DENSIMETRIYA (Latin densus dense + grech, metreo to measure, measure) — measurement of density of liquids and solids. In clinicodiagnostic and a dignity. - a gigabyte. laboratories determination of density biol, liquids both for the purpose of determination actually of density, and for definition of concentration of solutions is widely used.
Density of a body — physical. the size determined by body weight in unit volume: p = m/v, where p — density, m — the body weight, v — volume. According to the International System of Units (SI) density is expressed in kilograms per cubic meter (kg/m 3 ). In laboratory practice use the concept «relative density», edges are represented by the relation of density of any body to density of other body in certain reference conditions. Relative density is usually expressed the relation of density of a body at 20 ° to density a dist, waters at 4 °, to-ruyu it is possible to accept equal 1 kg/m 3 or 1 g/cm 3 .
At measurement of density of liquids the most exact method is measurement of mass of the known volume of liquid by means of the densimeter (see. Density ). For measurement of fluid density find the mass of the empty densimeter, then mass of the densimeter which is consistently filled a dist, water and ispytuyemy liquid. In such a way calculate the capacity of the densimeter and mass of ispytuyemy liquid in volume of the densimeter. Calculation is made on the following formula (on condition of the identical water temperature and ispytuyemy liquid at measurement):
pt = (m 3 - m 1 ) (p-D) / (m 2 - m 1 ) + D
where pt — density of ispytuyemy liquid at a certain temperature of t, p — density a dist, waters, m 1 — the mass of the weights counterbalancing the empty densimeter, m 2 — the mass of the weights counterbalancing the densimeter, the filled dist, with water, m 3 — the mass of the weights counterbalancing the densimeter filled with ispytuyemy liquid, D — density of air.
The methods of gravity test which gained the greatest distribution in laboratory practice (hydrostatic weighing, measurement by means of areometers), are based on Archimedes's law. At measurement of fluid density by hydrostatic weighing the glass float attached to a scale beam is weighed serially in air, a dist, water and in ispytuyemy liquid. By results of weighing determine the volume of a float and fuild mass in this volume, and according to these data count fluid density:
pt = (m 1 - m 3 ) (p-D) / (m 1 - m 2 ) + D
where pt — density of substance at a temperature of t, m 1 — the mass of the weights counterbalancing a float in air, m 2 — the mass of the weights counterbalancing a float in water, m 3 — the mass of the weights counterbalancing a float in ispytuyemy liquid, D — density of air, p — density a dist, waters at a temperature of t.
It is most widespread in practice a wedge, and a dignity. - a gigabyte. laboratories measurement of density of liquids with the help areometers (see) which are used for measurement actually of density (a densimetra of general purpose, galactometers, urometra etc.) and for measurement of concentration of solutions (alcoholometers, sakharometra etc.) since density of solutions with other things being equal depends only on concentration of solutes. For gravity test biol, liquids (urine, blood, cerebrospinal liquid, exudates, milk etc.) use areometers of the small size — the urometra having scale interval of 0,001 g/cm 3 .
Change of density of urine and according to its specific weight can serve as diagnostic test at a number of diseases. Specific weight of urine characterizes concentration ability of kidneys since it gives an idea of concentration of the substances dissolved in urine. When kidneys lose ability to concentrate urine, it is allocated in large numbers with low specific weight.
Methods of gravity test of solids are less exact, than liquids. Practically all these methods come down to immersion of a solid in inert liquid with the known density. Definition by means of volyumometr comes down to calculation of volume of a solid of the known weight on volume forced out by it in a volyumometra of liquid.
At gravity test of a solid by method of free flotation select liquid with a certain density, in a cut the solid is in a condition of neutral equilibrium (does not emerge and does not sink); in this case density of a solid is equal to density of this liquid.
Bibliography: Voskresensky P. I. Technology of laboratory works, M., 1973; Kiwi of l is S. M. Tekhnika of measurement of density of liquids and solids, M., 1959, bibliogr.; Physical methods of organic chemistry, under the editorship of A. Vaysberger, the lane with English, t. 1,> Mv 1950.
L. M. Pimenov.