FIELD: organic chemistry, chemical technology. SUBSTANCE: invention relates to a method of preparing iminodiacetic acid monosodium salt an aqueous concentrated solution that involves treatment of iminodiacetic acid disodium salt with a strong acid taken in the amount from 0.3 to 1.2 mole equivalent per 1 mole of iminodiacetic acid disodium salt. Method ensures to prepare the concentrated solution of iminodiacetic acid monosodium salt that can be store or transport without the use of a heat source for the crystallization prevention. Invention relates to also a method of preparing N- phosphonomethyliminodiacetic acid that involves treatment of iminodoacetic acid disodium salt with a strong acid to obtain a stable solution of iminodiacetic acid monosodium salt that can be store or transport before its the following treatment with an additional amount of a strong acid, phosphorous acid and formaldehyde. This method ensures to use the concentrated aqueous solution of iminodiacetic acid monosodium salt that can be store and transport easily without the use of additional measures for the crystallization prevention. EFFECT: improved method of preparing. 13 cl, 4 ex
