Measuring the density of spirits to calculate their alcohol content

Alcohol is an organic substance with a hydroxyl group -OH directly connected to a carbon atom. The simplest alcohol is methanol CH₃-OH with one carbon atom only while the component that is mostly referred to is ethanol (ethyl alcohol which contains two carbon atoms) C₂H₅-OH which is also the alcohol found in and used for alcoholic beverages such as wine and spirits. Subsequently, when using the term "alcohol" we refer to ethyl alcohol. This article focuses on measuring the alcohol content of beverages which are distillates (see below for a description). 

Spirits producers measure the alcohol content of their products to ensure that they conform to the label declaration of alcohol content and to establish the basis for the payment of tax.

Measuring the density followed by conversion into alcohol concentration using official alcohol tables is an officially recognized method for alcohol determination in distillates. Accepted instruments for measuring the density for subsequent alcohol determination include pycnometers, hydrometers, and digital density meters.

Alcoholic beverages produced directly by distillation and subsequent dilution with water (no addition of any other components, no storage in wood barrels, etc.) contain only alcohol and water. Typical examples of alcohol-water distillates are: vodka, fruit brandies, and freshly distilled beverages before storage in wood barrels or addition of additives. These liquids are officially considered pure alcohol-water mixtures, although small amounts of aroma substances, other alcohols, etc. can be present.

The alcohol content is calculated from the density values using alcohol/water density tables. For fiscal purposes, traces of volatile byproducts in the distillate are considered to contribute to the "strength" and not affect the calculation.

For quality control (but not acceptable for fiscal purposes) some alcoholic beverages with very low and very constant extract content (e.g. some whisky brands) can also be measured directly using digital density meters, pycnometers, or hydrometers. A correction must be applied to the alcohol concentration to compensate the influence of the extract (obscuration) on the density.

Besides containing alcohol and water, many alcoholic beverages also have various amounts of extract substances, aroma, and color components, etc. All these substances influence the density of a liquid and therefore direct alcohol determination using density measurement is not possible.

Fortunately, the extract components are not volatile, whereas the alcohol is volatile. For this reason, the liquid can be separated into an alcohol fraction and an extract fraction by performing a distillation analysis.

Heating the alcoholic beverage causes the alcohol to evaporate, the vapors are condensed and collected in a separate flask. All the extract remains in the residue of the liquid.

A precisely measured volume (or weight) of the alcoholic beverage is then filled into a suitable distillation apparatus. The distillation is performed and the alcohol fraction is filled up to the original volume (or weight) with distilled water. This fluid now is of the identical alcoholic strength as the original sample, but it does not contain extract any more. Therefore, the alcohol concentration can be determined via density measurement using one of the methods listed below. The distillation analysis is usually performed according to national or international standard regulations.

Depending on the country and sector, alcohol concentration is given in different units:

Alcohol %v/v:

This unit is influenced by temperature changes, therefore temperature should always be quoted together with the alcohol concentration (example: 41.90 %v/v at 20 °C corresponds to 41.82 %v/v at 15 °C). Results obtained using different density/alcohol tables may be slightly different. Therefore it is advisable to mention the alcohol table together with the results.

Alcohol %w/w:

This unit is not influenced by temperature (example: 40.82 %w/w at 20 °C is identical to 40.82 %w/w at 15 °C).

Grams per 100 mL or per L:

The temperature must be quoted together with the results in g/mL (or g/L) as the results are influenced by the temperature.

°Proof:

This unit is found in the US and several other countries. °Proof is the alcohol concentration in %v/v at 60 °F (15.56 °C) multiplied by two. Pure alcohol (100 %) therefore corresponds to an alcohol content of 200 °Proof (US °Proof = ABV * 2).

The unit °Proof in Great Britain (UK) is not as common as it is in the US and it is different from the American °Proof degrees (UK °Proof = ABV * 1.75).

There are a variety of different tables available. Some of them differ considerably in the higher concentration range. These are:

OIML Table (Organisation Internationale de Métrologie Légale): internationally the most frequently used table; it lists density versus %v/v or %w/w alcohol, temperature usually 20 °C.

AOAC Table (Association of Official Analytical Chemists): alcohol concentration %v/v at 60 °F.

IUPAC Table (International Union of Pure and Applied Chemistry): alcohol in %v/v or %w/w at 20 °C.

Syuseido Table (Japanese Alcohol table): alcohol in %v/v or %w/w at 15 °C.

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