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11-10-2011, 07:41 PM   #1
wesleyjo
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 Determination of Alcohol Content of anything

So I recently pulled a 750 ml bottle of a liquor out of the freezer which I know to be approximately 75-90% ethanol. The bottle contained about 750 ml liquor at room temp. Upon taking it out of the cold freezer however, the liquid inside had shrunk to a size of around 730 ml (an estimate).

This got the gears in my head turning. Since water and alcohol obviously have different expansion properties with cold/heat, could you use this to accurately determine alcohol content of just about anything? (Assuming the major substituents were just ethanol and water). This would simply include measuring the density of a liquid at room temperature, then measuring the density at near freezing. (Ice crystals are less dense than water and would skew calculations). The greater the density change, the greater the concentration of ethanol.

Obviously things would become kind of dicey with low alcohol substances (beer) because of the smaller amount of expansion would leave more room for error. Theoretically all you would need for this process is an accurate scale, an accurate way to measure volume, and a freezer.

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11-10-2011, 07:54 PM   #2
goatchze
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Quote:
 Originally Posted by wesleyjo So I recently pulled a 750 ml bottle of a liquor out of the freezer which I know to be approximately 75-90% ethanol. The bottle contained about 750 ml liquor at room temp. Upon taking it out of the cold freezer however, the liquid inside had shrunk to a size of around 730 ml (an estimate). This got the gears in my head turning. Since water and alcohol obviously have different expansion properties with cold/heat, could you use this to accurately determine alcohol content of just about anything? (Assuming the major substituents were just ethanol and water). This would simply include measuring the density of a liquid at room temperature, then measuring the density at near freezing. (Ice crystals are less dense than water and would skew calculations). The greater the density change, the greater the concentration of ethanol. Obviously things would become kind of dicey with low alcohol substances (beer) because of the smaller amount of expansion would leave more room for error. Theoretically all you would need for this process is an accurate scale, an accurate way to measure volume, and a freezer.
If you assume it's a binary mixture of ethanol and water, measuring the density at any temperature will give you the alcohol content.

Trying to get the alcohol content accurately by what you're describing probably isn't very practical. You'd have to assume that the only thing in the solid phase is water (it will contain some ethanol).

The volumes of the liquid changes with temperature as well. For example, the density of a 50/50 mixture, by mass, at 50C is 903.32 kg/m^3. If we drop the temperature down to 0C (still a liquid...solids won't form in this mixture until -7C), the density will increase to 935.37 kg/m^3...an increase of 3.5%.

Lots of uncertainties would lead to fairly inaccurate results. Accurately determining the weight (you'll have condensation on the outside of the container) an especially the volume would be very difficult.
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11-10-2011, 08:22 PM   #3
ajdelange
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As noted, if the mixture contains only water and ethanol it is enough to know the density at any temperature as the density of water ethanol mixtures has been tablulated over a range of tempeatures and concentrations by the OIML and AOAC (and doubtless other metrology bodies). People putting up distilled spirits thus put a sample into an accurate density meter (electronic) which is programmed with the tables and which read concentration directly in whichever units the TTB (AOAC) or other regulatory body demands.

If something like beer or liquers is to be measured the other stuff would render this method terribly inaccurate so the alcohol is separated off by distillation and made up to the same volume (or weight) as the original sample with DI water and the density of that mix measured.

Conceptually your scheme is doable as any mix will have a "warm up curve" unique to its concentration. The measurement would be simple enough to do with an electronic densitometer as these have Peltier heater/cooler junctions attached to the measurement cell. With a balance you would use a pycnomter with incorporated thermometer. You would want to work at two (or more) temperatures all above the dew point in your lab. But as you must, in your scheme, find density as a route to volume, you have already solved the problem. It is sufficient to know density and temperature to know ABV or ABW.

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