Item

Supreme grade

First grade

Qualified grade

Appearance

no visible mechanical impurity

no visible mechanical impurity

no visible mechanical impurity

N content(dry basis),%       ≥

21.0

20.8

20.5

Water content(H₂O),%         ≤

0.5

0.8

1.0

Free acid(H₂SO₄)content,%    ≤

0.03

0.05

0.20

Fe content, %                 ≤

0.007

-

-

As content,%                  ≤

0.00005

-

-

Heavy metal(Pb)content,%    ≤

0.005

-

-

Water insolubles,%            ≤

0.01

-

-