Relative density is the ratio of the density (mass of a unit volume) of a substance to the density of a given reference material (i.e., water). It is usually measured at room temperature (20 Celcius degrees) and standard atmosphere (101.325kPa). It is unitless. You can often find it in the section 9 of a safety data sheet (SDS).
Regulatory Implications of Relative Density
Relative density is often used to calculate the volume or weight of samples needed for preparing a solution with a specified concentration. It also helps us understand the environmental distribution of insoluble substances (i.e, oil spill) in aquatic eco-system (on water surface or bottom sediment) if the substance is released to water.
Relative density test is not required for every chemical. Under REACH, the study does not need to be conducted if:
· the substance is only stable in solution in a particular solvent and the solution density is similar to that of the solvent. In such cases, an indication of whether the solution density is higher or lower than the solvent density is sufficient, or
· the substance is a gas. In this case, an estimation based on calculation shall be made from its molecular weight and the Ideal Gas Laws.
