PURPOSE: To measure the concn. of ammonium salts at every ammonium salt having different chemical structures by using tetraethyl ammonium bromide as an internal standard material and measuring 14N-NMR spectra by a multinuclide Fourier transform nuclear magnetic resonance (FT-NMR) device.

CONSTITUTION: The zinc bromide electrolyte contains the quaternary ammonium and primary W tertiary amines added thereto as a bromine complexing agent or said salts and the ammonium salt added as a conductivity improving agent. Tetraethyl ammonium bromide is added thereto at a specific amt. as the internal standard material and thereafter the 14N-NMR spectra are measured. The concn. of the added ammonium salts is determined from the integration ratio of the peak area between the spectra of the standard material and said spectra. The condition of the electrolyte is thus understood in details, by which the control of the electrolyte is made easy and the accuracy thereof is improved.