Past exam of the medicine course of the University of Oxford / 1867 / First examination / Chemistry / 10 / Solution

Iodine is a dark, lustrous crystalline halogen which gives a violet vapour; historically, an important source was the ash of iodine-rich seaweeds. Elemental iodine commonly occurs as diatomic ${\mathrm{I}}_2$. It melts at about $114{}^{\circ }\mathrm{C}$ and boils at about $184{}^{\circ }\mathrm{C}$ at ordinary atmospheric pressure. It also undergoes sublimation below its melting point, depositing crystals again on a cooler surface.^[ref]

Iodine is sparingly soluble in water, giving a brownish solution, and dissolves more freely in ethanol and in an aqueous iodide solution. In the latter case the chemical equilibriumforms triiodide. Iodine with starch, under suitable conditions, gives a sensitive dark-blue colour; this is a useful recognition test, but oxidants that liberate iodine from added iodide must be distinguished from iodine initially present. Reaction with reactive metals gives iodides; in such compounds the iodine usually has oxidation state $-1$.

For the historical seaweed route, collect and burn iodine-accumulating seaweeds to produce kelp ash, then extract the soluble salts with water. Crystallization removes much of the common salt content, leaving an iodide-enriched mother liquor. Oxidation releases iodine, which can be distilled or sublimed and collected on a cool surface. One representative net chemical equation for liberation from iodide isThe manganese dioxide is the oxidizing agent; the seaweed does not contain a supply of ready-made violet iodine crystals. This describes the chemical principle of a historical extraction, rather than prescribing one unique factory recipe.

Iodine also occurs in natural brines and in iodate-bearing deposits associated with nitrate ores. Modern industrial supply relies heavily on brines and iodate ores; it should not be equated with the seaweed source emphasized in nineteenth-century teaching.^[ref]