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4. Operation
4.19. 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, when very high precision is required.
+
sensION
™
MM 374
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 13) 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 (158 ˚F)
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 (158 ˚F) will be 5.15.
b
P2
P1
6
7 pH
4.20. Recognized solutions
4.20.1. Buffer solutions
+
The sensION
™
has in memory the following data (pH
MM 374
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
1.668 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
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
4.20.2. Molar EC standards
Table of conductivity values according to temperature
Values stored in the memory of sensION
°C
°F
µS/cm
15.0
59
119
16.0
60.8
122
17.0
62.6
125
18.0
64.4
127
19.0
66.2
130
20.0
68
133
21.0
69.8
136
22.0
71.6
139
23.0
73.4
142
24.0
75.2
145
25.0
77
147
26.0
78.8
150
27.0
80.6
153
28.0
82.4
156
29.0
84.2
159
30.0
86
162
31.0
87.8
165
32.0
89.6
168
33.0
91.4
171
34.0
93.2
174
35.0
95
177
pH
mV
7.12
9.52
10.30
--
7.06
9.38
10.17
245
7.02
9.26
10.06
228
7.00
9.21 10.01
220
6.99
9.16
9.96
212
6.97
9.06
9.88
195
6.97
8.99
9.82
178
6.98
8.93
9.76
160
7.00
8.88
--
142
7.04
8.83
--
--
7.09
8.79
--
--
pH
--
11.449
--
--
--
--
--
--
+
™
.
MM 374
µS/cm
mS/cm
mS/cm
1147
10.48
92.5
1173
10.72
94.4
1199
10.95
96.3
1225
11.19
98.2
1251
11.43
100.1
1278
11.67
102.1
1305
11.91
104.0
1332
12.15
105.4
1359
12.39
107.9
1386
12.64
109.8
1413
12.88
111.8
1440
13.13
113.8
1467
13.37
115.7
1494
13.62
--
1522
13.87
--
1549
14.12
--
1581
14.37
--
1609
14.62
--
1638
14.88
--
1667
15.13
--
1696
15.39
--
21
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