1 Gram per litre, in 10 per cent. ethyl alcohol.

The alcoholic mixture, purified by extraction with petroleum ether as already described (p. 170), is diluted with water or mixed with ethyl alcohol, as the case may require, until it contains 10 per cent. of total alcohols.

To 5 c.c. of this prepared liquid contained in a wide test-tube are added 2 5 c.c. of the permanganate solution, and then 0 2 c.c. of strong sulphuric acid, and the liquids mixed. When the reaction has proceeded about three minutes, 0.5 c.c. of the oxalic acid solution is added, to dissolve the precipitated manganese oxide On shaking, the liquid becomes clear and nearly colourless. 1.0 C.c. of strong sulphuric acid is now run in and well mixed with the solution, which is finally treated with 5 c.c. of the Schiff's reagent. A violet colour is developed in the course of a few minutes if methyl alcohol was present, unless in mere traces, when twenty or thirty minutes may be required.

A preliminary experiment carried out as described serves to detect the presence of methyl alcohol (if this is not already known) and to give some idea of the quantity. According to the indications thus obtained, another part of the prepared liquid is further diluted, if necessary, with ethyl alcohol of 10 per cent. strength until it contains from 0 001 to 0 004 gram of methyl alcohol in 5 c.c.; and the experiment is repeated, side by side with two or more standards for comparison. These contain 0 001, 0002, 0 003, etc., gram of methyl alcohol in 5 c.c. of 10 per cent. ethyl alcohol. The colours produced are compared in small Nessler-tubes (25 c.c), or in a suitable colorimeter.

With properly sensitive Schiff's reagent, as little as 0 0003 gram of methyl alcohol in the 5 c.c. of liquid taken is readily detected.

The best depths of colour for comparison, however, are those given by the formaldehyde produced in the manner described from quantities of 0 001 to 0 004 gram of methyl alcohol.

It is convenient to keep a standard solution (1 gram per litre) of methyl alcohol in 10 per cent. ethyl alcohol. This is diluted as required with 10 per cent. ethyl alcohol to form the standards for comparison. The strength of the alcoholic mixture for oxidation is chosen at 10 per cent. for convenience, as the distillates ordinarily obtained are of higher alcoholic strength, and can thus be diluted down to the required degree instead of having to be concentrated or mixed with stronger alcohol.

The process has the advantages of (1) being rapidly executed, (2) requiring only a small quantity of material, and (3) being directly applicable to relatively weak distillates. It is especially suitable for the estimation of small quantities of methyl alcohol - say from 0 1 up to 1 per cent. - in ethyl alcohol.

Formaldehyde, of course, must be absent from the unoxidised solution of alcohols, or else its effect must be determined and allowed for. Glycerol must also be absent.