This section is from the book "Alcohol, Its Production, Properties, Chemistry, And Industrial Applications", by Charles Simmonds. Also available from Amazon: Alcohol: Its Production, Properties, Chemistry, And Industrial Applications.
The ordinary methods of estimating ethyl alcohol when mixed with water only are dealt with in the-chapter on Alcoholometry. In the following pages various special processes are given, including methods for the determination of ethyl alcohol in mixture with ether, chloroform, acetone, and other substances.
A series of six mixtures of dilute alcohol is prepared, containing respectively 0.2, 0.15, 0.1, 0.066, 0.05, and 0.033 per cent. of alcohol by volume. Of each of these mixtures, 5 c.c. are placed in test-tubes, 2 to 3 c.c. of strong sulphuric acid added, and then a solution of potassium dichromate (2 0 grams per 100 c.c.) is run in from a burette. Into the strongest alcoholic solution (0.2 per cent.) 2.0 c.c. of the dichromate are run, into the next 1.5 c.c, into the third 10 c.c, and so on. The dichromate is reduced, and the colour of the solution in each tube, after mixing its contents, becomes yellowish-green.
Six other tubes are prepared similarly, but adding now 0 1 c.c. of dichromate less than before in each case - i.e., 1.9 c.c, 1.4 c.c, 0.9 c.c, and so on. These twelve tubes serve as standards.
The dilute solution of alcohol to be estimated is now treated in the same way, namely, to 5 c.c of the alcohol sulphuric acid and dichromate solution are added. If 2 c.c. of the latter make the liquid yellow, the dichromate is in excess, and a smaller quantity is used in a repetition until the yellowish-green tint is obtained, after which the tube is compared with the standard most nearly approaching it in depth of colour. If the 2 c.c. of dichromate make the liquid green, the alcohol is in excess, and must be diluted accordingly before repeating the experiment.
Pozzi-Escot1 found with Nicloux's method that it was essential to maintain absolutely identical conditions in the comparative experiments, and even then somewhat discordant results might be obtained in duplicate experiments. It is obvious that no substances other than alcohol which would reduce dichromate may be present.
Bordas and Raczkowski shortened the foregoing procedure by omitting the standards. They added 2.5 c.c. of strong sulphuric
1 Ann. Chim. Anal., 1902, 7, 11.
P acid to 5 c.c. of the liquid to be tested, then 0.1 c.c. of the dichromate solution, and boiled the mixture. From what precedes, 1 c.c. of dichromate indicates 0.l per cent. of alcohol when the transition-tint is obtained, and the experiment is repeated with more or with less of the dichromate, or with the sample further diluted, until the proper colour is given.
Blank and Finkenbeiner describe a modification of the dichromate oxidation process, which they apply to the estimation of small quantities of methyl alcohol, but which can equally be used for determining ethyl alcohol.1 A very dilute aqueous solution of the alcohol is prepared, and mixed with an excess of decinormal dichromate acidified with sulphuric acid. After standing for some hours at the ordinary temperature, potassium iodide is added, and the mixture titrated back with decinormal thiosulphate solution. Each c.c. of dichromate used up represents 0 004 gram of methyl alcohol or 0.00575 gram of ethyl alcohol, as calculated from the equation: -
CH3 CH1OH + O2 = CH3.COOH + H1O for ethyl and the corresponding equation for methyl alcohol.
"Aeration" process. - Dox and Lamb2 describe an "aeration" method of expelling alcohol from fermentation mixtures, absorbing it in sulphuric acid, and determining its amount by oxidation with dichromate. The acetic acid produced is distilled off and titrated.
The alcoholic liquid is saturated with neutral ammonium sulphate (80 grams in 100 c.c), and is contained in a gas-washing bottle. This is connected at one end with a guard-flask of oxidising solution, and at the other with two cylinders containing concentrated sulphuric acid - the first about 18 c.c. and the second 8 to 10 c.c. Air is drawn through the apparatus at the rate of about 25 litres per hour; this removes the alcohol from the solution, and the vapour of the alcohol is absorbed by the acid. The aeration is generally complete in eight to ten hours, but slightly higher results are obtained by aeration at a slower rate for twenty-four hours.
After the aeration is finished, the sulphuric acid is mixed with 10 to 15 grams of potassium dichromate in water in a distilling flask, and the cylinders well rinsed with water (free from carbon dioxide). The mixture is allowed to stand for about fifteen minutes, and then distilled carefully over a free flame, using asbestos board with a small circular opening for the bottom of the flask. To prevent bumping and foaming, a glass bead and two small pieces of pumice stone are placed in the liquid. The volume of liquid in the flask should not become so low as to allow sulphuric acid to distil over. The distillate is titrated with decinormal alkali, and the distillation repeated with additions of water freed from carbon dioxide until the last titration shows less than 0.5 c.c. of decinormal alkali; this is generally reached after four or five distillations Each c.c. of alkali = 0.004606 gram of alcohol.
1 J. Soc. Chem. Ind., 1906, 25, 500. 2 J. Amur. Chem. Soc, 1916, 38, 2565.
Amounts of alcohol up to 2 grams can be determined accurately by this method, according to the authors' statements. In the presence of 0.5 gram or more, the sulphuric acid and dichromate solutions must be mixed carefully and cooled. The process has been used for the determination of alcohol in various kinds of silage, with very satisfactory results. The oxidising solution in the guard-flask is composed of dichromate, sulphuric acid, and water in the proportions of 1: 2: 7 by weight.