tion reactior'i are sulphur monochloride; sulphur dichloride; sodium thiosulphate; potassium thiosulphate; hydrogen sulphide and compounds such as sodium, potassium or calcium sulphide which when added to the acidic reaction mixture will react to form hydrogen sulphide; mercaptans and thiophenols such as ethyl mercaptan, thiophenol, p-methylthiophenol, o-ethyl-thiophenol, thiohydroquinone, thio-na