2. According to the different iron content, there are two forms: when the iron content is 14.5% ~ 16.0%, it is green; When the iron content is 16.5% ~ 18.5%, it is brown. They are all double salts composed of iron, ammonium and citric acid. Both of them are easily deliquescent and moldy.
3. The former is transparent green flake, granular crystal or crystalline powder. No taste, iron and metal taste. Paper coated with green ammonium ferric citrate and red blood salt (potassium ferricyanide) is called blue blueprint. When exposed, the trivalent iron in citrate is reduced to bivalent, and when it meets water, it generates tengshi blue, so the light-receiving part becomes blue, and the opaque part is still white, and a white line pattern with a blue background can be obtained. Under light, green scales are more easily reduced to ferrous salts.
4. The latter is brown or reddish brown transparent thin scaly or granular crystals, or crystalline powder, odorless, with iron metal taste.