Alcohol is an excellent solvent. It is used for many purposes in pharmaceutical practice and in formulation of cosmetic etc. In the United States Pharmacopoeia, all alcohol concentrations are expressed as volume-in-volume, based on the quantity of absolute alcohol present, as determined at 15.56 °C. Proof spirit was official in B.P. 1885. London Proof spirit mean that mixture of ethyl alcohol and water which weighs exactly 12/13^{th} part of an equal volume of distilled water at 51°F. Proof spirit has a sp. gr. of 091976 at 15.5 °C and contains 57.1% V/V of ethyl alcohol to 100 volume of proof spirit.

Proof spirit is therefore an aqueous solution containing 57.1% v/v of absolute alcohol. Any alcohol solution, which contains 57.1% v/v alcohol, is a proof spirit and it is said to be 100° Proof.

Over proof (°0/P) means strength of alcohol above the proof strength.

Under proof (°U/P) means strength of alcohol below proof strength.

Thus 25 (°U/P) mean that 100 volume of such alcohol are equivalent to 100-25 = 75 volume of proof sprit. 35 (°0/P) mean that 100 volume of such alcohol are equivalent to 100 + 35 = 135 volume of proof spirit.

In India the rates of excise duty are prescribed in terms of rupees per litre of proof alcohol.

**Example:**

Convert 90% V/V alcohol into proof spirit.

As 57.1 volume of ethyl alcohol = 100 volume of proof spirit

1 volume of ethyl alcohol = 100/57.1 = 1.7513 volume of proof spirit

90 volume of ethyl alcohol = 90 X 1.7513 volume of proof spirit

Hence Proof strength of 90% v/v alcohol = (90 X 1.7513) - 100 = 57.6°0/P

Similarly Proof strength of 30% v/v alcohol can be calculated as follows.

Proof strength of 30% V/V alcohol = (30 X 1.7513) - 100 = -47.5 or 47.5°U/P

Thus 70 °0/P = 100 + 70/ 1.7513 == 97% v/v of ethyl alcohol, and

70 °U/P = 100 - 70 / 1.7513 = 17.13% v/v of ethyl alcohol.

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