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

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
mV
150
a
c
100
50
pH 5.00
pH 5.15
0
pH 5
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 .
18
b
P2
P1
6 7 pH

4.16. Recognized buffers

Buffer solutions
+
The sensION
™ has in memory the following data (pH
PH31
values as a function of the temperature) .
Technical buffers (DIN 19267)
°C °F
0 32 2 .01 4 .01
10 50 2 .01 4 .00
20 68 2 .00 4 .00
25 77 2.00 4.01
30 86 2 .00 4 .01
40 104 2 .00 4 .03
50 122 2 .00 4 .06
60 140 2 .00 4 .10
70 158 2 .01 4 .16
80 176 2 .01 4 .22
90 194 2 .01 4 .30
Buffer solutions DIN 19266
°C °F
5 32 1 . 6 68 4 . 0 04 6 . 9 51 7 . 0 87 9 . 3 95 10 . 2 45 13 . 2 07
10 50 1 . 6 70 4 . 0 00 6 . 9 23 7 . 0 59 9 . 3 32 10 . 1 79 13 . 0 03
20 68 1 . 6 75 4 . 0 01 6 . 8 81 7 . 0 16 9 . 2 25 10 . 0 62 12 . 6 27
25 77 1.679 4.006 6.865 7.000 9.180 10.012 12.454
30 86 1 . 6 83 4 . 0 12 6 . 8 53 6 . 9 87 9 . 1 39 9 . 9 66 12 . 2 89
40 104 1 . 6 94 4 . 0 31 6 . 8 38 6 . 9 70 9 . 0 68 9 . 8 89 11 . 9 84
50 122 1 . 7 07 4 . 0 57 6 . 8 33 6 . 9 64 9 . 0 11 9 . 8 28 11 . 7 05
60 140 1 . 7 23 4 . 0 85 6 . 8 36 6 . 9 68 8 . 9 62
70 158 1 . 7 43 4 . 1 26 6 . 8 45 6 . 9 82 8 . 9 21
80 176 1 . 7 66 4 . 1 64 6 . 8 59 7 . 0 04 8 . 8 85
90 194 1 . 7 92 4 . 2 05 6 . 8 77 7 . 0 34 8 . 8 50
pH mV
7 .12 9 .52 11 .45 --
7 .06 9 .38 11 .20 245
7 .02 9 .26 11 .00 228
7.00 9.21 10.90 220
6 .99 9 .16 10 .81 212
6 .97 9 .06 10 .64 195
6 .97 8 .99 10 .48 178
6 .98 8 .93 10 .23 160
7 .00 8 .88 10 .19 142
7 .04 8 .83 10 .06 --
7 .09 8 .79 9 .93 --
pH
-- 11 . 4 49
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-- --
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