The liquid to be tested is mixed with an equal volume of 50 per cent. sulphuric acid, and introduced into a small flask in which has been placed 3 or 4 grams of powdered potassium dichromate and about 5 c.c. of water. After mixing and allowing to stand a few minutes, the contents of the flask are diluted with an equal volume of water and distilled. Formic acid is produced by the oxidation: -

CH3OH + O2 = H.COOH + H1O.

The distillate is neutralised with sodium carbonate and evaporated to expel any formaldehyde or acetaldehyde that may have been formed during the oxidation. A portion is then tested with ferric chloride solution; if a formate is present this gives a red coloration, destroyed by hydrochloric acid. But as a similar red colour is also given by acetic acid (which will be present if the original liquid contained ethyl alcohol) the result must be confirmed. To do this add solution of silver nitrate to another portion of the evaporated liquid; on warming, a black or brownish-black deposit of reduced silver is produced.

Or a portion of the evaporated liquid may be heated after adding to it a little mercuric chloride solution; a white precipitate of mercurous chloride is given if a formate is present.

For a quantitative method of estimating methyl alcohol by conversion into formic acid, see p. 194.