C 2257 (OTTO) Chromium(III) potassium sulphate, GR Cas 7788-99-0 - used for tanning of leather and as a photographic film hardener. C 2257 (OTTO) Chromium(III) potassium sulphate, GR Cas 7788-99-0 - used in mordantin in printing and dyeing industry and it could also be used as analytical reagent.