When the solute is present in trace quantities, the following expression is used:

Parts per million

(1) Gram per million (g/million): This expression refers to the concentration of a solute in grams per one million units of the solution. It represents the mass of the solute present in a given volume of solution. However, it is not commonly used to express trace concentrations as it may result in large values.

(2) Nanogram per cent (ng/%): This expression signifies the concentration of a solute in nanograms per one hundred units of the solution. It represents a very small amount of the solute per unit volume. However, it is not commonly used to express trace concentrations and may not be as widely recognized or standardized as other units.

(3) Microgram per cent (μg/%): This expression indicates the concentration of a solute in micrograms per one hundred units of the solution. It signifies a small quantity of the solute per unit volume. Like nanograms per cent, it is not commonly used for trace concentrations and may not be as widely adopted or standardized.

(4) Parts per million (ppm): This is a widely used unit for expressing trace concentrations. It represents the number of units of solute per one million units of the solution. It is a dimensionless quantity, meaning it does not have any specific units associated with it. For example, if a solute is present in a concentration of 1 ppm, it means there is 1 unit of solute per one million units of the solution.

Parts per million is often used in various fields, including chemistry, environmental science, and industry, to express very low concentrations of substances, such as pollutants, contaminants, or trace elements. It provides a convenient and standardized way to describe the presence of substances in small amounts within a larger solution.