Chemical test to distinguish between sulphurous acid and sulphuric acid

[kemensindia]

What would be a chemical test to distinguish between sulphurous acid and sulphuric acid? I think Sulphuric acid is an oxidising agent and sulphurous acid is a reducing agent but i am not able to come up with a common test that will distinguish the two.

[weiming1998]

Add a couple of ml of the unknown acid that you're testing to a dilute solution of potassium permanganate or dichromate. If it is sulfurous acid, the bright orange/purple colour will change. If it is sulfuric acid, the colour won't change.

Carefully smell the acid. Sulfuric acid hardly has a smell at room temperature, while sulfurous acid (which is just SO2 in water) has an odour resembling burnt matches (when dilute) and stings your nose (when more concentrated)

Sulfurous acid has a bleaching effect on some dyes, while sulfuric acid doesn't.

[carbonnanotube]

Last time I used sulpherous acid it was pretty apparent from the smell what it was. That being said you could always measurethe ORP if you have the equipment.

 

The test weimming suggested will also work if you have sample to sacrifice.

 

As a side note that was a fun project. We were trying to take "Red Mud" which is tailing from the bayer process and extract the REEs. Their idea was to use scrubber water from a near by coal plant to do the extraction. It did not work particularly well.

[ajkoer]

If using litmus paper is considered a chemical test, 1st test the solutions pH with litmus paper. Boil and re-test. If the solution has become more basic, it is H2SO3.

 

Reaction logic:

 

H2SO3 ---Heat--> SO2 (g) + H2O

 

and the pH approaches that of water.

 

Boiling H2SO4 will not raise its pH.

--------------------------------------------------------

 

Another test, add a large amount of zinc foil.

 

If the Zn dissolves and no fine precipitate, it is ZnSO4 and the acid is H2SO4.

 

If the Zn is only very slowly attacked, and forms a fine precipitate, it is ZnSO3 and the acid is H2SO3.

Edited April 26, 2013 by ajkoer

[elementcollector1]

If anything, boiling a solution of H2SO4 lowers the pH - the water boils off first, leaving a greater concentration of H2SO4 (to a point) behind.

[John Cuthber]

If anything, boiling a solution of H2SO4 lowers the pH - the water boils off first, leaving a greater concentration of H2SO4 (to a point) behind.

and...?

[elementcollector1]

and...?

When boiling the solution:

pH goes up - H2SO3

pH goes down - H2SO4

 

Just another thing to add to the test.

[ajkoer]

I did not mention the lowering of the pH upon boiling H2SO4 because the question asks what 'chemical tests' to employ, and this requires the use of somewhat imprecise litmus paper. If the H2SO4 is very dilute, upon boiling it may be difficult to discern a lowering of the pH via litmus paper, in my opinion.