Specific Gravity of Ethanol and Alcohol Solutions

What Is the Specific Gravity of Ethanol?

100% ethanol has a specific gravity of .785 which is lighter than water with a specific gravity of 1.0

 

A 50/50 mixture of water and ethanol (100 proof / 50%) will have the following specific gravity.

(.5L x 1.0) + (.5 L x 0.785) = 0.8925

 

A 75/25 mixture of water and ethanol (50 proof / 25%) will have the following specific gravity

(.75 L x 1.0) + (.25 L x 0.785) = 0.9463

 

As you can see the specific gravity of the mixture is inversely proportional to the alcohol concentration. As the alcohol concentration decreases the specific gravity increases and the hydrometer floats higher in the solution.

 

The alcohol hydrometer is calibrated in two scales % alcohol and proof. (1% alcohol = 2 proof) The manufacturer used the specific gravity of alcohol at various concentrations to calibrate the instrument.

 

How to Calculate Alcohol Concentration from Specific Gravity

To determine the concentration of a 1 liter solution of alcohol and water using specific gravity:

 

1) Measure the specific gravity of the solution

 

Let X = unknown volume of water

Let (1-X) = unknown volume of alcohol

 

Then X + (1-X) = 1 Liter

 

Specific Gravity of water = 1.0

Specific Gravity of ethanol = 0.785

 

(X)(1.0) + (1-X)(0.785) = Sp. G of solution

 

Solve for X

 

Worked Example

Assume the measured specific gravity is 0.9463

 

X (1.0) + (1-X)(0.785) = 0.9463

X + (0.785 - 0.785X) = 0.9463

0.215X + 0.785 = 0.9463

 

Measuring It on the Bench

Specific gravity is read directly with an alcohol hydrometer, and the calculation above is what the instrument's proof scale is doing for you. For spirits work, the thermo-hydrometer with the 0 to 100 percent proof Tralle scale reads temperature and proof together. If your sample is not at 60 °F, correct the reading against the IRS alcohol hydrometer correction table.