Hydrogen Overvoltage in Concentrated Sulfuric and Phosphoric Acid Solutions

Abstract

The overvoltage for the discharge of hydrogen on a mercury cathode has been measured at 25°C in sulfuric acid solutions in the concentration range 0.125 to 9.65 molar. In dilute solution, the overvoltage is independent of the acid concentration, the cathode reaction being the discharge of a proton from water. At concentrations in excess of 2.83 molar, the overvoltage decreases approximately as a linear function of the weight percentage. Evidence is presented to show that in this region, the cathode reaction involves the hydrogen sulfate molecule, H2SO4. Preliminary results on phosphoric acid solutions also show a drop in the overvoltage as the concentration is increased beyond 1 molar.

Recent Developments in the Study of Hydrogen Overpotential.

“…Jofa (103,104) found that at concentrations above 0.1 N m hydrochloric acid the overpotential becomes less negative with increasing hydrogen-ion concentration. This decrease with increasing hydrogen-ion concentration ivas also observed by Béthune and Kimball (8) in sulfuric and phosphoric acid solutions up to 9.65 M sulfuric acid and 5.65 M phosphoric acid. Bockris and Parsons (24) found that in methyl alcohol solutions of hydrogen chloride the hydrogen overpotential at a dropping-mercury electrode was affected by pH change to a larger extent than that in aqueous solution and the change with concentration began at a notably higher pH than in aqueous solutions.…”

“…The effect of electropolishing the metal surface has been examined by Bockris and Azzam (19), who find that metals of high overpotential such as copper and lead are little affected by electropolishing but that for metals of lower overpotential, such as tungsten and nickel, electropolishing increases the overpotential by an order of about 0.1 v. at medium or high current densities. 8. Effect of diffusion through the cathode Earlier work (e.g., particularly 47 and 130) on the transfer of hydrogen overpotential through a thin film of metal produced contradictory results, probably owing to the presence of impurities in solution which are known to affect adsorption markedly.…”

“…(38) 8 The order of the reaction should be 2 or 1, according to whether the slow combination is + -> H2(gas) or + H -> H2(gas). where a is a constant at constant T.…”

Charge Transfer Reactions in Electrochemical and Chemical Processes

“…Later, de Bethune and Kimball [126] adhered to this point of view. However, their experimental data are quite unreliable and can hardly serve as a justification for this hypothesis.…”

The Dissociation of Strong Electrolytes.

“…Rao's table of his results contains several inconsistencies, and this author has not been able to derive satisfactory figures. De Béthune and Kimball (30) attempted to locate one error and calculated concentrations of the hydrosulfate ion.…”