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Iodphenol is obtained by the action of iodine and iodic acid on phenol dissolved in a dilute solution of caustic potash.

With iodine compounds, iodic acid is likely to be formed, and hence the solution must be reduced with sulphurous acid before precipitation with silver nitrate.

In absence of the latter, and therefore in presence of compounds which destroy or absorb hydriodic acid—e.g. iodic acid—there results a brown addition product.

The chemical reaction by which the iodine is liberated from this compound is a complicated one, depending on the fact that sulphurous acid acts upon iodic acid, setting iodine free.

The iodic acid test is very delicate, but requires great care, and may be used in the presence of organic matter.