... or small crystals, with a strong smell of SO2, the proportion of 1.4, soluble in water, acidic aqueous ... ; In food processing it is used as preservative, bleach, loosening agent. Technical grade:Appearance: White or yellow crystalline powderQuality standard ... Item IndicatorsPurity (as Na2S2O5 ) % 96.5Iron (Fe)% ≤ 0.003ClarityPassed the experimentArsenic (As) % ≤ 0.0001Heavy metal (Pb) % ≤ 0.0005 2012 ...