Parts per million

Parts per million (ppm) is a unit of measurement used in many areas of science and engineering to express the level of impurity in a substance or the concentration of a substance in a mixture. It is a dimensionless quantity, meaning it does not have a specific unit of measurement. Instead, it represents a ratio of the part of the substance of interest to one million parts of the whole.

Definition

Parts per million is defined as the number of units of a substance (the "part") per one million units of the whole mixture. It is often used to express very small concentrations of a substance. For example, a concentration of 1 ppm means there is one unit of the substance for every one million units of the whole.

Usage

Parts per million is used in a variety of scientific and engineering fields, including chemistry, physics, environmental science, and engineering. It is commonly used to express the concentration of pollutants in air, water, and soil. For example, the United States Environmental Protection Agency (EPA) often reports the levels of pollutants in the environment in ppm.

In chemistry, ppm is used to express the concentration of a solute in a solution. This is particularly useful when the concentration is very small. In this context, 1 ppm is equivalent to 1 milligram of solute per liter of solution (mg/L).

In medicine, ppm is used to express the concentration of a drug or other substance in the body. This can be important for understanding the effects of a drug, monitoring its levels in the body, or diagnosing disease.

Calculation

The calculation of parts per million depends on the nature of the substance and the mixture. In general, it is calculated as the mass of the substance divided by the total mass of the mixture, multiplied by one million:

ppm = (mass of substance / total mass of mixture) x 1,000,000

For solutions, the calculation is often based on volume rather than mass. In this case, ppm is calculated as the volume of the solute divided by the total volume of the solution, multiplied by one million:

ppm = (volume of solute / total volume of solution) x 1,000,000

See also

• Parts per billion
• Parts per trillion
• Concentration
• Dimensionless quantity

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