Testing for anions

From a salt to its two ions

A salt is made of a cation and an anion. Qualitative analysis names both. We usually start with the anion, because its tests are quick and need no separation. We sort the ten anions of this lesson into three sets by the reagent that acts on them.

Before any reagent, look at the salt. Note its colour, its smell and what it does when heated. A blue or green salt suggests a copper compound. A smell of vinegar or ammonia is already a clue.

Set A: dilute sulphuric acid. Warm a little of the salt with dilute sulphuric acid in a test tube. A gas or vapour comes off if the salt is a carbonate, sulphide, sulphite, nitrite or acetate.

Set B: concentrated sulphuric acid. Warm a little of the dry salt with concentrated sulphuric acid. Chlorides, bromides, iodides and nitrates give a vapour.

Set C: an independent test. Sulphate gives no gas with either acid. A solution test with barium chloride finds it.

Set A: dilute sulphuric acid

• Carbonate, $\text{CO}_{3}^{2-}$. Brisk effervescence. The colourless, odourless gas is carbon dioxide. Pass it into lime water, and the water turns milky. $\text{Na}_{2}\text{CO}_{3}\ +\ \text{H}_{2}\text{SO}_{4}\ \rightarrow \ \text{Na}_{2}\text{SO}_{4}\ +\ \text{H}_{2}\text{O}\ +\ \text{CO}_{2}$
• Sulphide, $\text{S}^{2-}$. The gas is hydrogen sulphide, with the smell of rotten eggs. It turns a paper dipped in lead acetate black, from lead sulphide. $\text{Na}_{2}\text{S}\ +\ \text{H}_{2}\text{SO}_{4}\ \rightarrow \ \text{Na}_{2}\text{SO}_{4}\ +\ \text{H}_{2}\text{S}$ and $(\text{CH}_{3}\text{COO})_{2}\text{Pb}\ +\ \text{H}_{2}\text{S}\ \rightarrow \ \text{PbS}\ +\ 2\text{CH}_{3}\text{COOH}$
• Sulphite, $\text{SO}_{3}^{2-}$. The gas is sulphur dioxide, with the choking smell of burning sulphur. It turns a paper dipped in acidified potassium dichromate from orange to green. $\text{Cr}_{2}\text{O}_{7}^{2-}\ +\ 3\text{SO}_{2}\ +\ 2\text{H}^{+}\ \rightarrow \ 2\text{Cr}^{3+}\ +\ 3\text{SO}_{4}^{2-}\ +\ \text{H}_{2}\text{O}$ Sulphur dioxide also turns lime water milky, so smell and dichromate tell it from carbon dioxide.
• Nitrite, $\text{NO}_{2}^{-}$. Light brown fumes of nitrogen dioxide. The nitrous acid first formed breaks up: $2\text{NaNO}_{2}\ +\ \text{H}_{2}\text{SO}_{4}\ \rightarrow \ \text{Na}_{2}\text{SO}_{4}\ +\ 2\text{HNO}_{2}$, then $3\text{HNO}_{2}\ \rightarrow \ \text{HNO}_{3}\ +\ 2\text{NO}\ +\ \text{H}_{2}\text{O}$, and the nitrogen oxide meets air: $2\text{NO}\ +\ \text{O}_{2}\ \rightarrow \ 2\text{NO}_{2}$.
• Acetate, $\text{CH}_{3}\text{COO}^{-}$. On warming, the smell of vinegar. $\text{CH}_{3}\text{COONa}\ +\ \text{H}_{2}\text{SO}_{4}\ \rightarrow \ \text{CH}_{3}\text{COOH}\ +\ \text{NaHSO}_{4}$

Confirmatory tests.

• For sulphite: barium chloride gives a white precipitate of barium sulphite that dissolves in dilute hydrochloric acid with the smell of sulphur dioxide.
• For nitrite: fresh iron(II) sulphate with dilute acetic acid gives a dark brown colour. Potassium iodide with acetic acid and starch gives blue.
• For acetate: neutral iron(III) chloride gives a deep red colour. Warming the salt with ethanol and a few drops of concentrated sulphuric acid gives the fruity smell of ethyl ethanoate.

Set B: concentrated sulphuric acid

• Chloride, $\text{Cl}^{-}$. Colourless, pungent hydrogen chloride gas that gives dense white fumes of ammonium chloride with a rod dipped in ammonia solution. $\text{NaCl}\ +\ \text{H}_{2}\text{SO}_{4}\ \rightarrow \ \text{NaHSO}_{4}\ +\ \text{HCl}$ With silver nitrate the chloride gives a white precipitate that dissolves in ammonia solution.
• Bromide, $\text{Br}^{-}$. Reddish brown vapours of bromine. $2\text{KBr}\ +\ 2\text{H}_{2}\text{SO}_{4}\ \rightarrow \ \text{K}_{2}\text{SO}_{4}\ +\ \text{Br}_{2}\ +\ \text{SO}_{2}\ +\ 2\text{H}_{2}\text{O}$ Silver nitrate gives a pale yellow precipitate, sparingly soluble in ammonia solution.
• Iodide, $\text{I}^{-}$. Violet vapours of iodine. $2\text{NaI}\ +\ 2\text{H}_{2}\text{SO}_{4}\ \rightarrow \ \text{Na}_{2}\text{SO}_{4}\ +\ \text{I}_{2}\ +\ \text{SO}_{2}\ +\ 2\text{H}_{2}\text{O}$ Silver nitrate gives a yellow precipitate that does not dissolve in ammonia solution.
• Nitrate, $\text{NO}_{3}^{-}$. With copper turnings added, reddish brown fumes of nitrogen dioxide and a blue solution of copper(II) nitrate. $\text{Cu}\ +\ 4\text{HNO}_{3}\ \rightarrow \ \text{Cu}(\text{NO}_{3})_{2}\ +\ 2\text{NO}_{2}\ +\ 2\text{H}_{2}\text{O}$

The chromyl chloride test for chloride. Mix the dry salt with a little solid potassium dichromate and warm with concentrated sulphuric acid. Orange-red vapours of chromyl chloride, $\text{CrO}_{2}\text{Cl}_{2}$, come off. Pass them into sodium hydroxide, and a yellow solution of sodium chromate forms. Acidify it with acetic acid and add lead acetate. A yellow precipitate of lead chromate shows the chloride.

$4\text{NaCl}\ +\ \text{K}_{2}\text{Cr}_{2}\text{O}_{7}\ +\ 6\text{H}_{2}\text{SO}_{4}\ \rightarrow \ 2\text{KHSO}_{4}\ +\ 4\text{NaHSO}_{4}\ +\ 2\text{CrO}_{2}\text{Cl}_{2}\ +\ 3\text{H}_{2}\text{O}$

The layer test for bromide and iodide. Add chlorine water drop by drop and a little carbon tetrachloride. Shake. Bromide gives an orange-brown lower layer. Iodide gives a violet one.

The brown ring test for nitrate. Add iron(II) sulphate solution to the nitrate solution. Pour concentrated sulphuric acid slowly down the wall of the tilted tube. The acid sinks and forms a layer below. A brown ring forms where the two liquids meet.

$\text{NO}_{3}^{-}\ +\ 3\text{Fe}^{2+}\ +\ 4\text{H}^{+}\ \rightarrow \ \text{NO}\ +\ 3\text{Fe}^{3+}\ +\ 2\text{H}_{2}\text{O}$

$[\text{Fe}(\text{H}_{2}\text{O})_{6}]^{2+}\ +\ \text{NO}\ \rightarrow \ [\text{Fe}(\text{H}_{2}\text{O})_{5}\text{NO}]^{2+}\ +\ \text{H}_{2}\text{O}$

Nitrite also gives a brown colour with iron(II) sulphate, even with only dilute acetic acid. Destroy the nitrite first, for example by warming with ammonium chloride or sulphamic acid, and then test for nitrate.

Set C: sulphate

Add barium chloride solution to a solution of the salt. A white precipitate that does not dissolve in dilute hydrochloric acid is barium sulphate. $\text{Ba}^{2+}\ +\ \text{SO}_{4}^{2-}\ \rightarrow \ \text{BaSO}_{4}$ The acid is added to remove carbonate and sulphite, whose barium salts dissolve in it. Lead acetate also gives a white precipitate of lead sulphate that dissolves in ammonium acetate solution.

Choose sodium sulphide and 1 g. Then choose sodium carbonate. The lighter the molar mass, the more moles in a gram, and the more gas.

Practise with new numbers

Try it in Python

The program takes a salt and a mass, and works out the gas that dilute acid frees and the precipitate it can make.

Common mistakes

• Mixing carbon dioxide and sulphur dioxide. Both turn lime water milky. Only sulphur dioxide has a smell and turns dichromate paper green.
• Adding barium chloride to a carbonate or sulphite and calling it sulphate. Add dilute hydrochloric acid. Only the sulphate precipitate stays.
• Forgetting that nitrite also gives a brown colour with iron(II) sulphate. Test for nitrate only after nitrite is out of the way.
• Pouring the concentrated acid in quickly. The brown ring is lost if the layers mix.

Remember these

• Dilute acid: carbonate (milky lime water), sulphide (black lead paper), sulphite (green dichromate paper), nitrite (brown fumes), acetate (vinegar).
• Concentrated acid: chloride (white fumes with ammonia), bromide (brown vapours), iodide (violet vapours), nitrate (brown fumes with copper).
• Sulphate: white barium sulphate that stays in dilute hydrochloric acid.
• Silver halides: white, pale yellow and yellow.

References

• NCERT Chemistry Lab Manual for Class 11, the experiments on the analysis of an inorganic salt.
• Vogel's Qualitative Inorganic Analysis, the chapters on the reactions of anions.

Check yourself

Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.

1. A gas turns lime water milky. How can you tell whether it is carbon dioxide or sulphur dioxide?

2. Why does the brown ring form only where the two layers meet?

3. A salt gives dense white fumes with a rod dipped in ammonia solution after warming with concentrated sulphuric acid. Which anion is present?

4. Why is dilute hydrochloric acid added after barium chloride in the test for sulphate?