How to determine if mystery liquid is dilute H2SO4, H2SO3 or both?

Question

I have 150ml of solution from a student's experiment in producing H2SO4. The student used a variation of Glauber's method for producing a small amount of H2SO4. (burning sulfur & NaNO3 mix to oxidize sulfur into SO2. combining SO2 with air to form SO3, then bubbling the mixture into water. This is the 17th century method. No sweet vanadium catalyst required.

Question 1: He didn't have any NaNO3, so he substituted KNO3.
Am I right that this should still work? i.e. both oxidizers will oxidize sulfur to SO2 when burned?

Now the big question. How can I determine (chemically, electrically, numerologically -anything but orally or sexually :) ) whether this 150ml solution is dilute H2SO4, H2SO3, or a mix of both?

I know both chemicals have distinguishing physical properties (H2SO3 = on-your-*** sulfurous odor), but this is so diluted, none are apparent. All I have to go on is, it's 150ml of clear liquid with no odor and pH <1.

Thanks in advance.. Again.

Answer 1

KNO3 would work just as well.
Test a little of the sample with BaCl2. A white precipitate will tell you whether you have (SO4)2- ions. You have reported the pH, so that's fine - you have H2SO4.
But if there's any H2SO3 present, it will be a reducing agent.
Add a few drops of KMnO4 solution. If the mixture stays purple, you've got just H2SO4. If it takes several drops before finally going purple, you've got some H2SO3 as well.

Answer 2

Sulphurous acid is unstable and has never been isolated as a pure compound...