Water hardness determination

What is hardening?

Generally hardness of water is defined as the measure of capacity of water to precipitate soap i.e., the capacity of the water to form leather or soap. Hard water contains dissolved minerals such as Ca2+, Mg2+, Fe3+, SO42-, etc. The degree of hardness is measured in Parts per Million (ppm) or Grams per Gallon (GPG). Water with different degrees of hardness is used for different purposes such as household and industrial purposes.

Why determining hardness?

It s essential to know the hardness of water since it defines the purpose of it. Hard water is not health hazard but it cannot be used in industrial applications such as boilers, pipes, etc. It has the tendency of forming CaCO3 precipitate which later results in sludge/scale formation.

Determination of Hardness

The hardness of water can be estimated by the methods such as gravimetric method, EDTA titration, atomic absorption etc. In the above methods, EDTA titration is most inexpensive and simple way of determining the hardness. There are two types of hardness: Temporary and Permanent. Temporary hardness is due to the bicarbonate ions(Ca, Mg bicarbonates) being present in water. It can be removed by heating the water:

Ca (HCO3)2 ———> CaCO3 + CO2 + H2O

Permanent hardness is due to chlorides and sulphates of Ca2+, Mg+2, Fe3+and SO4 ions and it can be removed by boiling.

EDTA Titration

Permanent hardness is usually determined by titrating it with a standard solution of ethylene-diamine tetra-acetic acid (EDTA). The EDTA is a complexing, chelating agent used to capture the metal ions. This causes the water to become softened, but the metal ions are not removed from the water. This method includes a series of titrations to determine the total, permanent, temporary, Ca, Mg hardness of the given water sample. First the EDTA solution is standardized by titrating it against a standard CaCl2 solution and its normality is found out. The endpoint is the appearance of a steel blue color.

Reviewed by:
Dr. Muhammad Adnan Asghar (Ph.D.)
Chinese Academy of Sciences Fuzhou, Fujian