In the silver nitrate test, dilute nitric acid is added to the sample before the silver nitrate. It removes other anions that would also form precipitates with silver ions and give a false positive result.Carbonate ions (CO32−) would form a pale precipitate of silver carbonate, which could be mistaken for a silver halide:
2Ag+(aq) + CO32−(aq) → Ag2CO3(s)
Nitric acid reacts with carbonate ions to give carbon dioxide and water, so they are removed:
2H+(aq) + CO32−(aq) → CO2(g) + H2O(l) Hydroxide ions (OH−) would form a brown precipitate of silver oxide. Nitric acid neutralises them:
H+(aq) + OH−(aq) → H2O(l) Why nitric acid and not another acid?
Hydrochloric acid contains chloride ions, which would react with the silver nitrate to give a white precipitate of silver chloride, even if the sample contained no chloride.
Sulfuric acid adds sulfate ions, and silver sulfate is only slightly soluble, so it may also precipitate.
Nitric acid adds only nitrate ions, and silver nitrate is soluble, so no extra precipitate forms.Worked example: a student tests sodium carbonate solution with silver nitrate but forgets the nitric acid. A pale precipitate forms. Explain the result.
The carbonate ions have reacted with silver ions to form silver carbonate, Ag2CO3. This is a false positive: no halide is present. Adding dilute nitric acid first would have removed the carbonate as carbon dioxide (seen as fizzing), and no precipitate would then form.
Nitric acid reacts with carbonate ions to give carbon dioxide and water, so they are removed:
Hydrochloric acid contains chloride ions, which would react with the silver nitrate to give a white precipitate of silver chloride, even if the sample contained no chloride.
Sulfuric acid adds sulfate ions, and silver sulfate is only slightly soluble, so it may also precipitate.
Nitric acid adds only nitrate ions, and silver nitrate is soluble, so no extra precipitate forms.Worked example: a student tests sodium carbonate solution with silver nitrate but forgets the nitric acid. A pale precipitate forms. Explain the result.
The carbonate ions have reacted with silver ions to form silver carbonate, Ag2CO3. This is a false positive: no halide is present. Adding dilute nitric acid first would have removed the carbonate as carbon dioxide (seen as fizzing), and no precipitate would then form.