Weight per Milliliter (Wt. Per ml):- The weight per milliliter of a liquid is the weight, in gm., of 1 ml of a liquid when weighed in air at 25°C, unless otherwise specified.

• Method: – Select a thoroughly clean and dry pycnometer. Calibrate the pycnometer by filling it with recently boiled and cooled water at 25°C and weighing the contents. Assuming that the weight of 1 ml of water at 25°C when weighed in air of density 0.0012 g per ml is 0.99602 g, calculate the capacity of the of the pycnometer. Adjust the temperature of the substance being examined, to about 20°C and fill the pycnometer with it. Adjust the temperature of the filled pycnometer to 25°C, remove any excess of the substance and weigh. Subtract the tare weight of the pycnometer from the filled weight of the pycnometer. Determine the weight per milliliter by dividing the weight in air, in g, of the quantity of liquid, which fills the pycnometer at the specified temperature, by the capacity expressed in ml, of the pycnometer at the same temperature.

Relative Density:- The relative density of a substance is the ratio of the mass of a given volume of the substance to the mass of an equal volume of water, both weighed at 25° , unless otherwise specified.

• Method:- Proceed as described under Weight per milliliter. Divide the weight of the substance in the pycnometer by the weight of water contained, both determined at 25°, unless otherwise directed in the individual monograph.

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Reference links:-
http://apps.who.int/phint/pdf/b/Jb.7.1.3.pdf
http://www.drugfuture.com/Pharmacopoeia/BP2010/data/972.html