From wikipedia: Upon exposure to air, it oxidizes to form a corrosive brown-yellow coating of basic ferric sulfate, which is an adduct of ferric oxide and ferric sulfate: $$\ce {12FeSO4 + 3O2 -> 4Fe2 (SO4)3 + 2Fe2O3}$$ The Pubchem page for ferrous sulphate says that this reaction occurs rapidly in moist air and that the rate of oxidation is increased by increasing the pH, temperature, or by ...
Yes, you are quite right. It has something to do with prevention of oxidation of nitrites. By controlling the pH, the nitrite present is reduced to nitric oxide ($\ce {NO}$) which reacts with ferrous sulfate to form a brown-colored complex ion, $\ce {FeSO4.NO}$. Nitrates do not react in such way and thus not registered in the test. It uses a special agent to prevent color formation or ...
From your description I'd say you were titrating ferrous sulphate, $\ce {FeSO4}$ solution (the analyte), with potassium permanganate, $\ce {KMnO4}$ solution (the titrant), in acid conditions (dilute $\ce {H2SO4}$ present). The permanganate ion $\ce {MnO4^ {-}}$ is a strong oxidiser and oxidises the ferrous ion $\ce {Fe^ {2+}}$ to the ferric ion $\ce {Fe^ {3+}}$ very easily. In dilute solution ...
In the final compound $\ce {FeSO4}$ the iron is ferrous, i.e. at the oxidation number + $2$. In this equation, the iron atom is reduced from + $3$ to + $2$. This is impossible in an oxidation with $\ce {O2}$. The oxidative action of $\ce {O2}$ cannot lead to a decrease of the oxidation number of an atom present in the reaction.
The iron supplement ferrous sulfate is an iron salt with the chemical formula FeSO 4. Iron salts are one type of the mineral iron. People often use them as a supplement to treat iron deficiency.