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Chemistry Forums for Students => High School Chemistry Forum => Topic started by: yeung on September 23, 2005, 10:34:04 PM

Title: acidifying reagents
Post by: yeung on September 23, 2005, 10:34:04 PM

Why do we use dilute sulphuric acid to acidify potassium permangate solution, but not hydrochloric acid or nitric acid?

Title: Re:acidifying reagents
Post by: xiankai on September 24, 2005, 03:41:02 AM

the latter are sufficiently stronger reducing agents than the former.

using them would defeat the purpose of KMnO4 solution.

Title: Re:acidifying reagents
Post by: Borek on September 24, 2005, 10:01:38 AM

Quote from: xiankai on September 24, 2005, 03:41:02 AM

the latter are sufficiently stronger reducing agents than the former.

Cl^- can be oxidised to Cl₂ - that's OK.

But what do you want NO₃^- to be oxidised to?

I would rather expect NO₃^- to add to MnO₄^- oxidation, making stoichiometry of the reaction dubious.

Title: Re:acidifying reagents
Post by: yeung on September 25, 2005, 12:52:12 AM

Why do we need to acidify the solution?

Title: Re:acidifying reagents
Post by: Donaldson Tan on September 25, 2005, 02:03:27 AM

MnO₄^- + 8H⁺ + 5e <-> Mn²⁺ + 4H₂O

the oxidising power of potassium permanganate is governed by the above equation. The forward reaction is favoured (Le Chateliter's Principle) in an acidic medium.

The only reason why dilute sulphuric acid is the preferred acid medium is because not only it provides H⁺, but also HSO₄^- and SO₄^2- are not easily oxidised/reduced to yield unwanted products.