# Preparation of standard Fe from FeSO4 • 7H2O

We were assigned to prepare a standard Fe for spectrophotometry determination of Fe. What we did is basically obtain a mass of FeSO4•7H2O, add a small a mount of distilled water before adding 18M of H2SO4 then dilute.

My question is what exactly is the purpose of H2SO4? I asked my prof about it and she said it was to prevent Fe2+ from oxidizing. I'm not really satisfied with that answer. Maybe because we did add hydoxylamine hydrochloride later to reduce the Fe3+ present so I don't see the need to do that addition aside from a different purpose. Can anyone provide the redox reaction of it if it really is a redox reaction happening? Thank you so much for any help much appreciated

• Forget redox. Acid is intended to prevent hydrolysis. – Ivan Neretin Sep 25 at 9:37
• And hydrolysis favours iron oxidation, as standard redox potential decreases for hydrolyzed forms. Mohr salt (NH4)2Fe(SO4)2 is preferred, AFAIK. – Poutnik Sep 25 at 13:00
• I think I already answered your question yesterday. Acid is not added to prevent oxidation (your teacher has a misconception). It is added to prevent hydrolysis and precipitation of iron hydroxides which happens over a period of time. The hydroxides are easily oxidized. In this sense the teacher is right. One could add any dilute acid (HCl, $\ce{HNO3}$) to achieve the same purpose. There is no redox reaction. Conc. sulfuric acid acts as an oxidizing agent at really high temperatures. Guess what, if sulfate ion were an oxidizing agent, it has to reduce itself as well to something? Right? – M. Farooq Sep 25 at 13:20