Methods of Determining Specific Gravity

Methods of Determining Specific Gravity

A number of experimental methods for determining the specific gravities of solids, liquids, and gases have been devised. A solid is weighed first in air, then while immersed in water; the difference in the two weights, according to Archimede’s principle, is the weight of the water displaced by the volume of the solid. If the solid is less dense than water, some means must be adopted to fully submerge it, e.g., a system of pulleys or a sinker of known mass and volume. The specific gravity of the solid is the ratio of its weight in air to the difference between its weight in air and its weight immersed in water.

Two methods are commonly used for determining the specific gravities of liquids. One method uses the hydrometer, an instrument that gives a specific gravity reading directly. A second method, called the bottle method, uses a “specific-gravity bottle,” i.e., a flask made to hold a known volume of liquid at a specified temperature (usually 20°C). The bottle is weighed, filled with the liquid whose specific gravity is to be found, and weighed again. The difference in weights is divided by the weight of an equal volume of water to give the specific gravity of the liquid. For gases a method essentially the same as the bottle method for liquids is used. Specific gravities of gases are usually converted mathematically to their value at standard temperature and pressure .

Hydrometer , device used to determine directly the specific gravity of a liquid. It usually consists of a thin glass tube closed at both ends, with one end enlarged into a bulb that contains fine lead shot or mercury to cause the instrument to float upright in a liquid. In the glass tube is a scale so calibrated that the reading on it level with the surface of the liquid in which the hydrometer is floating indicates the number of times heavier or lighter the liquid is than water, i.e., the specific gravity of the liquid. The hydrometer is based on Archimedes' principle. The level at which the hydrometer floats depends only on the density of the liquid. Hence this level can be used to measure both the density and the specific gravity, which is proportional to it. Commercial hydrometers are usually calibrated for ordinary room temperature, which is taken to be 20°C (68°F), or for 4°C (39.2°F). Because of the variation in the depth to which the instrument sinks in heavy and in light liquids, one type is made for use in liquids more dense than water and another for use in those less dense than water. The so-called bulb hydrometer consists of a small commercial hydrometer contained in a larger glass tube into which the solution to be tested is drawn by the action of a rubber bulb. It is used to measure the specific gravity of the sulfuric acid solution in automobile batteries. The two Baumé hydrometers (invented by Antoine Baumé), one for specific-gravity determinations in liquids denser than water and the other for liquids less dense than water, are calibrated with the special Baumé scale (also constructed by him).

A hydrometer is an instrument used to measure the specific gravity (or relative density) of liquids; that is, the ratio of the density of the liquid to the density of water.

A hydrometer is usually made of glass and consists of a cylindrical stem and a bulb weighted with mercury or lead shot to make it float upright. The liquid to be tested is poured into a tall jar, and the hydrometer is gently lowered into the liquid until it floats freely. The point at which the surface of the liquid touches the stem of the hydrometer is noted. Hydrometers usually contain a paper scale inside the stem, so that the specific gravity can be read directly.

The operation of the hydrometer is based on the Archimedes principle that a solid suspended in a liquid will be buoyed up by a force equal to the weight of the liquid displaced. Thus, the lower the density of the substance, the lower the hydrometer will sink. (See also Relative density and hydrometers.) Some historians credit Hypatia of Alexandria with the invention of the hydrometer although there is little evidence to support this.

In low density liquids such as kerosene, gasoline, and alcohol, the hydrometer will sink deeper, and in high density liquids such as brine, milk, and acids it will not sink so far. In fact, it is usual to have two separate instruments, one for heavy liquids, on which the mark 1.000 for water is near the top of the stem, and one for light liquids, on which the mark 1.000 is near the bottom. In many industries a set of hydrometers is used — covering specific gravity ranges of 1.0–0.95, 0.95–0.9 etc — to provide more precise measurements.