Hach sensION+ PH 31 - Isopotential, Phiso; Recognized Buffers

50
0
100
150
mV
567 pHpH
4. Operation
4.15. Isopotential (pHiso)
Definition
Potential (mV) of one pH electrode that does not alter with the
temperature. This is the intersection point of the different cali-
bration lines at different calibration temperature. Normally, this
value corresponds to pH 7. Nevertheless, in reality the value is
slightly different.
Application
It is recommended to determine the pH of the real isopotential
(≠ 7) when after calibration at room temperature, the measure-
ments will be performed at different temperature, and in addi-
tion a very high precision is required.
sensION
™
+ PH31
The instrument offers three possibilities for the application of pHiso:
pH 7: A value generally accepted by all the manufacturers.
Introducing the real value: If the pHiso is known, it can be
introduced by the user.
Value calculation, by the instrument.
Procedure:
- Calibrate the instrument in 2 or 3 points with the buffers at
room temperature, following the common procedure.
- Heat the buffers. Their temperature must differ with 10 or more
degree centigrade from the calibration temperature.
- Select “Calculate” from the menu “Measuring conditions” (see
pH measurement re-programming on page 11) and follow the
instructions of the pH-meter.
- The instrument will calculate the pHiso of the electrode in use
and will memorize and apply it in the corresponding calculation
for the temperature compensation in the measurements.
Example for electrode behavior
a) Electrode calibration line at room temperature
b) A calibration line with pHiso=7 to apply when measure at
70°C
c) Real calibration line of the electrode at 70°C (158 °F)
P1) Classic isopotential, pHiso 7
P2) Real isopotential of the electrode, pHiso 6.
Error: Appling the classic isopotential (pHiso7) the pH measure-
ment in buffer 5 at 70°C will be 5.15.
4.16. Recognized buffers
Buffer solutions
The sensION
™
+ PH31 has in memory the following data (pH
values as a function of the temperature).
Technical buffers (DIN 19267)
Buffer solutions DIN 19266
18
°C °F pH mV
0 32 2.01 4.01 7.12 9.52 11.45 --
10 50 2.01 4.00 7.06 9.38 11.20 245
20 68 2.00 4.00 7.02 9.26 11.00 228
25 77 2.00 4.01 7.00 9.21 10.90 220
30 86 2.00 4.01 6.99 9.16 10.81 212
40 104 2.00 4.03 6.97 9.06 10.64 195
50 122 2.00 4.06 6.97 8.99 10.48 178
60 140 2.00 4.10 6.98 8.93 10.23 160
70 158 2.01 4.16 7.00 8.88 10.19 142
80 176 2.01 4.22 7.04 8.83 10.06 --
90 194 2.01 4.30 7.09 8.79 9.93 --
°C °F pH
5 32 1.668 4.004 6.951 7.087 9.395 10.245 13.207
10 50 1.670 4.000 6.923 7.059 9.332 10.179 13.003
20 68 1.675 4.001 6.881 7.016 9.225 10.062 12.627
25 77 1.679 4.006 6.865 7.000 9.180 10.012 12.454
30 86 1.683 4.012 6.853 6.987 9.139 9.966 12.289
40 104 1.694 4.031 6.838 6.970 9.068 9.889 11.984
50 122 1.707 4.057 6.833 6.964 9.011 9.828 11.705
60 140 1.723 4.085 6.836 6.968 8.962 -- 11.449
70 158 1.743 4.126 6.845 6.982 8.921 -- --
80 176 1.766 4.164 6.859 7.004 8.885 -- --
90 194 1.792 4.205 6.877 7.034 8.850 -- --
a
c
b
pH 5.00
pH 5.15
P2
P1