Carbon, hydrogen and nitrogen: the sodium fusion extract

A powder with no label

You find a white powder on a bench. You heat a little of it in a spoon, and it chars and gives off smoke. A good guess is that it is an organic compound. But a guess is not a proof. How can we show that it holds carbon and hydrogen? And how can we find the other elements, such as nitrogen, that organic compounds often carry?

This is qualitative analysis: finding out what is present, not how much. A chemist turns each element into a simple substance that gives a clear, easy test.

Carbon and hydrogen: the copper(II) oxide test

Aim. To show that an organic compound contains carbon and hydrogen.

Apparatus. A hard glass test tube with a delivery tube, a second test tube with lime water, a Bunsen burner, a little anhydrous copper(II) sulphate on cotton wool, and a spatula.

Steps.

1. Mix about 0.2 g of the dry compound with about 1 g of dry copper(II) oxide. Put the mixture in the hard glass tube.
2. Place a plug of cotton wool with a little anhydrous copper(II) sulphate near the mouth of the tube.
3. Fit the delivery tube so that its end dips into lime water in the second tube.
4. Heat the mixture strongly.

What you see. The lime water turns milky. White anhydrous copper(II) sulphate turns blue.

Why. Hot copper(II) oxide gives its oxygen to the compound. Carbon becomes carbon dioxide, and hydrogen becomes water.

$\text{C}\ +\ 2\text{CuO}\ \rightarrow \ 2\text{Cu}\ +\ \text{CO}_{2}$

$2\text{H}\ +\ \text{CuO}\ \rightarrow \ \text{Cu}\ +\ \text{H}_{2}\text{O}$

Carbon dioxide turns lime water milky because of insoluble calcium carbonate:

$\text{CO}_{2}\ +\ \text{Ca}(\text{OH})_{2}\ \rightarrow \ \text{CaCO}_{3}\ +\ \text{H}_{2}\text{O}$

Water turns white copper(II) sulphate into blue crystals with five molecules of water:

$\text{CuSO}_{4}\ +\ 5\text{H}_{2}\text{O}\ \rightarrow \ \text{CuSO}_{4}\cdot 5\text{H}_{2}\text{O}$

Precautions.

• Dry everything. A trace of water in the compound or in the copper(II) oxide gives a false test for hydrogen.
• Do not pass the gas for too long. An excess of carbon dioxide clears the milkiness: $\text{CaCO}_{3}\ +\ \text{CO}_{2}\ +\ \text{H}_{2}\text{O}\ \rightarrow \ \text{Ca}(\text{HCO}_{3})_{2}$. A student might then think there was no carbon.
• Lift the delivery tube out of the lime water before you stop heating. Otherwise the liquid is sucked back into the hot tube.

Raise $n$ from 1 to 8 at a mass of 1 g. A heavier alkane has more carbon in each gram, so the blue line rises. The red line falls a little. Why? The ratio of hydrogen to carbon gets smaller as the chain grows.

Why a sodium fusion?

Nitrogen, sulphur and the halogens sit in organic compounds as atoms joined by covalent bonds. Silver nitrate cannot find a chlorine atom that is bonded to carbon. We need ions in water. Lassaigne's test (1843) gives them. The compound is fused with sodium metal, and sodium turns each element into a sodium salt:

$\text{Na}\ +\ \text{C}\ +\ \text{N}\ \rightarrow \ \text{NaCN}$

$2\text{Na}\ +\ \text{S}\ \rightarrow \ \text{Na}_{2}\text{S}$

$\text{Na}\ +\ \text{X}\ \rightarrow \ \text{NaX}$ (X is Cl, Br or I)

If sulphur and nitrogen are both present and the sodium is not in excess, they form sodium thiocyanate instead: $\text{Na}\ +\ \text{C}\ +\ \text{N}\ +\ \text{S}\ \rightarrow \ \text{NaSCN}$.

Steps for the extract.

1. Cut a piece of sodium the size of a small pea. Dry it with filter paper. Put it in a dry fusion tube (a small hard glass tube).
2. Heat the tube until the sodium melts and its vapour rises.
3. Drop in about 50 mg of the compound. Heat the tube until it is red hot for about two minutes.
4. Plunge the red hot tube into about 10 mL of distilled water in a china dish. The tube breaks. Stand back and wear safety glasses.
5. Crush the glass with a rod, boil for a few minutes and filter.

The clear filtrate is the sodium fusion extract, or Lassaigne's extract. It is alkaline because of the sodium hydroxide that unused sodium makes.

Testing for nitrogen: Prussian blue

Steps.

1. Take about 2 mL of the extract in a test tube.
2. Add about 1 mL of freshly prepared iron(II) sulphate solution. A dirty green precipitate of iron(II) hydroxide forms.
3. Warm the mixture gently for a minute.
4. Cool, then add dilute sulphuric acid drop by drop until the precipitate dissolves.

What you see. A deep blue colour, or a blue precipitate, is Prussian blue. Nitrogen is present.

Why. The cyanide ions combine with iron(II) ions and make hexacyanoferrate(II) ions. Some iron(II) is oxidised by air to iron(III) during the warming. The iron(III) ions then join the hexacyanoferrate(II) ions to make Prussian blue.

$\text{Fe}^{2+}\ +\ 6\text{CN}^{-}\ \rightarrow \ [\text{Fe}(\text{CN})_{6}]^{4-}$

$3[\text{Fe}(\text{CN})_{6}]^{4-}\ +\ 4\text{Fe}^{3+}\ \rightarrow \ \text{Fe}_{4}[\text{Fe}(\text{CN})_{6}]_{3}$

The acid dissolves the green and brown iron hydroxides that would hide the blue colour.

A negative test is not always "no nitrogen". Hydrazine, $\text{N}_{2}\text{H}_{4}$, and hydroxylamine, $\text{NH}_{2}\text{OH}$, have no carbon, so no cyanide can form. A diazonium salt gives off nitrogen gas when it is heated, before the nitrogen can reach carbon. These compounds give no Prussian blue although they contain nitrogen.

Practise with new numbers

Try it in Python

The first program works out what the copper(II) oxide test can give from an alkane.

Common mistakes

• Testing wet apparatus. Water from the glass gives a false test for hydrogen.
• Passing carbon dioxide for too long. The milkiness goes away and the carbon test seems negative.
• Adding no acid to the iron(II) sulphate test. Green and brown hydroxides hide the blue.
• Saying the test shows nitrogen in every nitrogen compound. Hydrazine and diazonium salts fail it.
• Using old iron(II) sulphate for no reason. Use a fresh solution. An old solution is already oxidised and may hold much iron(III).

Remember these

• Carbon: carbon dioxide turns lime water milky. Hydrogen: water turns white copper(II) sulphate blue.
• Sodium fusion converts the elements to $\text{NaCN}$, $\text{Na}_{2}\text{S}$, $\text{NaX}$ or $\text{NaSCN}$.
• Nitrogen: iron(II) sulphate, warm, then acid gives Prussian blue, $\text{Fe}_{4}[\text{Fe}(\text{CN})_{6}]_{3}$.
• Hydrazine and diazonium salts give no Prussian blue.

References

• NCERT Chemistry Class 11, the chapter Organic Chemistry: Some Basic Principles and Techniques, section on qualitative analysis.
• A standard laboratory manual of organic qualitative analysis, such as Vogel's Textbook of Practical Organic Chemistry.

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 a test done on the sodium fusion extract and not on the organic compound itself?

2. A student bubbles carbon dioxide into lime water. It turns milky and then clear. Does this mean the compound had no carbon?

3. In the nitrogen test, where do the iron(III) ions come from?

4. Does urea, $\text{NH}_{2}\text{CONH}_{2}$, give Prussian blue? Does hydrazine?