Concentration refers to the amount of one component in a substance relative to the total amount of all components. It can be measured in various ways, including weight by weight, volume by volume, and weight by volume. Some measurements require units, while others do not. The term “proof” is used to refer to the concentration or strength of alcoholic beverages, and it is closely related to percent concentration.
In chemistry, the term “concentration” refers to the amount of one component in a substance, relative to the total amount of all components in that substance. There are several ways to enumerate concentration, the most common of which is a form of “percentage concentration”. For example, on a bag of solid fertilizer the percentages of nitrogen, phosphorus and potassium are usually listed in terms of weight by weight (w/w) or “percent by weight”. The alcohol in wine is listed on a volume-by-volume (v/v) or “percentage by volume” basis. Saline solution – salt dissolved in water – is listed as a percent concentration by weight per volume (w/v).
If the percent concentration is of the w/wov/v variety, there is generally no associated unit of measure. For example, sea salt contains about 98% sodium chloride (NaCl) plus trace amounts of other minerals. Each 10 ounces (oz) (283.5 g) of sea salt therefore contains approximately 9.8 ounces (277.83 g) NaCl. Weight percent needs no units, since sodium chloride in the numerator is in ounces and total weight in the denominator is also in ounces – ounces cancels ounces. If one measures 100 pounds (45.36 kg) of sea salt instead, it contains 98 pounds (44.45 kg) of NaCl and the units disappear again: pounds cancel pounds.
Similarly, a 10% v/v solution of glycerin in water requires no units, whether the volume is a dram (dr) or a liter (l). The units in the numerator are, again, the same as those in the denominator; they cancel each other out. On the other hand, a 15% w/v sugar solution has a unit of weight in the numerator, but a unit of volume in the denominator. It is defined as a percentage, somewhat artificially; a 1% solution contains 1 gram per 100 milliliters (ml) of solution. This form of percent concentration is used in the biological sciences.
Sometimes, as is the case with an active ingredient in a drug, whether it’s a tablet, ointment or syrup, concentration is compared to strength. It is similar for alcoholic beverages, although the concentration or strength of the “spirits” is usually referred to as “proof”. That term refers very closely to percent concentration; in the US, 80 proof is exactly 40% by volume. The term, however, was first defined in the UK as 7/4, not 1/2, of the percent alcohol concentration. This is because the desirability of spirits has been demonstrated by igniting a mixture with gunpowder, which first occurs successfully at a concentration of just over 57 percent by volume, about a 7/4 ratio of the desired alcohol to the ‘waterfall.
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