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4.9 DETERMINATION OF WEIGHT PER MILLILITRE, SPECIFIC GRAVITY AND RELATIVE DENSITY

4.9 DETERMINATION OF WEIGHT PER MILLILITRE, SPECIFIC GRAVITY AND RELATIVE DENSITY 

Weight per Millilitre​

      The weight per millilitre of a liquid is the weight in g of 1 ml of a liquid when weighed in air at 20º, unless otherwise specified in the monograph.

      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.

Specific Gravity

      Unless otherwise stated in the individual monograph, the specific gravity determination is applicable only to liquids, and, unless otherwise stated, is based on the ratio of the weight of a liquid in air at 25º to that of an equal volume of water at the same temperature. Where a temperature is specified in the individual monograph, the specific gravity is the ratio of the weight of liquid in air at the specified temperature to that of an equal volume of water at the same temperature. When the substance is a solid at 25º, determine the specific gravity of the melted material at the temperature directed in the individual monograph, and refer to water at 25º.

      Unless otherwise stated in the individual monograph, the density is defined as the mass of a unit volume of the substance at 25º, expressed in kilograms per cubic metre or grams per cubic centimetre (1 kg.m–3= 10–3 g.cm–3).

      Unless otherwise directed in the individual monograph, use Method I.

Method I Select a scrupulously clean, dry pycnometer that previously has been calibrated by determining its weight and the weight of recently boiled water contained in it at 25º. Adjust the temperature of the liquid to about 20º, and fill the pycnometer with it. Adjust the temperature of the filled pycnometer to 25º, remove any excess of the liquid, and weigh. When the monograph specifies a temperature different from 25º, filled pycnometers must be brought to the temperature of the balance before they are weighed. Subtract the tare weight from the filled weight.

      The specific gravity of the liquid is the quotient obtained by dividing the weight of the liquid contained in the pycnometer by the weight of water contained in it, both determined at 25º unless otherwise directed in the individual monograph.

Method II The procedure includes the use of the Oscillating transducer density meter. The apparatus consists of the following:

      ---- a U-shaped tube, usually of borosilicate glass, which contains the liquid to be examined;

      ---- a magneto-electrical or piezo-electrical excitation system that causes the tube to oscillate as a cantilever oscillator at a characteristic frequency depending on the density of the liquid to be examined;

      ---- a means of measuring the oscillation period (T), which may be converted by the apparatus to give a direct reading of density or used to calculate density by using the constants A and B described below; and

      ---- a means to measure and/or control the temperature of the oscillating transducer containing the liquid to be tested. The oscillation period is a function of the spring constant (c) and the mass of the system:

where ρ is the density of the liquid to be tested, M is the mass of the tube, and V is the volume of the filled tube.

      Introduction of two constants, A = C/(4π2 × V) and B = M/V, leads to the classical equation for the oscillating transducer:

ρ = A × T2 – B

      The specific gravity of the liquid is given by the expression: 

ρLW,

where ρL and ρW are the densities of the liquid and water, respectively, both determined at 25º, unless otherwise directed in the individual monograph.

      CALIBRATION The constants A and B are determined by operating the instrument with the U-tube filled with two different samples of known density (e.g., degassed water and air). Perform the control measurements daily, using degassed water: the results displayed for the control measurement using degassed water do not deviate from the reference value (ρ25 = 0.997043 g.cm–3) by more than its specified error. Precision is a function of the repeatability and stability of the oscillator frequency. Density meters are able to achieve measurements with an error on the order of 1 × 10–3 g.cm–3 to 1 × 10–5 g.cm–3 and a repeatability of 1 × 10–4 g.cm–3 to 1 × 10–6 g.cm–3. For example, an instrument specified to ±1 × 10–4 g.cm–3 must display 0.9970±0.0001 g.cm−3 in order to be suitable for further measurement, otherwise a readjustment is necessary. Calibration with certified reference materials should be carried out regularly.

      PROCEDURE Using the manufacturer’s instructions, perform the measurements using the same procedure as for Calibration. If necessary, equilibrate the liquid to be examined at 25º before introduction into the tube to avoid the formation of bubbles and to reduce the time required for measurement. Factors affecting accuracy include the following:

      ---- temperature uniformity throughout the tube,

      ---- nonlinearity over a range of density,

      ---- parasitic resonant effects, and

      ---- viscosity, if the oscillating transducer density meters used do not provide automatic compensation of sample viscosity influence.

Relative Density

      The relative density   of a substance is tha radio of the mass of a certain volume of a substance at temperature t the mass of an equal volume of water at temperature t2. Unless otherwise indicated, the relative density  is used. Relative density is also commonly expressed as 

      Density ρ20, defined as the mass of a unit volume of the substance at 20º, may also be used, expressed in kilograms per cubic metre or grams per cubic centimetre (1 kg.m–3 = 10–3 g.cm–3). These quantities are related by the following equations where density is expressed in grams per cubic centimetre:

      Relative density or density is measured with the precision to the number of decimals prescribed in the monograph using a density bottle (solids or liquids), a hydrostatic balance (solids), a hydrometer (liquids) or a digital density meter with an oscillating transducer (liquids and gases). When the determination is made by weighing, the buoyancy of air is disregarded, which may introduce an error of 1 unit in the third decimal place. When using a density meter, the buoyancy of air has no influence.

      Proceed as directed under “Specific Gravity, Method II.” 

APPENDICES • 4.9 DETERMINATION OF WEIGHT PER MILLILITRE, SPECIFIC GRAVITY AND RELATIVE DENSITY
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หมายเหตุ / Note : TP II 2011 PAGE 427-428