From Big Medical Encyclopedia

HYDROGEN ELECTRODE — a standard electrode, reversible concerning hydrogen ions. Century e. widely apply in laboratory practice as a reference electrode, at measurement of EMF of any galvanic device, at measurement of pH of the environment etc. of V. e. represents the platinum plate or a wire saturated with gaseous hydrogen and shipped in the solution containing hydrogen ions. The hydrogen absorbed by platinum partially dissociates on ions on the equation: H 2 ⇆ 2H + + 2e. As a result such platinum plate gains ability to exchange hydrogen ions with solution. The size of the potential arising at the same time depends on concentration (more precisely than activity) hydrogen ions in solution and from pressure of gaseous hydrogen, at Krom there is a saturation of platinum. Quantitatively this dependence is expressed by Nernst's equation.

where e — the potential of the hydrogen electrode in volts, e 0 — normal potential of the hydrogen electrode in volts, R — the gas constant in number equal of 8,314 J / grad-mol, T — the absolute temperature, F — Faraday's number, to an equal 96 491 pendent; ln — a sign of natural logarithms, aH + — hydrogen ion activity in gram ions on liter of solution, P H 2 — pressure of gaseous hydrogen in the atmospheres.

Century e., in Krom pressure of gaseous hydrogen is equal to 1 atm, and hydrogen ion activity is equal in solution 1, call the standard hydrogen electrode, and its potential is conditionally taken for zero.

Normal V. e. use as a standard electrode, in relation to Krom measure potentials of other electrodes.

See also Hydrogen ion exponent , Electrodes .

Bibliography: Bates R. Definition of pH, the theory and practice, the lane with English, L., 1968; Schwab K. Bases of the technology of measurement of pH, the lane with it., M., 1962, bibliogr.

V. A. Popkov.