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Guide to ethanol purity measurement

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A quick guide to ethanol’s various purity measurement systems

Reading about alcohol can be confusing. There are even multiple synonyms for the word alcohol itself (neutral spirits, ethyl alcohol, rectified alcohol, anhydrous ethanol, etc) though in broad terms alcohol is used to describe products for human consumption and ethanol to describe products for other uses.

But you also quickly notice that various terms are used to describe essentially the same thing: the alcoholic content of a solution. So what’s the difference between percentage, ABV and ABW? And what does it mean for a liquid to be 40 or 200 proof? A quick read through this short guide will uncover all the mysteries of ethanol’s, sometimes baffling, terminology.

What is ABV?

ABV (alcohol by volume or alc/vol) is the percentage of pure alcohol contained in a liquid. Meaning, if a litre bottle of whisky is 40% ABV, it contains 400ml of pure alcohol. More precisely, the ABV is the number of millilitres of pure ethanol present in 100 ml of solution at 20°C, or the mass of the ethanol divided by its density at 20°C.

The solutions with the highest ABV are generally designed for various purposes. They could be denatured to make them unpalatable, as they would be dangerous for human consumption. The most common are rectified spirits (also known as neutral spirits, rectified alcohol or ethyl alcohol), which range from 85% to 97.2% ABV. The purest ethanol is anywhere from 99.5% ABV to just a few parts-per-million water.

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What’s the difference between ABV and ABW?

As the name suggests, ABW (alcohol by weight) is the amount of alcohol in a solution expressed as a percentage of the liquid’s total weight.

Converting ABV to ABW or vice-versa is complicated, as it involves variables such as the relative miscibility of the water and alcohol. However, it is handy to know that at 0% and 100%, ABV = ABW. Otherwise, ABV is always higher than ABW. For low ABV values, ABW is approximately 80% of ABV. So 4.8% ABW is about 6% ABV. The difference is at its greatest at about 60% ABV, when ABW is approximately 47%.

ABW is far less common than ABV, but is used in various parts of the world including, for example, parts of the US and parts of India.

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How is ABV calculated?

As mentioned above, ABV is the percentage of pure alcohol a liquid contains. To calculate a liquid’s ABV, its original gravity (OG) is measured before fermentation and final gravity (FG) measured after fermentation is complete. The following basic formula is then applied:

(OG – FG) x 131.25 = ABV. For example, (1.050 - 1.010) x 131.25 = 5.25% ABV.

So why 131.25? The details are complex, but essentially 131.25 is derived from a chemical formula that has been used by brewers and others for centuries, involving the density of ethanol and number of ethanol grams generated per gram of CO2 released.

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What does proof mean?

Alcohol proof is another way of measuring the amount of ethanol a solution contains. It is generally used for alcoholic beverages designed for human consumption, especially spirits and fortified wines. The higher the proof, the stronger the drink.

The origins of the proof system

The word ‘proof’, in the context of describing the alcoholic content of drinks, originated in 16th century England. It was required because the government had begun taxing different alcoholic drinks above a certain strength. Officials and others needed a handy method to test alcoholic content, so the government came up with a simple ‘burn/no burn’ test: if the liquid contained enough alcohol to burn it was ‘above proof’ for tax purposes and if it didn’t it was ‘under proof’. A liquid with an alcoholic content just sufficient enough to maintain combustion was said to be ‘100 proof’.

This methodology is made complicated by the fact that the point at which a liquid combusts, its so-called flash point at 100 proof, depends not just on its alcoholic content, but also on its temperature. Therefore, 100 proof would be 20% ABW at 36°C and 96% ABW at 13°C. At 24°C, 100 proof would be 50% ABW.

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Use of proof today

In the United States, a liquid that’s 50% ABV is considered 100 proof. So a 40% ABV whisky is 80 proof and a 12% ABV wine is 24 proof. In other words, a liquid’s proof is double its ABV.

In the UK on the other hand, proof is 1.75 times its ABV. So a 16% ABV sherry is 28 proof in the UK and 32 proof in the USA. However, since 1980 in the UK, proof has been replaced by ABV as the universal measure of alcoholic content. Canada had taken the same step back in 1972. But in the US, while you are legally required to specify alcohol content as an ABV percentage, you will still find US proof levels on practically all spirit labels and informally people still use proof more often than ABV to describe the strength of spirits.


High-proof ethanol products

Proof is also used to describe ethanol products with industrial purposes, such as 190 proof ethanol, which might typically be used in solvents, cleaning products or disinfectants.

Pure alcohol — also known as anhydrous ethanol, anhydrous alcohol, absolute alcohol, absolute ethyl alcohol, pure ethyl alcohol or extra neutral anhydrous alcohol — is 199/200 proof. It has various specialist uses, such as cleaning sensitive electronic equipment that cannot come in contact with water.