Class 11 Chemistry — Chapter 302: Basic Analytical Techniques

Key concepts: Basic Analytical Techniques (Class 11 Chemistry)

Before any substance can be analysed it must first be separated from the mixture it occurs in and then shown to be pure. This chapter surveys the standard laboratory techniques for doing exactly that — filtration and crystallisation for solids, sublimation for solids that vaporise directly, the four kinds of distillation (simple, fractional, steam and reduced-pressure) for liquids, solvent extraction using a separating funnel, and chromatography in its paper, thin-layer and column forms — and then sets out the criteria of purity, namely a sharp melting point for a solid and a constant boiling point for a liquid.

Filtration

Separates an insoluble solid from a liquid by passing the mixture through filter paper: the solid held back is the residue and the liquid collected below is the filtrate. Suction through a Buchner funnel speeds it up for fine or gelatinous precipitates.

Crystallisation

Purifies a solid by dissolving it in a minimum of hot solvent and cooling the saturated solution, so that the desired substance separates as crystals while the small amount of impurity stays behind in the mother liquor. It works best when solubility rises steeply with temperature.

Sublimation

The direct conversion of a solid into vapour on heating, and back to solid on cooling, without an intervening liquid state. It separates a sublimable component such as camphor, naphthalene, iodine or ammonium chloride from a non-sublimable one such as common salt.

Simple distillation

Boils a liquid and condenses its vapour back in a Liebig condenser. It is suitable when the impurity is non-volatile, or when a second liquid differs in boiling point by a wide margin, so that one component vaporises far more readily than the other.

Fractional distillation

Uses a fractionating column between the flask and the condenser, in which the ascending vapour repeatedly condenses and re-vaporises. Each cycle enriches the vapour in the lower-boiling component, which is what makes separation possible when boiling points are close.

Steam distillation

Passes steam through the mixture so that a steam-volatile substance immiscible with water distils over well below its own boiling point, as the total vapour pressure of the two liquids reaches atmospheric pressure sooner. Aniline is the standard example.

Distillation under reduced pressure

Also called vacuum distillation, this lowers the pressure above the liquid so that it boils at a much lower temperature. It is essential for substances such as glycerol that would decompose if heated to their normal boiling point at atmospheric pressure.

Solvent extraction

Shakes an aqueous solution with an immiscible organic solvent, usually ether, in a separating funnel. The solute distributes itself in favour of the solvent in which it is more soluble, and the two layers are then run off separately through the stopcock.

Chromatography

A family of techniques that separate components by their differing distribution between a fixed stationary phase and a moving mobile phase. Weakly held components travel far with the mobile phase while strongly adsorbed ones lag behind, so the mixture spreads into bands.

Paper chromatography

The stationary phase is water adsorbed on the cellulose of the chromatography paper and the mobile phase is the developing solvent that climbs the paper by capillary action, carrying the spotted components to different heights above the baseline.

Thin layer chromatography (TLC)

A thin film of silica gel or alumina coated on a glass or plastic plate serves as the stationary phase. It is faster than paper chromatography, needs very little sample, and is routinely used to follow whether a reaction has gone to completion.

Column chromatography

The mixture is run down a glass column packed with alumina or silica gel and washed through with an eluent. The most weakly adsorbed component leaves first and the most strongly adsorbed one leaves last, so preparative amounts can be collected as separate fractions.

Rf value

The retardation factor, defined as the distance travelled by the component divided by the distance travelled by the solvent front from the same baseline. It is dimensionless, never exceeds one, and is characteristic of a substance only for a stated solvent and stationary phase.

Criteria of purity

A pure solid melts sharply over about one degree, and a pure liquid boils at a constant temperature at a stated pressure. Impurities lower and broaden a melting point, so a mixed melting point test can confirm the identity of a known compound.

Qualitative analysis basics

Detecting the elements in an organic compound begins with Lassaigne's sodium fusion, which converts nitrogen, sulphur and halogens into ionic sodium salts; the extract then gives Prussian blue for nitrogen, a violet colour with sodium nitroprusside for sulphur, and silver halide precipitates for halogens.