How are chemical methods of determining salinity dependent on the principle of constant proportions?

Salinity can be determined in many different ways including evaporating water and measuring the leftover solids (or salts). The principle of constant proportions is considered when determining salinity because the ratio of dissolved solids in the ocean is constant. Because of the principle of constant proportions the measurement of only one major constituent is necessary to find the salinity of a water sample. Chlorinity is used to determine salinity by using the standard formula:Salinity in ‰ = 1.80655 X Chlorinity in%.