The Units ppm, ppb, and ppt

Abstract

Dimensionless quantities, which are also sometimes called quantities of dimension one, are generally defined as the ratio of two quantities of the same kind. Examples are refractive index n (defined as the ratio of the speed of light in vacuum to the speed in a medium, n = c 0 /c ); and mole fraction x i – more properly called amount of substance fraction (defined as the ratio of the amount of a particular substance i in a mixture to the total amount of substance x i = n i / Σ j n j ). Thus the value of a dimensionless quantity is simply a number, and it would seem that no unit is required. The International System of units, the SI, includes no units for dimensionless quantities as it is at present defined.