Electrode slope

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The electrode slope indicates how the potential of an electrode and thus the voltage of a galvanic cell or during electrolysis changes when the concentration of a dissolved reaction partner is changed by a factor of ten. The theoretical value of the electrode slope is also called the Nernst factor or Nernst slope . It appears in the Nernst equation and has the value R T ln (10) / z e F , i.e. it is equal to the product of the molar gas constant R , the absolute temperature T and the natural logarithm from 10, divided by the number of electron z e and the Faraday constant F . If the Nernst equation is written with natural logarithms, the conversion factor ln (10) ≈ 2.302585 does not occur, and a “natural Nernst factor” can be specified which indicates the change in potential in the event of a change in concentration by the factor e with which Euler's number e ≈ 2.71828.

The electrode slope is important for calculating voltages, especially for

Particularly important values ​​are those at room or standard temperature and those for body temperature of 37 ° C.

If the measured electrode slope of an electrode (e.g. that of a pH electrode with z e = 1, measured in buffers with different pH values) deviates significantly from the Nernst factor, the electrode may be dirty and should be cleaned. Many suitable measuring devices offer the possibility of performing a slope correction by means of a calibration, i. H. instead of using the theoretical Nernst factor, the actual slope of the electrode is used to calculate a display value. If large deviations from the table values ​​remain after cleaning, the electrode may also have to be replaced. For example, it has been proposed to discard pH electrodes whose slope at room temperature is not in the range 55-61 mV per change of one pH unit.

Overview table

Temperature
t
absolute temperature
T
natural factor
R T / F
Nernst factor for z e = +1
ln (10) R T / F
Nernst factor for z e = +2
ln (10) R T / 2 F
Nernst factor for z e = +3
ln (10) R T / 3 F
0 ° C 273.15K 23.54 mV 54.20 mV 27.10 mV 18.07 mV
2 ° C 275.15K 23.71 mV 54.60 mV 27.30 mV 18.20 mV
4 ° C 277.15K 23.88 mV 54.99 mV 27.50 mV 18.33 mV
6 ° C 279.15K 24.06 mV 55.39 mV 27.69 mV 18.46 mV
8 ° C 281.15K 24.23 mV 55.79 mV 27.89 mV 18.60 mV
10 ° C 283.15K 24.40 mV 56.18 mV 28.09 mV 18.73 mV
11 ° C 284.15K 24.49 mV 56.38 mV 28.19 mV 18.79 mV
12 ° C 285.15K 24.57 mV 56.58 mV 28.29 mV 18.86 mV
13 ° C 286.15K 24.66 mV 56.78 mV 28.39 mV 18.93 mV
14 ° C 287.15K 24.74 mV 56.98 mV 28.49 mV 18.99 mV
15 ° C 288.15K 24.83 mV 57.18 mV 28.59 mV 19.06 mV
16 ° C 289.15K 24.92 mV 57.37 mV 28.69 mV 19.12 mV
17 ° C 290.15 K 25.00 mV 57.57 mV 28.79 mV 19.19 mV
18 ° C 291.15K 25.09 mV 57.77 mV 28.89 mV 19.26 mV
19 ° C 292.15K 25.18 mV 57.97 mV 28.98 mV 19.32 mV
20 ° C 293.15K 25.26 mV 58.17 mV 29.08 mV 19.39 mV
21 ° C 294.15K 25.35 mV 58.37 mV 29.18 mV 19.46 mV
22 ° C 295.15K 25.43 mV 58.56 mV 29.28 mV 19.52 mV
23 ° C 296.15K 25.52 mV 58.76 mV 29.38 mV 19.59 mV
24 ° C 297.15K 25.61 mV 58.96 mV 29.48 mV 19.65 mV
25 ° C 298.15K 25.69 mV 59.16 mV 29.58 mV 19.72 mV
26 ° C 299.15K 25.78 mV 59.36 mV 29.68 mV 19.79 mV
27 ° C 300.15K 25.86 mV 59.56 mV 29.78 mV 19.85 mV
28 ° C 301.15K 25.95 mV 59.75 mV 29.88 mV 19.92 mV
29 ° C 302.15K 26.04 mV 59.95 mV 29.98 mV 19.98 mV
30 ° C 303.15K 26.12 mV 60.15 mV 30.08 mV 20.05 mV
31 ° C 304.15K 26.21 mV 60.35 mV 30.17 mV 20.12 mV
32 ° C 305.15K 26.30 mV 60.55 mV 30.27 mV 20.18 mV
33 ° C 306.15K 26.38 mV 60.75 mV 30.37 mV 20.25 mV
34 ° C 307.15K 26.47 mV 60.95 mV 30.47 mV 20.32 mV
35 ° C 308.15K 26.55 mV 61.14 mV 30.57 mV 20.38 mV
36 ° C 309.15K 26.64 mV 61.34 mV 30.67 mV 20.45 mV
37 ° C 310.15 K 26.73 mV 61.54 mV 30.77 mV 20.51 mV
38 ° C 311.15K 26.81 mV 61.74 mV 30.87 mV 20.58 mV
39 ° C 312.15K 26.90 mV 61.94 mV 30.97 mV 20.65 mV
40 ° C 313.15K 26.99 mV 62.14 mV 31.07 mV 20.71 mV
45 ° C 318.15K 27.42 mV 63.13 mV 31.56 mV 21.04 mV
50 ° C 323.15K 27.85 mV 64.12 mV 32.06 mV 21.37 mV
55 ° C 328.15K 28.28 mV 65.11 mV 32.56 mV 21.70 mV
60 ° C 333.15K 28.71 mV 66.10 mV 33.05 mV 22.03 mV
65 ° C 338.15K 29.14 mV 67.10 mV 33.55 mV 22.37 mV
70 ° C 343.15K 29.57 mV 68.09 mV 34.04 mV 22.70 mV
75 ° C 348.15K 30.00 mV 69.08 mV 34.54 mV 23.03 mV
80 ° C 353.15K 30.43 mV 70.07 mV 35.04 mV 23.36 mV
85 ° C 358.15K 30.86 mV 71.06 mV 35.53 mV 23.69 mV
90 ° C 363.15K 31.29 mV 72.06 mV 36.03 mV 24.02 mV
95 ° C 368.15K 31.72 mV 73.05 mV 36.52 mV 24.35 mV
100 ° C 373.15K 32.16 mV 74.04 mV 37.02 mV 24.68 mV

literature

  • Erich Meister: Basic practical physical chemistry. 2nd edition, VDF Hochschulverlag AG at ETH Zurich, Zurich 2012, ISBN 978-3-7281-3709-8 .

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