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.
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