High Precision pH Measurement with the Excellence Titrator T90 and Rondo 20 Sample Changer

High Precision pH Measurement with the Excellence
Titrator T90 and Rondo 20 Sample Changer
The pH of a sample solution is often a critical measure for process control. In all industries,
optimum pH conditions can increase process yields, maximize efficiency and lead to significant cost reductions. When automated systems are used, these savings are greater still.
S. Vincent
The accurate measurement of pH depends
on many factors. Key amongst these are:
• The response time of the sensor
• The calibration slope and offset
• The temperature of the samples
• The sample composition
K = [H3O++ ] [A- - ]
Kaa= [H3O
] [A ]
[HA]
[HA]
Temperature dependence of
the sensor slope
This equilibrium constant (also known
as the acid dissociation constant) is temperature dependent, and the Van’t Hoff
equation describes this variance as follows:
E = E° --2.3
RT
{pH}
2.3RT
RT{pH}
{pH}
EE= =E°E°- 2.3
zF
zFzF
The measured response of a pH sensor is
given by the Nernst equation:
As can be seen, the sensor slope factor
contains a temperature term. The same
sensor will therefore respond differently
at different temperatures. This effect can
be compensated for, with measured values adjusted according to the temperature corrected slope factor.
Perhaps the least understood, and most
often ignored of these is the effect of
dlnK
ΔH°
dlnK
m H°
a =a Δ
m
dlnK
H°
a
= =Δ
m
2 2
sample temperature. The temperature
dTdT RTRT
dT
RT2
influences pH measurement in 4 differFrom this, it is clear that changes in the
ent ways:
measurement temperature lead to altera• The temperature coefficient of the
tions to the true solution pH. As these
measured solution
pH changes are real, and not caused by
• The temperature dependence of the
measuring error alone, it is not possible
sensor slope
• The position of the isothermal point
to correct for them. For this reason it is
vital, when quoting a pH value for a samfor the given sensor
ple, that the temperature of the measure• Differing response time for the sensor
ment is also quoted.
(equilibration effects)
These are all discussed in the following
sections, after which an automated method is described to minimize the common
sources of pH measurement error.
Isothermal intersection point
The isothermal point for a given sensor is
that point at which the calibration lines
for different temperatures intersect. An
‘ideal’ sensor would behave in such a
+ mV
Real isothermal
intersection point
Every solution has a characteristic variation of pH with temperature. Looking at
the generic acid dissociation equilibrium
will help to understand this:
HA + H2OHA + H2O
H3O + A H3O + A
+
-
Figure 1:
Calibration line and
isothermal intersection points.
Theoretical isothermal
intersection point
Temperature coefficient
+
Eis
0
7
14
pH
-
Error
T1
- mV
}
The position of this dynamic equilibrium
governs the concentration of hydrogen
ions in solution, and hence the solution
pH. The equilibrium constant is a measure of the equilibrium position, and is
defined as:
T2
T2 > T1
METTLER TOLEDO
UserCom 1/2008
21
Although modern pH sensor design has
allowed the deviations from ideal behaviour to be kept to a minimum, this temperature compensation error still exists.
Also, the larger the temperature difference between the temperature of calibration and the temperature of measurement, the larger the error will be.
way that this intersection point coincided
with the zero point of the pH sensor (pH 7
= 0 mV) (Fig. 1).
Expert Tips
As the potential recorded by a combination pH sensor is the sum of many
contributing potentials within the sensor, each with its own temperature
dependence, the real isothermal point
rarely coincides with the theoretical one
(Fig. 2).
For the most accurate pH measurement,
it is therefore essential that the tem-
Figure 2: Different
potentials of a combined pH sensor.
E4
E5
Reference
electrolyte
E6
E3
Inner buffer
E2
E1
Figure 3: Effect on
stability time.
pH units
80 °C
0.5
pH units
30 °C
80 °C
30 °C
Symmetrical design
Symmetrical design
Conventional
design
Conventional
design
0 0
10
10
Asymmetrical lead-off
system of a conventional
sensor
Asymmetrical lead-off
system of a conventional
sensor
UserCom 1/2008
t (min)t (min)
20
Symmetrical sensor
Asymmetrical conventional
sensor
Asymmetrical conventional
sensor
20
The best pH measurements are made
when a sensor achieves rapid temperature equilibrium with its surroundings. The good quality glass sensors of
METTLER TOLEDO will always exhibit
this behaviour. They will also have the
same temperature coefficient and isothermal intersection at pH 7 and 0 mV.
The response time of a glass sensor to
changes in the pH of the solution is also
critical. A sufficiently long equilibration
time must be allowed to achieve a stable
mV signal as well as a stable temperature
reading. This effect is especially important when moving from one solution to
another of widely differing pH or temperature, as a high liquid junction potential
can result in this case (Fig. 3).
Automating a precise pH
measurement
0.5
METTLER TOLEDO
In practice, the buffer and sample solutions are left to equilibrate, in a water
bath, prior to measurement. During the
measurements, a water-jacketed vessel
maintains this temperature equilibrium.
A time saving automated approach to this
is described later in this article.
Response time of the sensor
E
22
perature during calibration and sample
measurement is identical.
To minimize all of these possible sources
of error, and still automate the measurement of a series of samples has been a
challenge. The following procedure for
calibration and subsequent measurement
of pH makes use of the flexible rinsing
and conditioning options offered by the
Rondo sample changer. Using it a series
of up to 120 samples of 100 mL volume
can be tested in a single run.
Symmetrical sensor
The sample temperature is adjusted
to 25 °C by means of a heat exchanger
immersed in the beaker. This is fed by
Sectional view
a circulating heater/chiller to allow for
of sensor
Symmetrical lead-off
adjustment of temperature to take place
system of a specialist
in either direction.
sensor
Symmetrical lead-off
system of a specialist
sensor
Sectional view
of sensor
For sensor calibration, the method
sequence is generally as shown in the
flow chart (Fig. 4).
The key stages are:
• The pre-rinsing of the sensor assembly
prior to calibration
• Equilibrate the sensor in pH 4.01
buffer solution.
• Transfer to a fresh sample of pH 4.01
buffer.
• Wait for the temperature to stabilize
at 25 ±1°C.
• Take mV reading in pH 4.01 buffer
• Rinse
• Equilibrate the sensor in pH 7.00
buffer solution.
• Transfer to a fresh sample of pH 7.00
buffer.
• Wait for the temperature to stabilize
at 25 ±1°C.
• Take mV reading in pH 7.00 buffer
• Rinse
• Equilibrate the sensor in pH 10.00
buffer solution.
• Transfer to a fresh sample of pH 10.00
buffer.
• Wait for the temperature to stabilize
at 25 ±1°C.
• Take mV reading in pH 10.00 buffer
• Rinse
• Perform calibration. Stop if outside
limits:
• -55.0 mV/pH ≤ Slope ≤ -60.0 mV/pH
• Record slope, offset and calibration
curve
• Move to new sample beaker with pH
7.00 buffer.
• Wait for the temperature to stabilize
at 25 ±1°C.
• Measure the pH
• Stop if outside of limits.
6.90 ≤ pH ≤ 7.10
Figure 4:
The flow chart of the
automated method
sequence.
Title
Rinse in fixed
rinse beaker
Sample
number
n=3
Sample
number
n= 1?
Yes
No
Yes
Sample
number
n= 2?
No
Conditioning in buffer pH 4.01
at fixed conditioning beaker
Conditioning in buffer pH 7.00
at special beaker 1
Conditioning in buffer pH 10.00
at special beaker 2
Stir
Measure temperature [°C]
24.5 < T < 25.4
Measure mV
Rinse in fixed
rinse beaker
End of Sample
Calibration
Instruction:
Slope out of limits
No
Slope OK?
Yes
Sample
Measure temperature [°C]
24.5 < T < 25.4
Measure pH
Instruction:
pH out of limits
Rinse
No
pH OK?
Yes
Rinse
Instruction:
pH OK
METTLER TOLEDO
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23
Expert Tips
The pre-equilibration steps and rinsing
take place at fixed positions on the sample rack of the Rondo 20 sample changer
(Fig. 5).
These are defined quickly and easily with
magnets placed in the coding ring under
the rack (Table 1). The method moves the
sensor to the appropriate position by the
use of conditional statements appended
to the method function:
The efficient PowerShower™ rinsing system ensures that there are no problems of
carry-over from one sample to another.
For all subsequent sample pH measurements, a similar approach is adopted.
The sample temperature is equilibrated
to 25 °C to match that of the calibration. In this way the temperature effects
described earlier will cause minimum
disruption to the measurement.
The time taken to achieve the desired
temperature is sufficient to allow for
stabilization of all the internal sensor
potentials.
Conclusion
By using the latest design pH sensors of
METTLER TOLEDO, and taking great
care over temperature and potential
equilibrium of both the calibration buffers and samples, reliable and accurate
pH measurements can be made.
Automation of the entire process is now
possible using the unrivalled flexibility
of the METTLER TOLEDO T90 Excellence
Titrator and Rondo 20 sample changer.
Figure 5:
Rondo 20 together
with the T90
Excellence Titrator.
Table 1:
Assignment of
beakers to special
Rondo 20 rack
positions.
24
METTLER TOLEDO
Rack position
Purpose
20
Equilibration in pH 4.01 buffer
19
Rinsing
18
Equilibration in pH 7.00 buffer
17
Equilibration in pH 10.00 buffer
UserCom 1/2008
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