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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?

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    $\begingroup$ 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? $\endgroup$ – M. Farooq Oct 31 at 22:58
  • $\begingroup$ @ 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 $\endgroup$ – Ornate Oct 31 at 23:17
  • $\begingroup$ Yes this sounds plausible. $\endgroup$ – M. Farooq Oct 31 at 23:49
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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.

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