50
0
100
150
mV
5
6
7 pH
pH
4. Operation
4.16. 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
™
+
MM340
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
70ºC (158 °F)
c)
Real calibration line of the electrode at 70ºC (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 70ºC (122 to 158 °F) will be 5 .15 .
4.17. Recognized buffers
Buffer solutions
The
sensION
™
+
MM340
has in memory the following data (pH
values as a function of the temperature) .
Technical buffers (DIN 19267)
Buffer solutions DIN 19266
19
ENGLISH
ºC
ºF
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
--
ºC
ºF
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
