[FraggleRock]

I've asked this question on other boards and No one has helped me yet. I have a salt mixture containing sodium acetate, sodium carbonate, and sodium sulfide. I have to determine the average sulfide ion. I have to dissolve it in water and react it with aqueous silver nitrate solution to form a percipitate. My problem is that carbonate ions form percipitates with silver ions as well. I have to remove the carbonate ion with an acid base reaction (choice between nitric acid or hydrochloric acid). I have to isolate the sulfide ion in the form of silver (I) sulfide. Which acid should I choose?

[Borek]

Acidifying the solution will most likely remove not only carbonate, but hydrogen sulfide as well. Unless you can find a pH at which Ag₂S can be precipitated, but not Ag₂CO₃. Can make an interesting equilibrium problem to solve.

Nitric acid is an oxidizer, which should be taken into account as well.

[CopperSmurf]

That's a very interesting problem. Maybe you could try adding some dilute nitric acid little by little to slowly remove the carbonate ions (by conversion into unstable carbonic acid and then to carbon dioxide gas). Don't use HCl since chloride ions will be present to precipitate with your silver, which will be a problem. *edit* but you still risk making H₂S by adding acid as Borek mentioned, so this isn't the best way to go about the problem.

If you don't absolutely have to use acid/base, I'd try a different approach. I would introduce Ca²⁺ ions into your solution to precipitate all of those carbonate ions first (maybe in the form of calcium nitrate if you have any or some other water-soluble calcium compounds). Then filter and go ahead to use your silver nitrate to isolate your sulfide ions.