For excise purpose, in India, strength of alcohol is indicated by degree “Over Proof” or “Under proof”.

**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.

Proof spirit has a specific gravity of 091976 at 15.5 °C and contains 57.1% V/V of ethyl alcohol to 100 volume of proof spirit.

57.1 v/v alcohol is considered to 100 volume of Proof Spirit(P.S.).

**100 P.S Alcohol.=57.1 % v/v (Standard)**

If alcohol strength is greater than 57.1 v/v => Over Proof or > 100 P.S

If alcohol strength is less than 57.1 v/v => Under Proof

**How to calculate Proof Strength of Alcohol **

Any percentage volume in volume of alcohol can be converted into proof strength and vice versa by using the following method.

Multiply the percentage strength of alcohol by 1.753 & Deduct 100 from the product.

- If the result is positive it is known as Over proof
- If the result is negative it is known as Under proof.

** Example 1: Find the Proof Strength ( P.S**.**) of 88 % v/v alcohol/ethyl alcohol? **

Solution:

Since **P.S. =1.753 X strength of alcohol in v/v %**

P.S..= 1.753 X 88 -100= 154.264

Example 2: Which of the alcohol is Over Proof ?

a) 59% v/v b) 49 % v/v c) 75.1 % v/v d) None

Ans: a

Solution:

**P.S. =(1.753 X strength of alcohol in v/v %** )**-100**

a) **Proof Strength ( P.S**.**) of** 59% v/v Alcohol:

= (**1.753 X** 59) **-100** = 3.427 (Value in positive means over proof)

b ) **Proof Strength ( P.S**.**) of** 49% v/v Alcohol:

= (**1.753 X** 49) **-100** = -14.14 ( Value in negative means under proof)

c) **Proof Strength ( P.S**.**) of** 57.1% v/v Alcohol:

= (**1.753 X** 57.1) **-100** = 0 (100 proof)

if P.S. > 100 means over proof & if P.S. < 100 means Under proof

So answer is “a”

Example 3: Find the strength of ethyl alcohol of 21^{o} Over Proof & 10^{o} Under Proof?

Solution:

**P.S. =(1.753 X strength of alcohol in v/v %** ) **-100**

So Strength 21^{o} Over Proof (Positive valve) alcohol in v/v is

21 =(1.753x **strength of alcohol in v/v %** ) – 100

21 + 100= **strength of alcohol in v/v %**

**strength of alcohol in v/v %** = 21 + 100= 121 P.S.

So Strength 10^{o} Under Proof (Negative value) alcohol in v/v is

**P.S. =(1.753 X strength of alcohol in v/v %** ) **-100**

-10 =(1.753x **strength of alcohol in v/v %** ) – 100

-10 + 100= **strength of alcohol in v/v %**

**strength of alcohol in v/v %** = -10 + 100= 90 P.S.

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