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 21o Over Proof & 10o Under Proof?
Solution:
P.S. =(1.753 X strength of alcohol in v/v % ) -100
So Strength 21o 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 10o 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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Great breakdown of alcohol strength and proof! I never knew how much the calculations varied depending on the spirit. This really clears up a lot of confusion for me—thanks for making it so easy to understand!