Certified reference material for ammonium ions in high-purity ammonium chloride: Influence of pH on coulometric titration of ammonium ions with electrogenerated hypobromite

No report has been published concerning reference materials of ammonium chloride in which ammonium ions were accurately determined, though ammonium chloride is widely used as a standard of ammonium ions. In the present paper, accuracy of the coulometric titration of ammonium ions with electrogenerated hypobromite was studied. The authors found that the pH of electrolyte solution significantly affected the accuracy of the assay of ammonium ions. Lower assays were obtained at pH more than 8.5 or less than 8.0: much higher pH values led to evaporation of ammonia and much lower pH ones led to insufficient hypobromite generation or insufficient conditional equilibrium constant of the reaction between ammonium ions and hypobromite. Ammonium ions in ammonium chloride were certified by coulometric titration under an appropriate condition, and chloride ions in it were also certified by gravimetric titration based on standard sodium chloride. The first certified reference material with ammonium assay was developed which enabled more accurate ammonium determination than that before.