FIELD: chemical industry, namely, production of fixing solutions for treatment of films. SUBSTANCE: concentrated ammonium solution is treated with hydrogen fluoride acid. Thus prepared concentrated solution of ammonium fluoride or concentrated solution of ammonium bifluoride is treated with pentahydrate of sodium thiosulfate being preliminary melt in crystallization water. The process is carried out in the presence of concentrated ammonium solution. Thus prepared pulp is then filtered, precipitate of sodium fluoride being separated. Solution of ammonium thiosulfate is treated by freshly precipitated magnesium carbonate to remove fluoride ions as impurities. Then two-stepped concentration is carried out, products of decomposition of thiosulfate ion being separated after the first step. Crystalline sodium sulfite is fed at the second step of concentration, its quantity being 0.5-2.0 mass % of mass of ammonium thiosulfate. EFFECT: improved quality of desired ammonium thiosulfate; decreased expense for production of ammonium thiosulfate.
