Content: Na 2 S 2 O 4 90% 88% 85% Fe ≤20ppm ≤20ppm ≤20ppm Zn ≤1ppm ≤1ppm ≤1ppm Heavy metal (as Pb) ≤1ppm ≤1ppm ≤1ppm Insoluble matter ≤0.05% ≤0.05% ≤0.05%