In a broad, general sense, the term "alcoholometry" signifies the determination of the proportion of alcohol contained in a liquid, whatever the nature of the liquid and whatever the method used. It is usually, however, employed in a somewhat narrower sense; thus chemical processes, such as the determination of alcohol by oxidation to acetic acid and finding the amount of this acid produced, would not generally be considered as coming under the head of "alcoholometry." Neither would such operations as the preliminary extraction, purification, and distillation, which many liquids must undergo in the determination of their alcoholic content, be regarded as operations of alcoholometry in the narrower and more usual sense. " The determination by physical methods of the proportion of alcohol in mixtures of alcohol and water " indicates fairly well what the term generally connotes.

The method almost invariably employed for the purpose is to determine the specific gravity of the mixture. This is done either directly, by means of a specific gravity bottle, a Sprengel tube, or other form of pyknometer; or else indirectly with a hydrometer or some form of hydrostatic balance. Next in importance to these means is the refractometer, which in certain cases can be usefully employed for estimating alcohol. Other physical methods are sometimes made use of, such as those depending, for example, upon the temperature of ebullition of the mixture, or on its vapour pressure; for these see Chapter VI (The Analytical Chemistry Of Methyl And Ethyl Alcohols).

Specific Gravity

In the laboratory the specific gravity bottle is generally the most convenient instrument to use for ordinary miscellaneous work; but if a large number of routine determinations have to be made, it is a common practice to use a hydrometer. A particular form of the latter instrument is alone used by revenue officers in assaying spirits for fiscal purposes, as is described later on (see "Hydrometry").

The specific gravity bottle is made in various sizes, 50 c.c, 100 c.c, or 1,000 grains being the most generally convenient. The form mostly used is fitted with a perforated stopper, which when pushed home allows the excess of liquid to escape through the perforation; and the bottles are carefully adjusted to hold the required weight of water at the proper temperature when the exuded excess of liquid has been removed.

In this country the temperature 60° F. (= 1556° C.) is the one usually adopted as the standard temperature in alcoholometry, and it is the practice in trade circles to refer specific gravities of alcoholic liquids to water at this temperature taken as 1,000. Thus beer of sp. gr. 1055 (water = 1) is said to have the sp. gr. 1,055; and dilute alcohol of sp. gr. 0.9842 is referred to as of sp. gr. 984.2. This has the practical advantage of making the chief figures integers, and reducing the decimals to one, or at most two.

For this reason also, in dealing with spirituous liquors for trade or fiscal purposes in this country, if many samples are examined it is convenient to use a 1,000-grain specific gravity bottle and take the weights in grains. This gives the sp. gr. without calculation. It also economises time in distillation, compared with what is required by the use of a 100 c.c. bottle, since in the former case it is only necessary to distil 75 c.c, whilst at least 110 c.c. must be distilled if the larger bottle is used. For occasional requirements, however, an ordinary 100 c.c. or 50 c.c. bottle with gram weights is perfectly satisfactory.

In using the bottle form of pyknometer, the liquid to be tested is first brought as nearly as practicable to the standard temperature, and the bottle filled nearly to the neck with it. A sensitive thermometer is then inserted, and the liquid brought exactly to the standard temperature by placing the bottle in an ice-bath, or by warming with the hand, as may be required; the liquid being stirred frequently with the thermometer. The latter is then removed, the bottle filled up to the top, the stopper inserted, excess of liquid at once wiped off the top of the stopper, the bottle carefully dried externally with the balance-cloth, and weighed.

The specific gravity having thus been determined, the corresponding percentage of alcohol is obtained by reference to a suitable table. The one appended (p. 237) is abridged from official tables published for the Board of Customs and Excise in 1912, and is reproduced here by permission of the Controller of H.M. Stationery Office. It is based upon a careful revision of the results obtained by Mendeleeff, the Kaiserliche Normal Eichungs Kommission, Blagden and Gilpin, and Drinkwater; it is believed to be one of the most accurate and trustworthy alcohol tables in existence. The revision was carried out at the Government Laboratory, London, under the auspices of Sir Edward Thorpe, who was at that time the principal chemist of the laboratory.

Fig. 32. - ordinary pyknometer.

With perforated stopper, as used for alcohol determinations.