# How does acidifying a compound in qualitative analysis make sure any interrupting compounds get removed?

So I've learnt that acidifying a compound for salt analysis is to remove any basic interference and remove interrupting compounds such as carbonates. How does it remove them or what reaction is taking place?

One of the fundamental reactions of chemistry is that acid + carbonate gives carbon dioxide + water plus the salt of the acid. The carbon dioxide bubbles off removing it from the system and you are left with an aqueous solution of the salt of the acid.

$$\ce{XCO3 + HY -> XY + CO2 + H2O}$$

We acidify salts for two reasons, either to remove offending compounds such as carbonates (which can give a false positive for the sulfate test since it too forms a white precipitate with $$\ce{BaCl2}$$). Additionally $$\ce{NaOH}$$ (example), if you add $$\ce{BaNO3}$$ to it (sulphate ions reagent) you’ll get $$\ce{Ba(OH)2 + NaNO3}$$, barium hydroxide is only slightly soluble at normal temperatures so you see precipitate. If we had added an acid it would’ve been fine, no false positive with the sulphate reagent. Another example would be $$\ce{AgNO3 + Na2CO3 -> AgCO3 + NaNO3}$$

Also we acidify to remove any basic interference for the halides. The solution needs to be acidic so that $$\ce{AgO}$$ doesn’t form, silver oxide forms in alkaline solutions. It is brownish in colour and can thus interfere with the precipitation of the halides since that depends on colour.

So if there was any basic interference in the solution of lets say $$\ce{NaCl}$$.

$$\ce{AgNO3 + LiOH -> AgOH + H2O}$$

$$\ce{2AgOH -> Ag2O + H2O}$$ (Highly favourable energetics)

And we use nitric acid especially since the salts it forms are unoffending, let me give an example. SO for the halides, if we have a solution of $$\ce{NaBr}$$, and we add $$\ce{HCl}$$ to the solution and maybe there is basic interference like $$\ce{NaOH}$$. $$\ce{HCl + NaOH}$$ would give us a chloride salt which would obviously interefere in the precipitates colour when we add $$\ce{AgNO3}$$.

But this is simply a precaution to add $$\ce{HNO3}$$, I myself have done ppt reactions without acidifying (it was a mistake, but it worked).

• Was the answer downvoted for not using the markdown formatting? – Serapion Mar 22 at 10:52