Index Name Index ValueNitrogen (N) content (based on dry basis),% ≥ 21.0Water content (H2O),% ≤ 0.2Free acid (H2SO4),% ≤ 0.05Iron (Fe),% ≤ 0.007Arsenic (As),% ≤ 0.00005Heavy metal (based on Pb),% ≤ 0.005Insoluble substance in water,% ≤ 0.05PH 2.5---3.5