Testing for sulphate ions?

Think of it like this:
You are given a white powder sample and you are asked to confirm whether there is a sulphate present in the sample. If you add some barium chloride solution , You get a white precipitate. So you conclude that a sulphate is present because of the following reaction:
BaCl2(aq) + SO4 2-(aq) → BaSO4(s) + 2Cl-(aq)

But is this correct? What would happen if the sample had actually been a carbonate? You add a solution of BaCl2 and you get a precipitate because of the following reaction:
BaCl2(aq) + CO3 2-(aq) → BaCO3(s) + 2Cl-(aq)

Now the only conclusion you can arrive at is that the BaCl2 test is not conclusive for sulphate - it will also give a precilitate if there is a carbonate present . What to do? If there is some way that you can eliminate the carbonate ( if present) then you can be sure that any precipitate comes from the presence is the sulphate only.
This elimination of the carbonate is done by reacting any carbonate present with an acid. The usual acid of choice is hydrochloric acid , Therefore you add some HCl to the solution of your unknown sample. If a carbonate is present the following reaction occurs:
2HCl(aq) + CO3 2- (aq) → H2O(l) + CO2(g) + 2Cl-(aq)
After the reaction with the HCl , there is no more carbonate in the test solution , so adding BaCl2 will not produce a precipitate.
If a precipitate is produced after the HCl reaction , you can be sure that this is only because a sulphate is present in the original sample.

The CO3 2- ion is called an interfering ion in the sulphate test , and it must be eliminated before BaCl2 is added.
You will note that if the SO4 2- ion is present in the original sample , it does not react with the HCl.

To answer your second question: I seem to remember that when I was learning these basic chemical test , we always used nitric acid to eliminate the possible carbonates , followed by BaCl2 to precipitate the sulphates. Likewise Ba(NO3)2 could be used in place of BaCl2
I cannot see any reason why any combination is HCl/HNO3 and BaCl2 / Ba(NO3)2 cannot be used. What is not permitted is to use H2SO4 as the acid - you are wanting to test for the sulphate ion - so you must not add this to the test solution.
I think that HCl / BaCl2 are used because they are both cheap and readily available in most laboratories.

Sulphate Ion

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Good question. Both the sulfate ion and the carbonate ion are holding multiple electron pairs on their oxygen atoms surround the central sulfur and carbon atoms. The Lewis acid base theory expands what can be thought of as acids and bases by stating that Lewis acids are electron pair acceptors and Lewis bases are electron pair donors, so both the carbonate and sulfate ions are potential Lewis bases. (possible electron pair donors) But some Lewis bases are better electron pair donors than others, or we say, "stronger Lewis bases" Since H2SO4, sulfuric acid is a strong acid, this shows that the H+ ions are able to "escape" the Lewis base SO4= very easily, so the SO4= ion must be a very weak Lewis base so incoming hydrochloric acid which is a strong H+ donor will not do anything with the sulfate ions. But since the carbonate ion does react with incoming H+ from the hydrochloric acid, it must be a stronger Lewis base, donating an electron pair which bonds with the incoming H+ to form HCO3- ions which can in turn also accept another H+ to form molecules of H2CO3, carbonic acid, which then breaks down into CO2 gas and H2O. The reason you want to remove the CO3= ion when testing for Ba++ ions is that both CO3= ions and SO4= ions form an insoluble compound with Ba.

1. Other possible precipitates are carbonates formed by reaction with slightly soluble carbon dioxide
-A carbonate reacts with acid to form carbon dioxide and water so acids will eliminate interference of experiment results


2. If you are testing for chloride ions, then it is possible and absolutely appropriate to use nitric acid. Same goes with nitrate test.