Qualitative analysis

Which elements are in it?

Every organic compound has carbon. Most have hydrogen. Many also hold oxygen, nitrogen, sulphur, a halogen or phosphorus. Qualitative analysis tells you which of these are present, without yet telling you how much.

The tests rely on one trick. A covalent compound gives no simple tests, because its atoms are not ions. So the element is first turned into an ion, and the ion is then detected with a reagent that gives a colour or a precipitate.

Carbon and hydrogen

Heat a little of the compound with dry copper(II) oxide. The oxide hands over its oxygen.

• Carbon becomes carbon dioxide: $\text{C} + 2\text{CuO} \rightarrow 2\text{Cu} + \text{CO}_2$
• Hydrogen becomes water: $2\text{H} + \text{CuO} \rightarrow \text{Cu} + \text{H}_2\text{O}$

Pass the gas into lime water. If it turns milky, there is carbon dioxide, so the compound held carbon.

• $\text{CO}_2 + \text{Ca(OH)}_2 \rightarrow \text{CaCO}_3 + \text{H}_2\text{O}$

The water condenses as droplets on the cooler part of the tube. Anhydrous copper sulphate is white. It turns blue on taking up water, so a blue colour shows there was hydrogen.

The sodium fusion test

Nitrogen, sulphur, the halogens and phosphorus are bonded covalently. To detect them, the compound is fused with a small piece of sodium metal. The mixture is heated until red hot, and then plunged into distilled water. It is boiled and filtered. The filtrate is called the sodium fusion extract, or Lassaigne's extract.

The sodium turns each element into a sodium salt that dissolves in water.

• $\text{Na} + \text{C} + \text{N} \rightarrow \text{NaCN}$
• $2\text{Na} + \text{S} \rightarrow \text{Na}_2\text{S}$
• $\text{Na} + \text{X} \rightarrow \text{NaX}$, where X is Cl, Br or I

The cyanide, the sulphide and the halide ions are what the tests find. Note that nitrogen forms cyanide only when carbon is also present, which is always so in an organic compound.

Testing for nitrogen

Boil a part of the extract with iron(II) sulphate. Then acidify it with concentrated sulphuric acid. A Prussian blue colour shows that nitrogen was present.

• $\text{Fe}^{2+} + 6\text{CN}^- \rightarrow [\text{Fe(CN)}_6]^{4-}$
• $3[\text{Fe(CN)}_6]^{4-} + 4\text{Fe}^{3+} \rightarrow \text{Fe}_4[\text{Fe(CN)}_6]_3$

The iron(III) ions come from the air oxidising a little of the iron(II).

Testing for sulphur

Two tests give the sulphide ion away.

• Lead acetate. Acidify part of the extract with acetic acid, and add lead acetate. A black precipitate of lead sulphide, PbS, shows sulphur.
• Sodium nitroprusside. Add a few drops to the extract. A violet colour shows sulphur.

When both nitrogen and sulphur are present, sodium can make the thiocyanate ion, $\text{SCN}^-$, instead. With iron(III) chloride, it gives a blood red colour. If a lot of sodium is used, the thiocyanate breaks down into cyanide and sulphide, and each gives its own test.

• $\text{Fe}^{3+} + \text{SCN}^- \rightarrow [\text{Fe(SCN)}]^{2+}$

Testing for the halogens

First boil the extract with dilute nitric acid. This drives off any cyanide and sulphide as gases. If they stayed, they would also give precipitates with silver ions, and spoil the test.

Then add silver nitrate solution.

• White precipitate, soluble in ammonia: chlorine, as silver chloride.
• Pale yellow precipitate, sparingly soluble in ammonia: bromine, as silver bromide.
• Yellow precipitate, insoluble in ammonia: iodine, as silver iodide.

The reaction is $\text{Ag}^+ + \text{X}^- \rightarrow \text{AgX}$.

Testing for phosphorus

The compound is heated with an oxidising agent, sodium peroxide. This turns the phosphorus into a phosphate. Boil the solution with nitric acid, and add ammonium molybdate. A yellow colour or precipitate shows that phosphorus was present.

Practise with new numbers

Try it in Python

The program turns the colours seen into a list of elements, with the blood red colour counting for both nitrogen and sulphur.

Common mistakes

• Skipping the nitric acid before silver nitrate. Cyanide and sulphide would give a false precipitate.
• Thinking blood red means iron. The iron is the reagent. The colour shows the thiocyanate ion, so it points to nitrogen and sulphur together.
• Writing that lime water turns milky for hydrogen. It tests for carbon dioxide, so it shows carbon.
• Mixing up the three silver halides. Think white, pale yellow, yellow in the order of chlorine, bromine, iodine.

Remember these

• Carbon: carbon dioxide turns lime water milky. Hydrogen: water turns white copper sulphate blue.
• Fusion with sodium turns N, S and the halogens into the ions CN, S and X.
• Nitrogen: Prussian blue. Sulphur: black lead sulphide or violet nitroprusside.
• Nitrogen and sulphur together: blood red thiocyanate.
• Halogens: boil with nitric acid first, then silver nitrate. Phosphorus: a yellow colour with ammonium molybdate.

Check yourself

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

1. Why is the extract boiled with nitric acid before silver nitrate is added?

2. A compound gives a blood red colour with iron(III) chloride. Which two elements does that show?

3. A white precipitate forms with silver nitrate and dissolves in ammonia. Which halogen was present?

4. Why is the compound fused with sodium before testing for nitrogen and sulphur?