For the determination of small quantities of methyl and ethyl alcohols in aqueous solution, J. Hetper1 proposes a method based upon two oxidations of the liquid with permanganate. One oxidation is carried out with acid permanganate, the other with an alkaline solution of this reagent.

Solutions required: -

(1). Acid N/2-KMnO4: 16 grams of KMnO4 + 40 grams of crystallised phosphoric acid in 1 litre of water.

(2). Alkaline KMnO4: 16 grams KMnO4 + 40 grams NaOH per litre.

These two solutions are standardised against iV/2-oxalic acid.

The liquid to be examined must not contain other substances which reduce permanganate. Previous to the estimation it must be diluted or concentrated until it contains from 0.1 to 0 25 per cent. by weight of total alcohols.

A preliminary experiment is made to determine the alcoholic strength of the liquid. Twenty-five c.c. of the solution (2) are diluted with 50 c.c. of water, heated on a steam-bath, and the alcoholic liquid added in small quantities until the colour of the permanganate is changed to green. The mixture is then heated for a period of twenty minutes, when 25 c.c. of N/2-oxalic acid solution are added, and the excess of oxalic acid titrated with N72 permanganate. Each c.c. of N/2 -permanganate required is equivalent to 0 0025 gram of alcohol. The sample is then diluted or concentrated accordingly.

For the alkaline oxidation, 25 c.c. of the solution (2) are diluted, heated to 40°, and 10 c.c. of the alcoholic liquid added. The mixture is at once heated on the water-bath, and kept at 94° for fifteen to twenty minutes. It is then treated hot with 25 c.c. of N/2 -oxalic acid, and titrated with the solution (1). The

1 Zeitsch. Nahr. Genussm., 1913, 26, 342.

2 quantity of (1) reduced should not be less than 7 c.c. nor more than 12 c.c.

For the acid oxidation, 30 c.c. of the solution (1) are diluted with water, from 10 to 20 c.c. of the alcoholic liquid added, and the mixture heated under a reflux condenser to 94° for two hours. Thirty c.c. of N/2-oxalic acid solution are then added, and the excess titrated with N/2-KMnO4. The volume of permanganate reduced should be from 8 to 14 c.c, and 0 1 c.c. is deducted from the quantity used up.

The calculation is made as follows:

then p =

Example: - 30 c.c. of sample required 9 c.c. of permanganate in the preliminary test.

9 X 0.0025 X 100 / 30 = 0075 gram of total alcohols per

100 c.c, = approximately 0.078 per cent. by weight. Therefore 100 c.c of the sample were distilled to 75 c.c in order to bring the strength within the required limits. In the oxidation: -

Hence

and

Multiplying these values of p, y, and z by 75/100 to correct for the preliminary concentration, we have: total alcohols, 0.0760; methyl alcohol, 00163; and ethyl alcohol, 0.0597 gram per 100 c.c. of the original sample.