# When disolving a iron pill for titration, why do I use sulfuric acid instead of nitric or hydrochloric acid?

In a titration to decide the iron content of a pill we dissolved the iron in sulfuric acid and then titrated it with cerium(IV) sulfate. But I just wondered what's wrong with using nitric and hydrochloric acid in this scenario?

• Hint: Ceric sulfate is a strong oxidizing agent, and so is nitric acid. HCl can be oxidized. Can you think of interferences in redox titrations? – M. Farooq Oct 31 '19 at 22:58
• @ M. Farooq Not really sure what to look for. If i used nitric acid it would already oxidise my iron to Fe3+ ions i suppose?. And if i were to guess when using HCL the Ceric sulfate would react with the CL- ions.? not really sure – Ornate Oct 31 '19 at 23:17
• Yes this sounds plausible. – M. Farooq Oct 31 '19 at 23:49

In this case, cerium(IV) sulfate is a strong oxidising agent, hence, using $$\ce{HCl}$$ would likely be oxidised to form $$\ce{Cl}$$ of higher oxidation state. Therefore, the titre would be vastly higher than expected.
$$\ce{HNO3},$$ on the other hand, is an oxidising agent and would oxidise $$\ce{Fe}$$ to a higher oxidation state than expected (of $$\ce{Fe^3+}$$). Hence, not as much cerium(IV) sulfate would be needed to reach the endpoint, resulting in a titre lower than expected.
Therefore, $$\ce{SO4^2-}$$, being a relatively unreactive species, is the best reactant in this case.