S-Diphenylguanidine may be purified by recrystallization to provide a satisfactory acidmetric standard. The results of titration of an alcoholic solution of the base to pH 5.2 for N acid or 5.6 for N/10 acid agree within experimental error with values obtained with sodium carbonate or borax. Three methods of preparing borax containing exactly 10 molecules of water of crystallization have been compared, viz., drying over sodium bromide, drying over a saturated solution of sugar and salt, and washing with alcohol and ether. Satisfactory products resulted, but they were found to be unstable on storage. Sodium bicarbonate has been found to be stable on long storage. The density of s-diphenylguanidine at 20 °C. was found to be 1.19.