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Recently, I have learnt about testing for ions in qualitative analysis (BBC Bitesize).

The link above which I used for reference mentions that for the test for sulfate anions, hydrochloric acid $(\ce{HCl})$ is used to ensure that there is no presence of carbonate ions. For the test for halide ions $(\ce{F-},$ $\ce{Cl-},$ $\ce{Br-},$ $\ce{I-},$ etc), nitric acid $(\ce{HNO3})$ is used instead, for the same purpose.

I would like to know why the acids used are different, since they serve the same purpose.

Update: $HCl$ cannot be used for removal of carbonates in halides since $HCl$ contains the $Cl-$ halide. However, as to why nitric acid cannot be universally applied to all, I am still unsure.

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    $\begingroup$ Wouldn't adding $\ce{HCl}$ interfere with a test for $\ce{Cl-}$? $\endgroup$ – MaxW Jul 17 at 15:33
  • $\begingroup$ @MaxW how about adding nitric acid in the test for sulfate ions? $\endgroup$ – QuIcKmAtHs Jul 17 at 15:34
  • $\begingroup$ Quoting the linked page: "Barium nitrate solution can be used instead of barium chloride solution. However, nitric acid is added first to acidify the test solution. Sulfuric acid cannot be used because it contains sulfate ions - these would interfere with the second part of the test." $\endgroup$ – Buck Thorn Jul 17 at 16:31
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The link above which I used for reference mentions that for the test for sulfate anions, hydrochloric acid (HCl) is used to ensure that there is no presence of carbonate ions. For the test for halide ions (F−, Cl−, Br−, I−, etc), nitric acid (HNO3) is used instead, for the same purpose.

Good question, but this is not a universal practice. In reality, the choice of acid depends on the actual sample. Qualitative tests, like these, have become merely academic exercises due the to advances in instrumental analysis these days. As you said, one cannot add HCl to destroy carbonates in a sample which is to be tested for halide ions. Dilute nitric is the only choice, because you would not use sulfuric acid in the case of halides because (i) your test sample may contain ions which will precipitate with sulfate (Ba, Ag, Pb, Ca etc.). Concentrated nitric acid would be a poor choice as well because it will oxidize iodide to free iodine.

An excellent book, which is far better than any online resource, is Vogel's Qualitative Inorganic Analysis. It should be available in any good library and it will give an in depth view of qualitative analysis.

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