increase in altitude, the amount of dissolved oxygen found
in water reduces.
To ensure that your dissolved oxygen is not affected by atmospheric
pressure, most instruments include barometric pressure compensation.
Advanced meters include a built in barometer that measures and adjusts
automatically. However, basic meters will make adjustments only after
the user inputs the appropriate pressure manually. Atmospheric pressure
correction charts included in with certain instrument manuals are also a
helpful reference.
BODt = Oxygen uptake during incubation period t
The measurement of DO requires a special DO electrode that is made up of
an anode, a cathode, electrolyte solution and a gas permeable membrane.
The material of the membrane is specially selected to permit oxygen to pass
through. Oxygen is consumed by the cathode which will create a partial
pressure across the membrane. Oxygen then diffuses into the electrolyte
solution. In short, a DO meter actually measures the pressure caused by
movements of oxygen molecules in water or any other medium. The higher the
partial pressure of the oxygen in solution, the higher the oxygen concentration.
Currently, galvanic and polarographic electrodes are the predominant methods
for measuring dissolved oxygen.
The Galvanic Cell consists of two metals, the positive anode and the negative
cathode, connected by a salt bridge between the individual half-cells. As the
metal electrodes leave electrons behind as they dissolve in the electrolyte.
The different properties of the two metals causes them to dissolve at different
rates, hence a potential is created when the number of electrons in either side
of the cell differs. The potential is translated into an electric current proportion
to the oxygen concentration in the electrolyte if an electrical circuit is created
between the two electrodes. The galvanic electrode does not need polarising
time and is able to assume operation immediately.
During this process, ions of the more active anode are transferred through
the electrolyte to the less active cathode, and deposited there as a plating. In
this way the anode is corroded. When the anode material eventually corrodes
away, the potential drops and the current halts.
D1
= DO of diluted sample immediately after preparation (mg/L)
D2
= DO of diluted sample after incubation period t (mg/L)
P
= Decimal volumetric fraction of sample used
BOD is similar to the Chemical Oxygen Demand (COD), which also measures
relative oxygen-depletion. However, the possible presence of non-biologically
oxidizable may render the COD test to be less accurate.
Dissolved Oxygen
What is Dissolved Oxygen?
The COD test is often used to measure the amount of organic compounds in
water by measuring the amount of oxygen required to oxidize and break down
an organic compound into carbon dioxide, ammonia and water. The basis of
the COD test is to determine what can be oxidized into carbon dioxide using a
strong oxidizing agent in acidic environments. A blank sample,
created by adding all reagents to distilled water is usually used as a control
in COD measurements.
Both the BOD and COD tests are means to measure the relative
oxygen-depletion effect of a waste contaminant, and are widely used to
monitor pollution levels. The BOD test measures the oxygen demand of
biodegradable pollutants whereas the COD test measures the oxygen demand
of biogradable pollutants plus the oxygen demand of non-biodegradable
oxidizable pollutants.
COD measures everything that can be chemically oxidized and not just the level
of biologically active matter that BOD measures. This is especially important
to keep in mind when understanding and comparing COD and BOD results that
may contain non-biological oxidizable components.
The Polarographic Cell consists of two electrodes placed in the electrolyte:
One with fixed potential called the reference electrode, and the other with a
variable potential called the polarizable electrode. As voltage is applied to the
polarizable electrode, a redox reaction occurs, where electrons break away
from the electrode to bond with oxygen in the electrolyte. The rate at which
the electrons break away from the polarizable electrode is linearly
proportionate to the amount of oxygen available in the electrolyte, hence this
movement of electrons is representative of the amount of dissolved oxygen left
in the electrolyte.
The advantage of a polarographic cell is that the cathode remains intact.
The current flow of the polarographic cell is also linearly proportional to the
amount of oxygen present in the electrolyte, enabling the cell to provide highly
accurate measurements at low oxygen levels.
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electrochemistry handbook
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© 2009 Thermo Fisher Scientific Inc. All rights reserved.
BN1210092
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