When only a small quantity of a very dilute solution of alcohol is available, an accurate estimation may be made by means of alkaline permanganate. In the following process, described by Barendrecht,1 the alcohol is completely oxidised to carbon dioxide and water, and the quantity calculated from the volume of permanganate used up. Other oxidisable (non-volatile) substances such as sugar or extractive matters may be present; but if the proportion of these is considerable in relation to the alcohol-content, the liquid may be distilled and the determination carried out on the distillate. Ten c.c. of a solution containing about 0.2 per cent, of alcohol will suffice for the experiment.

In a hard glass flask of about 700 c.c. capacity are placed 100 c.c. of potassium permanganate solution (9.75 grams per litre) and 40 c.c. of sodium hydroxide solution (150 grams per litre), and the mixture is heated to boiling. Five c.c. of the alcoholic liquid, diluted if necessary to contain about 0.2 per cent, of alcohol, are then rapidly run in, and boiled for one minute. The flame is then removed. One hundred c.c. of oxalic acid solution (20 grams per litre) are now added, followed by 40 c.c. of sulphuric acid (2 vols. of acid, sp. gr. 184, mixed with 5 vols. of water) and the liquid shaken; after which the excess of oxalic acid is forthwith titrated with permanganate solution of strength 3.182 grams per litre.

Another 5 c.c. of the dilute alcohol solution are evaporated in a hard glass dish on the water-bath to expel the alcohol, and 5 c.c. of a sucrose solution (1 gram in 250 c.c. of water) are added to the residue. The foregoing operations are then repeated on this mixture.

The sucrose added (0 02 gram) corresponds with 28.05 c.c. of the KMnO4 solution used in the titration. If A and B are the volumes in c.c. of permanganate used in the respective titrations, the weight of alcohol present is obtained (in mg.) by multiplying the expression A - (B - 28 05) by the factor 0 384. Alternatively, by using the factor 0483, the volume of the alcohol is given in c.mm.

1 Zeitsch. anal. Chem., 1913, 32, 304.