Densimetry

a group of methods for measuring the relative density of liquids and solids. The methods that are widely used in laboratory practice are based on Archimedes’ principle: the relative density is evaluated according to the depth of immersion of a hydrometer in the test liquid or from the results of weighing the liquid or solid on a hydrostatic balance.

Relative density can be measured with great accuracy by a pycnometer, first weighing the empty pycnometer, then when it is filled with distilled water, and finally when it is filled with the test liquid. The value of the density is obtained from the ratio of the mass of the test liquid to the mass of water. In industry the relative density is measured by means of various automatic densimeters that are located on processing lines. Their operation is based on the continuous weighing of a certain volume of liquid, on measuring the pressure of a column of liquid having a constant height (hydrostatic densimeters), on registering the variation of the velocity of the propagation of sound as a function of the liquid density (ultrasonic densimeters), and on measuring gamma-ray scattering (radioactive densimeters). There are also densimeters based on other operating principles.

Inasmuch as the relative density is constant for each chemically homogeneous substance and for solutions at a given temperature, it is possible to determine from the value of the density whether there are impurities in a substance as well as the concentration of a solution. It is therefore possible to employ densimetry in research and for production control in various branches of industry.