What is the method of expressing very dilute concentrations in terms of millions?

The method of expressing very dilute concentrations in terms of millions is known as Parts per million (ppm). This unit is commonly used in environmental science, including aquatic weed control, to indicate the concentration of a substance in a solution.

When expressing a concentration in ppm, it represents the number of units of the substance per million units of total solution. For example, if you have one milligram of a substance in one liter of water, that concentration can be expressed as 1 ppm, since one liter of water is roughly equal to one million milligrams.

Using ppm allows for practical communication of very low concentrations that are relevant in contexts like pesticide application in aquatic environments. Understanding concentrations in this way helps ensure safety and effectiveness, especially when dealing with potentially harmful substances.

Other methods of expressing concentrations, such as parts per billion (ppb) or parts per thousand (ppt), are used for different contexts and levels of dilution but do not specifically express concentrations in terms of millions. Concentration percentage is typically used for more concentrated solutions and not for expressing very dilute concentrations in millions. Therefore, parts per million is the appropriate choice for this context.