The weight per millilitre (weight per mL) of a liquid is the weight in g of 1 mL of a liquid when weighed in air at 20°C, unless otherwise specified in the monograph.

Note: This method is used, where indicated, in the assays in monographs for Oral solutions and Oral suspensions. The quantity of the oral solution or suspension taken for assay purposes is weighed accurately and the weight per mL determined in order to calculate the content as weight in volume (g per mL).

The weight per millilitre is determined by dividing the weight in air, expressed in g, of the quantity of liquid that fills a pycnometer at the specified temperature by the capacity, expressed in mL, of the pycnometer at the same temperature. The capacity of the pycnometer is ascertained from the weight in air, expressed in g, of the quantity of water required to fill the pycnometer at that temperature. The weight of a litre of water at specified temperatures when weighed against brass weights in air of density 0.0012 g per mL is given in the following table. Ordinary deviations in the density of air from the above value, here taken as the mean, do not affect the result of a determination in the significant figures prescribed for Pharmacopoeial substances.