Scientists in fields ranging from oceanography to soil science use the relationship between specific gravity and salinity to characterise solutions. Specific gravity represents the ratio between the density of a sample to the density of a reference solution. The reference solution is normally water at 4 degrees Celsius, which exhibits a density of 1.000 grams per millilitre. Salinity refers to the amount of dissolved salts in water. It is most commonly stated as the grams of dissolved salts per kilogram of solution and is expressed in units of parts per thousand, or ppt. Normally, scientists measure specific gravity and then use this value to estimate the salinity of the solution because specific gravity is easily determined with a hydrometer. The conversion tables, however, work in both directions, and thus measured salinity can be converted back to specific gravity.
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Things you need
- Salinity and temperature data
- Temperature-Salinity-Specific Gravity conversion table
Measure the temperature of the solution being measured. If you are performing the measurement, then this simply involves immersing a thermometer in the test solution and noting the temperature. If the measurement has been performed by someone else, then they should have noted the temperature at which it was taken. In the absence of such information, the conversion of salinity to specific gravity becomes impossible.
Locate a temperature-salinity-specific gravity table that include data for the temperature and salinity values being converted. Such tables are readily available on the Internet. See Resources for an example.
Convert the salinity value to specific gravity by first locating the data column corresponding to the temperature determined in Step 1. In the appropriate temperature column, locate the salinity value --- usually stated in ppt --- corresponding to the value to be converted. Then locate the specific gravity value --- usually listed in the left-hand column --- that corresponds to that salinity. As an example, using the conversion table given in the Resources, a salinity of 20.0 ppt at 22.0 degrees Celsius would correspond to a specific gravity of about 1.0140.
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