Anhydr salt for detecting and removing trace amounts of water from alcohols and other organic Compounds; as fungicide. Pentahydrate as agricultural fungicide, algicide, bactericide, herbicide; food and fertilizer additive; in insecticide mixtures; in manufacture of other Cu salts; as mordant in textile dyeing; in preparation of azo dyes; in preserving hides; in tanning leather; in preserving wood; in electroplating solutions; as battery electrolyte; in laundry and metal-marking inks; in petroleum refining; as flotation agent; pigment in paints, varnishes and other materials; in mordant baths for intensifying photographic negatives; in pyrotechnic compositions; in water-resistant adhesives for wood; in metal coloring and tinting baths; in antirusting compositions for radiator and heating systems; as reagent toner in photography and photoengraving; etc.