1
$\begingroup$

For the Determination of iron by Redox Titration experiment, I am finding the role of sulphuric acid and phosphoric acid. Then I found that other than $\ce{H3PO4}$ masks the yellow color of $\ce{Fe^3+}$ and makes the endpoint easier to see (Daniel C Harris, Quantitative Chemical Analysis 8ed. pg 350) by forming complex while $\ce{H2SO4}$ prevents hydrolysis and provide surplus $\ce{H+}$ ions in the solution to keep reaction proceeding; both can prevent air oxidation.($\ce{H2SO4}$, $\ce{H3PO4}$) Do both phosphoric acid and sulphuric acid prevent oxidation of ferrous ions in iron titration or is it just $\ce{H2SO4}$?

Titration against KMnO4

$\endgroup$

1 Answer 1

1
$\begingroup$

My understanding is that acidic solutions have a lower potential for the oxidation of ferrous ion by O2 so sulfuric will increase the oxidizing ability of permanganate and not accelerate side reactions. Phosphoric acid complexes ferric ion and has 3 effects. It increases the overall potential change at the equivalence point making endpoints easier to determine both potentiometrically and using a redox indicator and removes the ferric ion color. It should also hasten oxidation by O2. Phosphoric acid is more useful using dichromate oxidant than permanganate since permanganate is its own indicator. The best way to avoid reaction with O2 is to purge with N2 or add a pinch of sodium bicarbonate to the acidic solution and fill the flask with CO2, then add the phosphoric acid.

$\endgroup$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.