Specifications: Item 90% 88% 85% Food grade Na2S2O4 ≥90% ≥88% ≥85% ≥85% Fe ≤20ppm ≤20ppm ≤20ppm ≤20ppm Zn ≤1ppm ≤1 ppm ≤1ppm ≤ 1ppm Solution Appearance Clear Clear Clear Clear Other heavy metal (as pb) — — — — HCHO — — — ≤0. 05% Water insoluble matter ≤0. 05% ≤0. 05% ≤0. 05% — Reserve period (months) 12 12 12 12