This instrument is used to determine the concentration of the chemicals e.g. caustic soda, soda ash e.t.c. It is the chemical concentration that determines the quality of the final product. Likewise, the quality of the chemical solution in grams or kilograms depends on the chemical concentration determined by hydrometer which may call for addition of more water or chemical. To have accurate concentration so as to avoid the corrosive effect, the importance of hydrometer cannot be over emphasized. Hydrometer is fragile and such must be handled with care.

In using hydrometer, if the chemical solution level is above the expected number on the scale of the hydrometer, its concentration is said to be high, more water is added in order to achieve the desired concentration. On the other hand if the chemical solution is below the expected number on the scale of the hydrometer, its concentration is said to be low so more chemical is added to give the required concentration.