Why Naphthalene Balls Vanish From a Cupboard Without Ever Melting

An organic compound straight from a reaction or a plant is almost never pure. Before it can be studied, it must be separated from impurities, and then its elements must be identified and measured.

This part covers purification techniques, chromatography, qualitative detection of elements, and quantitative estimation.

Choose the method that exploits the biggest difference between the compound and its impurities: sublimation for solids that turn directly into vapour, crystallisation for solids with different solubilities, distillation for liquids with different boiling points, steam distillation for water-immiscible volatile compounds, and differential extraction for compounds more soluble in another solvent.

- Sublimation — camphor, naphthalene, benzoic acid
- Crystallisation — dissolve in a hot solvent in which the compound is much less soluble when cold; pure crystals form on cooling
- Simple distillation — boiling points far apart, such as chloroform ($334$ K) and aniline ($457$ K), a gap of $123$ K
- Fractional distillation — boiling points close together, as in separating crude petroleum into fractions
- Distillation under reduced pressure — liquids that decompose at their boiling point, such as glycerol, boil at a lower temperature when pressure is lowered
- Steam distillation — steam-volatile, water-immiscible substances such as aniline and essential oils
- Differential extraction — shaking a solution with an immiscible solvent in a separating funnel

An everyday example. Rose water (gulab jal) is made by distilling rose petals with steam, carrying the fragrant oils over without burning them.

The substance. Crystallisation works only if the solvent is chosen well — too soluble when cold, and the compound never crystallises.

**Chromatography separates components as a mobile phase carries them over a stationary phase; in adsorption chromatography components stick to a solid surface to different extents, in partition chromatography they distribute differently between a stationary liquid and a mobile liquid, and the retardation factor $R_f$ compares how far a spot moves with how far the solvent moves.**

$$R_f=\frac{\text{distance moved by the substance}}{\text{distance moved by the solvent front}}$$

- Adsorption: thin-layer and column chromatography on silica gel or alumina
- Partition: paper chromatography, where water held in the paper is the stationary phase

Worked example. The solvent front travels $8.0$ cm. Spot A moves $3.2$ cm and spot B moves $6.0$ cm.

$$R_f(\text{A})=\frac{3.2}{8.0}=0.40R_f(\text{B})=\frac{6.0}{8.0}=0.75$$

Spot A is held more strongly by the stationary phase; B moves more freely with the solvent.

An everyday example. Sketch-pen ink spreading on wet filter paper separates into bands of different colours — paper chromatography at home.

The substance. **$R_f$ is always less than $1$**, and it changes if the solvent or stationary phase changes.

Carbon and hydrogen are detected by heating with copper(II) oxide; nitrogen, sulphur and halogens are detected after fusing the compound with sodium in Lassaigne's test, which converts them into ionic sodium compounds that give coloured results; phosphorus is detected as phosphate.

Carbon and hydrogen. Heating with CuO turns carbon into **CO${}_2$, which turns lime water milky, and hydrogen into water, which turns anhydrous copper sulphate blue.

Lassaigne's test.** Fusing with sodium gives NaCN, Na${}_2$S and NaX; the fused mass is boiled with water to give the sodium fusion extract.

- Nitrogen: boil with FeSO${}_4$, then acidify with concentrated H${}_2$SO${}_4$ — a Prussian blue colour
- Sulphur: add sodium nitroprusside — a violet colour; or lead acetate — a black precipitate of PbS
- Nitrogen and sulphur together: NaSCN forms and gives a blood-red colour with Fe${}^{3+}$
- Halogens: boil with nitric acid, then add AgNO${}_3$ — white AgCl (soluble in ammonia), pale yellow AgBr, yellow AgI

Phosphorus. Oxidising gives phosphate, which forms a yellow precipitate with ammonium molybdate in nitric acid.

An everyday example. Blowing through a straw into lime water turns it milky — the same carbon dioxide test used to detect carbon.

The substance. **Boiling with nitric acid before adding AgNO${}_3$ is essential**, because it removes cyanide and sulphide ions that would otherwise give false precipitates.

Each element is converted into a weighable or measurable product, and its percentage is the fraction of that product's mass belonging to the element, divided by the sample mass; oxygen is usually found by difference, and dividing percentages by atomic masses gives the empirical formula.

Worked example 1 — carbon and hydrogen. $0.246$ g of a compound gives $0.198$ g of CO${}_2$ and $0.1014$ g of H${}_2$O:

$$\%\text{C}=\frac{12}{44}\times \frac{0.198}{0.246}\times 100\approx 21.95\%\%\text{H}=\frac{2}{18}\times \frac{0.1014}{0.246}\times 100\approx 4.58\%$$

Worked example 2 — nitrogen by Dumas method. $0.30$ g gives $50$ mL of N${}_2$ at STP:

$$\%\text{N}=\frac{28}{22400}\times \frac{50}{0.30}\times 100\approx 20.83\%$$

Worked example 3 — bromine by Carius method. $0.15$ g gives $0.12$ g of AgBr (molar mass $188$):

$$\%\text{Br}=\frac{80}{188}\times \frac{0.12}{0.15}\times 100\approx 34.0\%$$

Worked example 4 — empirical formula. A compound has $54.5\%$ C, $9.1\%$ H and $36.4\%$ O:

$$\frac{54.5}{12}\approx 4.54\frac{9.1}{1.008}\approx 9.03\frac{36.4}{16}\approx 2.28$$

Dividing by $2.28$ gives $2:4:1$, so the empirical formula is **C${}_2$H${}_4$O.

An everyday example. The nitrogen content printed on fertiliser bags is found by nitrogen estimation methods such as Kjeldahl's.

The substance. Kjeldahl's method does not work for nitro compounds, azo compounds or nitrogen in rings such as pyridine**, which do not change to ammonium sulphate.