pH Electrodes
pH Electrode
basics
The pH electrode, due to the nature of its construction,
needs to be kept moist at all times. In order to operate properly, glass needs to be hydrated. Hydration is required for
the ion exchange process to occur. If an electrode should
become dry, it is best to place it in some tap water for half
an hour to condition the glass.
pH electrodes are like batteries; they run down with time and
use. As an electrode ages, its glass changes resistance.
This resistance change alters the electrode potential. For
this reason, electrodes need to be calibrated on a regular
basis. Calibration in pH buffer solution corrects for this
change. Calibration of any pH equipment should always
begin with buffer 7.0 as this is the "zero point." The pH scale
has an equivalent mV scale. The mV scale ranges from
+420 to -420 mV. At a pH of 7.0 the mV value is 0. Each pH
change corresponds to a change of approx. ±60 mV. As pH
values become more acidic the mV values become greater.
pH electrodes have junctions which allow the internal electrolyte solution of the measuring electrode to leak out into
the solution being measured.
Glass Conic Tip Sensor
Glass Spheric Sensor
Epoxy Electrode
MA916B/1 (will be
replaced by SE100)
17 B /1 =1 m
120 mm
16 B /1 =1 m
/3 =3 m
M
A9
12 mm
12 mm
A9
18 B /1 =1 m
120 mm
M
A9
120 mm
M
12 mm
Model
MA916B/1 - MA916B/3
MA917B/1
MA918B/1
Measuring Range
Temperature Range
Shaft Material
Reference Electrolyte
Reference Junction
Reference Type
Shape of membrane
Max pressure
Connector Type
Cable length
Shaft length
Diameter
Application
0 to 13 pH
-5 to 100°C (23 to 212°F)
glass
KCl 3.5M + AgCl
ceramic, single
single, Ag/AgCl
spheric
0.1 bar
BNC
coaxial, 1 or 3 m
120 mm
12 mm
laboratory applications
0 to 14 pH
0 to 100°C (32 to 212°F)
glass
KCl 3.5M
ceramic, single
double, Ag/AgCl
spheric
0.1 bar
BNC
coaxial, 1 m
120 mm
12 mm
laboratory applications
0 to 12 pH
-5 to 100°C (23 to 212°F)
glass
KCl 3.5M + AgCl
ceramic, triple
single, Ag/AgCl
conic
0.1 bar
BNC
coaxial, 1 m
120 mm
12 mm
laboratory applications
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7
pH Electrodes
pH Electrode
basics
Single Junction
Electrode
Double Junction
Electrode
This junction can become clogged by particulates in the
solution and can also facilitate poisoning by metal ions present in the solution. If a clogged junction is suspected it is
best to soak the electrode in tap water to dissolve the material and clear the junction. When not in use it is best to store
the electrode in either buffer 4.0 or buffer 7.0. Never store an
electrode in distilled or deionized water as this will cause
migration of the electrolyte solution from the electrode.
Inner Tube
Housing the
Membrane
Sensing Wire
Electrolyte
Containing
Silver
Inner
Ceramic
Junction
Silver-Free
Electrolyte
Outer
Ceramic
Junction
M
A9
Silver
Silver/Chloride
Reference
Wire
How long a pH electrode will last will depend on how it is
cared for and the solutions it is used to measure. Typically, a
gel-filled combination pH electrode will last six months to 1
year depending on the care and application.
How long an electrode will last is determined by how well the
probe is maintained and the pH application. The harsher the
system, the shorter the lifespan. For this reason it is always
a good idea to have a back-up electrode on hand to avoid
any system down time. Calibration is also an important part
of electrode maintenance. This assures not only that the
electrode is behaving properly but that the system is operating correctly.
Ceramic
Junction
15 B /2 =2 m
/3 =3 m
M
A9
20 B /1 =1 m
M
A9
91 B /1 =1 m
75 mm
75 mm
120 mm
MA991B/1 (will be
replaced by SE120)
12 mm
6 mm
12 mm
Model
MA915B/2 - MA915B/3
MA920B/1
MA991B/1
Measuring Range
Temperature Range
Shaft Material
Reference Electrolyte
Reference Junction
Reference Type
Shape of membrane
Max pressure
Connector Type
Cable length
Shaft length
Diameter
Application
0 to 13 pH
-5 to 95°C
glass
polymer
ground glass
double, ground glass
spheric
3 bar
BNC
2 or 3 m
75 mm
12 mm
industrial applications
0 to 12 pH
0 to 50°C (32 to 122°F)
PVDF
Viscolene
open
single, Ag/AgCl
conic
0.1 bar
BNC
coaxial, 1 m
75 mm
6 mm
laboratory applications
0 to 13 pH
-5 to 100°C (23 to 212°F)
glass
KCl 3.5M
ceramic, single
single, Ag/AgCl
spheric
0.1 bar
BNC
coaxial, 1 m
più di 120 mm
12 mm
laboratory applications
8
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pH Electrodes
pH Electrode
basics
This means that the temperature is corrected to the value
expected at 25 Deg C. Automatic temperature compensation requires input from a temperature sensor and constantly sends a compensated pH signal to the display. Automatic
temperature compensation is useful for measuring pH in
systems with wide variations in temperature.
BNC Connector
M
05 B /3 =3 m
M
A9
13 B /3 =3 m
120 mm
A9
120 mm
M
DIN Connector
22 mm
A9
23 B /3 =3 m
120 mm
Temperature compensation: When measuring pH using a
pH electrode the temperature error from the electrode varies
based on the Nernst Equation as 0.03pH/10C/unit of pH
away from pH7. The error due to temperature is a function
of both temperature and the pH being measured.
Temperature compensation can be achieved manually or
automatically. Manual temperature compensation is usually
achieved by entering the temperature of the fluid being
measured into the instruments menu and then the instrument will display a "Temperature Compensated" pH reading.
12 mm
12 mm
Model
MA905B/3
MA913B/3
MA923B/3
Measuring Range
Temperature Range
Shaft Material
0 to 13 pH
-5 to 95°C
Reference Electrolyte
polymer
double, Teflon
0 to 13 pH
0 to 60°C (32 to 140°F)
Epoxy
gel
ceramic, single
single, Ag/AgCl
spheric
2 bar
BNC
coaxial, 3 m
120 mm
12 mm
water, waste water
±1999 mV
0 to 80°C (32 to 176°F)
Epoxy
gel
cloth
single, Ag/AgCl
spheric
pH: conic / ORP: Platinum sensor
3 bar
DIN
7-pole, 1 m
120 mm
14 mm
water, waste water
Reference Junction
Reference Type
Shape of membrane
Max pressure
Connector Type
Cable length
Shaft length
Diameter
Application
6 bar
3/4” NPT - BNC
3m
120 mm
22 mm
industrial applications
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9