Why doesn’t ferrous sulfate appear orange in vinegar?

When I mix ferrous sulfate crystals with water, the solution almost immediately turns orange. However, when I mix ferrous sulfate with vinegar, it doesn’t change colors.


Why is this?




As Poutnik suggests, water contains a small proportion of dissolved oxygen, about $\pu{10 mg/L}$. These $\ce{O2}$ molecules oxidize quickly some $\ce{Fe^{2+}}$ ions, and produce some $\ce{Fe^{3+}}$ ions which have a brownish color when included in aqueous complexes like $\ce{[Fe(OH)]^{2+}}$ according to :
$$\ce{4 Fe^{2+} + O2 + 2 H2O -> 4 Fe^{3+} + 4 OH^- -> 4 [Fe(OH)]^{2+}}$$In vineggar, or in any acidic solution, the brown complex $\ce{[Fe(OH)]^{2+}}$ is not formed, because the oxidation equation is $$\ce{4 Fe^{2+} + O2 + 4 H^+ -> 4 Fe^{3+} + 2 H2O}$$ The ferric ion $\ce{Fe^{3+}}$ may be included in a water complex like $\ce{[Fe(H2O)6]^{3+}}$, which is not brownish.


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