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Examination Of Commercial Alcohol

Specimens of "plain" spirit are frequently required to be tested as regards general purity and suitability for use in pharmacy, perfumery, or other arts. Defects due to imperfect rectification are not often met with in ordinary "patent-still " spirit of good quality; but even with such spirit impurities may have been introduced through careless storage or accidental admixture.


The nature of the general examination will of course depend to some extent upon the particular purpose for which the spirit is required. Usually, however, it will include at least a determination of the alcoholic strength of the sample, and proof of the absence of solid matters, dissolved oily substances, excessive acidity, and methyl alcohol. In the case of " absolute ' alcohol, it may be necessary to prove also that excess of water is not present.

1 "Studies on Fermentation" (Eng. trans., 1879), p. 78.

2 Compt. rend. trav. Lab. Carlsberg, 1911, 10, 99.

The "rectified spirit" of the Pharmacopoeia is required to pass the following tests: -

"Specific gravity 0.8337. Burns with a blue smokeless flame. Leaves no residue on evaporation (absence of non-volatile matter). Remains clear when mixed with water (absence of oily or resinous substances). A little exposed on clean white filter paper leaves no unpleasant smell after the alcohol has evaporated (absence of fusel oil and allied impurities)."

"One hundred c.c., with 2 c.c. of N/10 solution of silver nitrate, exposed for twenty-four hours to bright light and then decanted from the black powder which has formed, undergo no further change when again exposed to light with more N/10 solution of silver nitrate (absence of more than traces of amyl alcohol and of other organic impurities). When mixed with half its volume of an aqueous solution (1 in 5) of sodium hydroxide, the mixture does not immediately darken in colour (absence of more than traces of aldehyde). No immediate darkening in colour is caused by the addition of solution of ammonia (absence of tannin, excess of aldehyde, and other organic impurities)."

The French Pharmacopoeia prescribes the following tests for alcohol: - It should be colourless, and show no acid reaction when diluted with two volumes of water. It should leave no residue on evaporation.

Heated on a water-bath in a porcelain capsule, no foreign odour should be perceptible either during evaporation of the alcohol or subsequently. When diluted with two volumes of water, it should remain clear, and the odour and flavour should be those of ethyl alcohol, without extraneous admixture.

Distil 100 c.c. from a water-bath until 60 to 70 c.c. of distillate are collected. With this distillate and with the residue carry out the two following series of tests.

(1). Distillate (for "foreshot " impurities).

(a). Ten c.c. of the distillate in a test-tube are mixed with 5 c.c. of ammoniacal solution of silver nitrate (10 per cent. of AgNO3) and heated in the water-bath: the mixture should remain clear and colourless. If the alcohol is impure (aldehydes), a brown colour or a precipitate of metallic silver will be given.

(b). In a stoppered flask of 100 c.c. capacity place 50 c.c. of the distillate and 2 c.c. of solution of potassium permanganate (0 2 gram of KMnO4 per litre). Maintain the temperature at 15° to 18°. If the alcohol is pure, the rose-violet tint of the solution should persist during twenty minutes, before passing to a salmon-pink shade. (Absence of acetone, aldehydes, commercial methyl alcohol.)

(2). Residue from distillation (for "taillings" impurities).

(c). In a flat-bottomed flask cooled in a cold water bath, pour 10 c.c. of the residue, and add, little by little, with shaking,. 10 c.c. of pure sulphuric acid, in such manner as to prevent rise of temperature. With pure alcohol, the mixture remains colourless; if impure, it acquires a colour varying from yellow or light rose to granite red, violet, or even dark brown. {Higher alcohols, etc.)

(d). Introduce into a test-glass 2 c.c. of an acid and colourless solution of aniline acetate, made by mixing equal volumes of redistilled aniline and glacial acetic acid. On the surface of this pour carefully about 10 c.c. of the residual liquid from the distillation; the mixture should remain colourless. If a bright red colour is produced at the surface of contact, gradually penetrating into the bulk, furfural is present.

A third series of tests, carried out on the alcohol itself, has for its object the detection of nitrogen compounds.

To 50 c.c. of the sample add 1 or 2 drops of dilute sulphuric acid to give an acid reaction, and then 10 c.c. of distilled water. Evaporate the mixture on the water-bath down to a bulk of 10 or 12 c.c, and apply the following tests: -

(e). Make 5 c.c. of the liquid, in a test-tube, alkaline with a few drops of a 10 per cent. solution of potassium hydroxide, and pour in one or two drops of Nessler's reagent (alkaline solution of mercuric potassium iodide). If ammonia is present a yellow colour, or a reddish-brown precipitate, is produced.

(/). To 5 c.c. of the same residue add 5 c.c. of dilute sulphuric acid, and pour the liquid little by little, with shaking, into 5 c.c. of solution of bismuth-potassium iodide (10 grams of KI dissolved in 80 c.c. of water, and this solution divided into two equal parts. In one part 12 grams of sublimed BiI3 are dissolved, then the remaining part of the KI solution added, and the mixture filtered. Kept protected from the light). Pure alcohol remains clear; if pyridine bases are present an orange-red precipitate forms, which. is crystalline if amyl alcohol is absent.

The ordinary (95 per cent.) alcohol is usually transported in metal drums or other vessels, and often contains sensible traces of the common metals. These are tested for as follows.

To 100 c.c. of the alcohol add 10 c.c. of dilute acetic acid and an equal volume of water; and evaporate on the bath till reduced to 20 c.c. The residue thus obtained should give no precipitate or coloration with hydrogen sulphide or ammonia.

An objectionable, pungent odour which is sometimes present in both crude and refined alcohol appears to be due to an aldehydic constituent, which, according to E. Bauer,1 is probably acrolein. The presence of this body is, in general, attributable to the decomposition of the fermentation-glycerol during distillation. Traces of ammonia may also have a deleterious effect upon the quality of the spirit. A leek-like odour and flavour are traceable to sulphur compounds; these may be present as free hydrogen sulphide and as sulphur in organic combination. As the latter is chiefly responsible for the objectionable odour, it is usually sufficient in analysing the spirit to determine the total sulphur, which may be done in the following manner. Not less than a litre of the alcohol is treated with an excess of bromine-water, then diluted with 300 c.c. of water, the alcohol distilled off, and the residue concentrated to a conveniently small bulk - say about 50 c.c. The sulphuric acid present is then precipitated with barium chloride solution in the usual manner, and the resulting barium sulphate weighed. Sulphur to the extent of 0 005 to 0 007 gram per litre was found by Bauer in crude molasses spirit.

The same author recommends that, in determining the acidity of alcohol, the spirit should first be heated to 63° to expel carbon dioxide. Cochineal and hematoxylin are preferable to phenol-phthalein as indicators; Congo red and rosolic acid are useless.

An opalescence shown by a rectified spirit on dilution with water may be due to the presence of caoutchouc or other substances extracted from rubber, if the alcohol has been in contact with rubber tubing or washers.

Detection of water in "absolute" alcohol. - If the alcohol is shaken with anhydrous copper sulphate, the latter will become blue if the liquid contains water to the extent of about 0 8 per cent.

This test is applied to the "Absolute Alcohol" of the Pharmacopoeia in the following form: "Anhydrous copper sulphate shaken occasionally during two or three hours in a well-closed vessel with about fifty times its weight of absolute alcohol does not assume a decidedly blue colour (absence of excess of water)." The limit of water allowed in the B.P. absolute alcohol is 1 per cent. by weight.

A still more delicate test is to shake the alcohol with a crystal or two of potassium permanganate. This salt is insoluble in absolute alcohol, but dissolves sufficiently to give the liquid a pink tinge if as little as 0.4 or 05 per cent. of water is present.

Calcium carbide may also be used to detect the presence of water in alcohol. Acetylene gas is liberated by the action of the water, and calcium hydroxide formed, rendering the liquid turbid.

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