What instrument measures alcohol content?

Answer 1

Alcohol lowers the specific gravity of a solution which contains it. If the solution is almost pure water and alcohol, you can determine the specific gravity with a hydrometer, and look up the % alcohol on a table,.

Because many Alcoholic Beverages contain other ingredients which alter specific gravity (such a sugars which RAISE the specific gravity) the alcohol must be purified before taking the specific gravity. This requires distillation with some specific pieces of apparatus.

For instance, 100 mL of beverage is measured into a boiler. 50 mL of water is added as a surplus water vehicle. Add a couple of boiling chips. Fit with a condensor cooled with running cold water. Distill off slowly into a (preweighed) 100 mL volumetric flask (fitted with a glass funnel). When you have almost filled the volumetric flask with distillate, remove from the still, adjust the temperature to 4 C, and add sufficient distilled water to equal exactly 100.00 mL. You weigh the filled flask and subtract the weight of the empty flask. Since water at 4 C weighs exactly 100.00 g, the weight of the distillate divided by 100 yields the specific gravity. (Alternatively, pour a little of the 100.00 mL of the distilate into a narrow glass, and drop a hydrometer into it. Read the specific gravity off of the stem. Now go find the table to see what % alcohol it is.

PREVIOUS ANSWER:

Alcohol content is measured by finding the specific gravity of the brew or whatever before and after fermentation. The instrument is called a hydrometer.