The amount of ferrous sulfate in the same substance reacts with ammonium sulfate to produce ammonium ferrous sulfate with less solubility.

Ferrous sulfate. (ammonium sulfate) 2SO4 .6H2O, light green crystal], the trade name of this crystal is called molar salt.

In this experiment, iron filings are first dissolved in dilute sulfuric acid to obtain ferrous sulfate.

Iron (iron filings)+sulfuric acid (dilute) = ferrous sulfate+H2

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Then ferrous sulfate with the same amount of substance is added to react with ammonium sulfate to generate ferrous sulfate with less solubility.

Ammonium, heating and concentrating, cooling to room temperature, ammonium ferrous sulfate can be separated out.

Ferrous sulfate +(NH4)2so 4+6H2O = ferrous sulfate (NH4)2so 4·6H2O

Ammonium ferrous sulfate is a double salt,

Generally, ferrous salts are easily oxidized in air,

But it is more stable after forming double salt.

Fixed, not easy to be oxidized, so it is often used to prepare standard solutions of ferrous ions in quantitative analysis.