What Is a Concentration Unit?

M. McGee

A concentration unit is a way of showing the amount of material present within another substance. There are a staggering amount of terms for the different methods of noting a concentration unit, but it will generally fall into one of three different areas. A common marketing term is the use of ‘concentrated’ and ‘diluted’ as a method of showing relative and non-specific concentration. The most common general method is describing the solute in terms of the solvent; basically, ‘five parts per million.’ The last method, molarity and molality, is the most specific and is used heavily in chemical mixing.

Woman holding a book
Woman holding a book

Of the actual specific concentration unit measurements, ‘solute parts per solvent amount’ is generally the most common format. A mixture of two materials is called a solution and the solute is the smaller of the two while the solvent is the larger. This description is typically in parts per million, billion or trillion (ppm/b/t). The measurement doesn’t use actual defined amounts in order to provide scalability. If one person wants a part to be a single drop and another wants it to be full glass, the figures are just scaled up and down to match.

For laymen and marketing purposes, the exact makeup of a material is generally unimportant. When a shopper purchases orange juice, it is more important that he knows whether it is concentrate or premixed rather than the amount of one component versus another. For this reason, the most common general concentration unit is simply ‘concentrated.’ This label is placed on a wide array of products to show the material is separate from the unprocessed versions of its kind. The opposing label, “diluted,’ is less common but it has the same idea.

If an experimenter were to put drops of ink into water, then drops would be the part, the solute would be ink and the water is the solvent. One ppm of ink would go into about 14 gallons (53 liters) of water, or about a quarter of a standard 55-gallon bucket. One ppb would need an entire tanker truck and one ppt would require over 12,000,000 gallons (over 45,000,000 liters) of water.

The last common type of concentration unit uses moles, a base measurement used in chemistry. In many ways, it is like the ppm format — it simply uses moles as a specific amount over the more flexible parts descriptor. In this style, the molarity is the number of moles of solute compared to the entire solution and molality is the number of moles of solute compared to the solvent.

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