Alcohol by Volume Using Distillation/Hydrometry Technique

One of the myriad of lab tests taught at UC Davis was the ABV (Alcohol by Volume) using the bench distillation apparatus followed by hydrometry. The principles of distillation are universal; essentially, you are using the concept that alcohol and water solution behaves differently from either pure alcohol or pure water (called an azeotrope in the language of chemists and distillers) alone. Alcohol and water solution boils and vaporizes much sooner (less than 80°C) than water alone (100°C). This allows the alcohol and water in the wine mixture to “boil off or vaporize,” leaving the residue in the wine flask behind. This residue would have interfered with the subsequent hydrometry test.

The distilling apparatus with cooling ice under the counter

The distillation apparatus up close

If you look closely at the photo on the right, the Graham condenser is the long angled tube with the little spring-shaped glassware in it; the condenser allows the vaporized solution to condense back into a liquid which falls into the little collection vessel on the right. Why does it condense? During the vaporizing and boiling process, ice cold water is pumped through the condenser’s yellow tubing from the ice water basin hidden under the table. There is a small aquarium pump in the ice water. The ice water effects the condensation of the vaporized alcohol and water, and the condensate (also called distillate) drips into the collection beaker on the right. To better understand this concept, think of boiling water on the stove. If the steam from your soup pot hits the colder cupboard above, it will form droplets that drip back onto the stove or into the pot.

After the distillate is collected, the next step is to bring the level of the distillate back to the same volume of wine you started with by adding enough distilled water. For example, if you started with 250 ml of wine, and collected 200 ml of distillate, you would add back 50 ml of distilled water. This works because at that point, all of the alcohol is distilled out along with only some of the water and to create a situation of equal measure, you have to equalize the solutions. And at that point, you no longer have the issue of the other wine components interfering with the delicate hydrometry test.

Then you measure the alcohol by volume with a special instrument called an ABV Hydrometer. The hydrometer is based the principles of density; there are special ones with a narrower range and wider bulb for accuracy. Essentially, you are measuring the density and then using tables to convert to actual ABV, expressed as alcohol v/v or alc/vol. There are also instant calculators that will allow you to input the measured density, and they will calculate the ABV for you. In Canada, the Excise department accepts only certified calibrated instruments for the purpose of density hydrometry testing. We also had to purchase a certified thermometer to determine the temperature of the final distillate. In addition to the ABV tables, there are also “temperature correction tables” to adjust the density reading if the temperature is not exactly 20°C. The final ABV is what will be written on the bottle. Initially, we are required to verify this reading by an independent laboratory, but once we prove that we are able to accurately determine the readings, we will be permitted to conduct the test without verification.

Density and Concentration Calculator for Mixtures of Ethanol and Water at 20°C

Below is a video representation of the information above.