Diffusion potential

Under a diffusion potential refers to the difference in electrical potential, which in the phase boundary between the two different electrolytic solutions occurs due to the uneven distribution of electrolyte. There can be differences in the solutions with regard to the chemical nature of the electrolyte or ion and in the concentration of a certain ion. The phase boundary can be produced by a membrane or a frit or can also be designed as a boundary between superimposed layers of different densities.

Diffusion potentials occur, for example, in galvanic cells , for example in the case of the Gravity-Daniell element , where they are separated in a density-layered manner as the difference in galvanic voltages - from zinc in zinc sulfate (ZnSO ₄ ) on the one hand and copper in copper sulfate (CuSO ₄ ) on the other Let sulphate solutions describe.

Diffusion potentials also occur in all living cells , for example as diffusion potential for potassium ions (K ⁺ ), the uneven distribution of which between the cell interior and the external environment creates a voltage difference on the cell membrane that is essential for their membrane potential .

Minimization of diffusion potentials in measuring arrangements

Diffusion potentials can interfere with measurements with electrochemical sensors ( ion-selective electrodes such as pH combination electrodes ) or with equilibrium potentials of galvanic elements. To minimize this influence, suitable salt bridges are used , which are mostly filled with concentrated potassium chloride solution.

Examples of the size of diffusion potentials

A diffusion potential of 28.52 mV occurs between a 0.1 molar hydrochloric acid and a 0.1 molar potassium chloride solution; this relatively high diffusion potential can be explained by the high mobility of the protons. The diffusion potential that is established between different hydrochloric acid concentrations is 85.3 mV for 1 M HCl and 11.5 M HCl. The diffusion potentials in galvanic cells such as the Daniell element mentioned above are significantly lower, since the differences in concentration are only small and the mobilities, in this case those of copper and zinc ions, are closer together.

For the theory and calculation of diffusion potentials

With the activities of all ions known, the diffusion potential is given by the equation ${\displaystyle a_{i}}$ ${\displaystyle \Delta \varphi _{\mathrm {Diff} }}$

${\displaystyle \Delta \varphi _{\mathrm {Diff} }=-{\frac {RT}{F}}\sum _{i=1}\int \limits _{a_{i}(\mathrm {I} )}^{a_{i}(\mathrm {II} )}{\frac {t_{i}}{z_{i}}}\mathrm {d} \ln(a_{i})}$

given. This includes the the transport numbers that the charge numbers and T the absolute temperature . The natural constants are the gas constant R and the Faraday constant F , for the factor RT / F see also electrode slope . ${\displaystyle t_{i}}$ ${\displaystyle z_{i}}$

In the simplest case, when the diffusion potential only arises due to differences in concentration, because the same components are contained on both sides, when it comes to binary electrolytes and when one can count on concentrations instead of activities, the simple equation applies

${\displaystyle \Delta \varphi _{\mathrm {Diff} }=-{\frac {RT}{F}}({\frac {t_{+}}{z_{+}}}-{\frac {t_{-}}{z_{-}}})\ln({\frac {c_{2}}{c_{1}}})}$.

literature

• Peter W. Atkins : Physical Chemistry. (VCH, Weinheim, 2006). ISBN 3-527-31546-2
• Gerd Wedler : Textbook of physical chemistry. (VCH, Weinheim, 1987). ISBN 3-527-26702-6

Individual evidence

1. ^ ^{A b} Rudolf Brdička, Jiří Dvořák: Basics of physical chemistry . 15th, edited edition. Deutscher Verlag der Wissenschaften, Berlin 1990, ISBN 3-326-00099-5 , p. 645-649 .
2. ↑ Gerd Wedler: Textbook of physical chemistry . 5th, completely revised and updated edition. Wiley-VCH, Weinheim 2004, ISBN 3-527-31066-5 , pp. 475-477 .