Abstract 1) The relationship of pH–E1⁄2 (vs. S.C.E.) and of pH–il of nitrourea under the dropping mercury electrode was studied. 2) It is presumed from the experiment on the effect of the effective height of the mercury reservoir and of temperature upon ii that ii will follow the diffusion law in acidic solution. 3) The electron number consumed for reduction was calculated by assuming the diffusion coefficient.