Identification of Anions

Carbonates and hydrogencarbonates

Add dilute acid to a carbonate and it effervesces (fizzes). The gas is carbon dioxide.

To prove the gas is $\mathrm{CO_2}$, bubble it through limewater, $\mathrm{Ca(OH)_2(aq)}$. A white precipitate of calcium carbonate makes the limewater turn milky.

Why does this work? Carbonate is the salt of a weak acid. The $\mathrm{H^+}$ ions from the strong acid take the carbonate ion away as unstable carbonic acid, which breaks down into water and $\mathrm{CO_2}$.

Hydrogencarbonates do the same thing, so the gas test cannot tell the two anions apart:

Telling carbonate from hydrogencarbonate

Add magnesium chloride solution (or magnesium sulfate, provided the sample contains no sulfate or sulfite) to a solution of the salt, at room temperature. This is a common school test rather than the only one, so base your conclusion on what you observe.

• Carbonate: an immediate white precipitate of magnesium carbonate. $\mathrm{Mg^{2+}(aq) + CO_3^{2-}(aq) \rightarrow MgCO_3(s)}$

• Hydrogencarbonate: no precipitate in the cold. Magnesium hydrogencarbonate is soluble. On boiling, $\mathrm{HCO_3^-}$ breaks down to carbonate and a precipitate appears slowly.

Back to the start: the fizz in soda bread is the hydrogencarbonate reacting with the acid in the buttermilk, giving $\mathrm{CO_2}$ bubbles that make the bread rise.

Sulfite and the dichromate test

Dilute acid added to a sulfite (warm gently if the reaction is slow) gives sulfur dioxide, a colourless gas with a sharp, choking smell.

The test for $\mathrm{SO_2}$ is a piece of filter paper soaked in acidified potassium dichromate(VI), $\mathrm{K_2Cr_2O_7}$. It turns from orange to green.

Why does the colour change? The orange dichromate(VI) ions gain electrons and become green chromium(III) ions. They can only gain electrons if something supplies them: the $\mathrm{SO_2}$ loses electrons and is oxidised to sulfate. A substance that loses electrons (gets oxidised) and so causes another to be reduced is a reducing agent.

Precipitation tests: chloride, sulfate and phosphate

Some anions are found by adding a cation that forms an insoluble salt with them. The solid that forms is a precipitate. We write an ionic equation: only the ions that join up, with spectator ions left out.

Other silver halides help you to tell halides apart: silver bromide is cream and silver iodide is yellow, and these dissolve in ammonia only with difficulty (iodide not at all), while white silver chloride dissolves in dilute ammonia.

Why acidify first?

Carbonate and sulfite ions also form insoluble silver salts and insoluble barium salts, for example $\mathrm{Ag_2CO_3}$, $\mathrm{BaCO_3}$ and $\mathrm{BaSO_3}$. These would give false positives. Adding dilute acid first destroys $\mathrm{CO_3^{2-}}$ and $\mathrm{SO_3^{2-}}$ (see the sections above), so any precipitate that forms must be due to chloride or sulfate.

Nitrate: the brown ring test

Nitrates form no precipitates, and nearly all nitrates are soluble. So a nitrate is detected with a colour test, the brown ring test. This is one of the ion-identification investigations in the course: carry it out yourself, record your observations as primary data, and write the equations for each test. The brown ring test is a common school test rather than a required one, so always base your conclusion on what you actually observe.

1. Add freshly prepared iron(II) sulfate solution to the nitrate solution in a test tube.

2. Tilt the tube and carefully run concentrated sulfuric acid down the inside wall so that it forms a separate layer underneath.

3. A brown ring forms where the two liquids meet.

The acid and $\mathrm{Fe^{2+}}$ reduce the nitrate to nitrogen monoxide, NO. The NO then binds to more $\mathrm{Fe^{2+}}$ to give the brown complex ion $\mathrm{[Fe(NO)]^{2+}}$.

Putting the tests together

When you have an unknown salt, the order of tests matters. Test for the anion that can interfere with the others first, and remove it.

Notice that the problem was solved by elimination as well as by positive results. Always say what you expect to see for each salt.

Using a precipitate to measure concentration

A precipitation reaction can also measure how much anion there is. This is gravimetric analysis: precipitate all the anion, filter, dry, weigh, then use moles.

Summary

• Carbonate and hydrogencarbonate: dilute acid gives $\mathrm{CO_2}$, which turns limewater milky. Magnesium ions (e.g. magnesium chloride solution) give an immediate precipitate with carbonate only; hydrogencarbonate gives none in the cold, and a precipitate appears slowly on boiling.

• Sulfite: dilute acid gives $\mathrm{SO_2}$ (choking smell), which turns acidified dichromate(VI) paper orange to green. $\mathrm{SO_2}$ is the reducing agent.

• Chloride: acidify with dilute $\mathrm{HNO_3}$, add $\mathrm{AgNO_3}$: white $\mathrm{AgCl}$, soluble in dilute ammonia.

• Sulfate: acidify with dilute HCl, add $\mathrm{BaCl_2}$: white $\mathrm{BaSO_4}$, insoluble in acid. Phosphate: $\mathrm{AgNO_3}$ in neutral solution: yellow $\mathrm{Ag_3PO_4}$, which dissolves in acid.

• Nitrate: brown ring test with iron(II) sulfate and concentrated sulfuric acid. Handle the acid with great care.

• Acidify first to remove carbonate and sulfite. When combining tests, say what you expect to see for each possible anion, and write ionic equations for every precipitate.