Comparatively Speaking: Density vs. Specific Gravity

This edition of "Comparatively Speaking" by industry expert Tony O'Lenick compares a product's density with its specific gravity, which are important measurements for formulators to understand especially when considering product specifications.

Formulators are often presented with measurements listed in product specifications that are not always clear. The terms density and specific gravity are two such measurements. Density is defined as its mass per unit volume and is measured in kg/m3. The symbol most often used for density is ρ (the Greek letter Rho). Table 1 shows some example density measurements. Specific gravity, on the other hand, is the ratio of the density of a substance to the density of a given reference material, most commonly water (see Table 2).

Logically, if a substance's density is < 1, it is considered less dense than the reference. If it has a density > 1, it is considered denser than the reference. If the relative density = 1, the densities are equal; i.e., equal volumes of the two substances have the same mass.

If the reference material is water, then a substance with a relative density (or specific gravity) < 1 will float in water. For example, an ice cube, with a relative density of about 0.91, will float. A substance with a relative density > 1 will sink.

In relation to these measurements, a pycnometer1 (see Figure 1) typically is used to measure a known volume and an exact weight. This apparatus is usually made of glass and has a close-fitting ground glass stopper in it with a capillary tube through it to allow air bubbles to escape.

This apparatus enables a liquid's density to be measured accurately and compared with a working reference fluid, such as water or mercury, using an analytical balance. If the flask is weighed empty, then full of water, and finally filled with a liquid whose specific gravity is being investigated, the specific gravity of that liquid can easily be calculated.

The particle density of a powder, to which the usual method of weighing cannot be applied, can also be determined using a pycnometer. The powder is added to the pycnometer, which is then weighed, giving the weight of the powder sample.


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