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Determination of Weight Per Millilitre and Relative Density

Learn how to determine the Weight per Milliliter and Relative Density by using Pycnometer.

Weight per Milliliter (Wt. per ml.)

The weight per milliliter of a liquid is the weight, in g, of 1 ml of a liquid when weighed in the air at 25°, unless otherwise specified.


Weight Per Millilitre and Relative Density TableSelect a thoroughly clean and dry pycnometer. Calibrate the pycnometer by filling it with recently boiled and cooled water at 25° and weighing the contents. Assuming that the weight of 1 ml of water at 25°, when weighed in the air of density 0.0012 g per ml, is 0.997 g, calculate the capacity of the pycnometer (Ordinary deviations in the density of air from the value given do not affect the result of a determination significantly). Adjust the temperature of the substance under examination, to about 20° and fill the pycnometer with it. Adjust the temperature of the filled pycnometer to 25°, remove any excess of the substance and weight. 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.


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.

Ankur Choudhary is India's first professional pharmaceutical blogger, author and founder of, a widely-read pharmaceutical blog since 2008. Sign-up for the free email updates for your daily dose of pharmaceutical tips.
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