Density and Specific Gravity

Density and Specific Gravity are related concepts that are often confused. Density is defined as the ratio of the mass (weight) to volume of a substance. It is usually measured in grams per cubic centimeter, grams per milliliter, pounds per cubic foot or pounds per gallon, and is somewhat dependent on temperature and, to a lesser extent, pressure. Specific Gravity, on the other hand, is defined as the weight of a sample, at a given temperature, divided by the weight of the same volume of water, at the same temperature or a different specified temperature. Since Specific Gravity is a ratio, it has no units, but it is dependent upon both the temperature of the substance and the temperature of the water reference.

Density and Specific Gravity are never the same, because Density has units and Specific Gravity is dimensionless, but they are numerically equal when three conditions are met: (1) Density is measured in grams per cubic centimeter, grams per milliliter, or kilograms per liter; (2) Density and Specific Gravity are measured at the same temperature; and (3) the Specific Gravity is referenced to water at 4°C, where its Density is very close to 1 gram per cubic centimeter. The Density of a substance may be calculated by multiplying the Specific Gravity by the Density of water at the reference temperature.

There are also various arbitrary scales used to express Specific Gravity. Among them is the Baumé scale, developed by a French Chemist, Antoine Baumé, around the year 1800. It is used to express the Specific Gravity of liquids and involves two formulas, one for Specific Gravities less than one, and another for Specific Gravities greater than one. These two formulas are:

 

 

degrees Baumé (°Bé)  =   

140


Sp. Gr. at 60°F/60°F

  - 130     (Sp. Gr. < 1)

 

 

 

 

degrees Baumé (°Bé)  =   

145 -  

145


Sp. Gr. at 60°F/60°F

     (Sp. Gr. > 1)

 

Other common arbitrary scales for Specific Gravity are degrees API (American Petroleum Institute), or degrees Brix, a scale used mainly in the sugar industry, where °Brix is equal to the sucrose content (weight percent) of sucrose syrups.

Specific Gravity of liquids is most conveniently measured using a Hydrometer. For accurate results, however, care must be exercised to insure that measurements are made at the correct temperature, i.e., the temperature stated on the Hydrometer at which it was calibrated. Most Hydrometers are calibrated at 60°F/60°F (15.6°C/15.6°C). Hydrometers are commercially available to measure not only Specific Gravity, but concentrations of specific solutions such as salt and alcohol solutions, as well as °Brix, °API, and other Specific Gravity scales. It is important to remember that these Hydrometers are specific to certain types of solutions and do not read the concentrations of other solutions directly.

RICCA CHEMICAL COMPANY uses a pycnometer to accurately measure the Specific Gravity of our Specific Gravity Standards (Group Nos. 2330, 2340, and 7992.110 - 7992.150). These standards are traceable to the National Institute of Standards and Technology (NIST) national standards of mass and temperature. Solutions of different Specific Gravities and/or prepared with different substances can be custom manufactured to your specifications.