116. Solving a chemistry problem

4. Give an assessment of the quality of the cysteine substance (M.m. 121.16) in terms of quantitative content, taking into account the requirements of VFS 42-2633-95 (cysteine ​​should be at least 98.5%), if during the analysis 0.2510 g was added to a sample of 0.2510 g 25 ml of a 0.1 M solution of iodine UCH (1/2 l2), and the iodine residue was titrated with a 0.1 M solution of sodium thiosulfate (KP = 0.9978), which consumed 4.39 ml. 25.10 ml of 0.1M sodium thiosulfate solution was used for the control experiment.