Chemical Specifications Appearance White crystalline powder Assay (iodometric) 95 - 99.9% Iodometric,SO2 54.8 - 57.6% Appearance of solution Passes test pH (5% solution in water) 3 - 4.5 Chloride (Cl) Max 0.05% Heavy metal (as Pb) Max 0.001% Thiosulphate Passes test Copper (Cu) Max 0.0025% Zinc (Zn) Max 0.0025% Iron (Fe) Max 0.002% Selenium (Se) Max 0.0005% Arsenic (As) Max 0.0002% Mercury (Hg) Max 0.0001%