Just How Hard is Our Water?

Just How Hard is Our Water?


When acidic rainwater comes in contact with minerals like calcium, magnesium, manganese and potassium, it readily dissolves them into the water. The more mineral the water contains, the “harder” the water is said to be. Total hardness of water is defined as the sum of calcium and magnesium ion concentrates and is expressed as calcium carbonate in milligrams per litre.

Hardness in water is neither classified as a Primary or Secondary water contaminant because it does not seem to have any major adverse effect on health, though using hard water may cause the skin to become dry and itchy. Hard water also clogs hair follicles causing unnecessary hair loss. Hard water interferes with the function of soaps and detergents making them less effective in lathering. Ions of calcium and magnesium form insoluble salts when mixed with soap or detergent. Hard water can very well stain plumbing fixtures with a whitish “soap scum”, when the water evaporates leaving behind the mineral precipitates.It is this mineral staining which appears on bathroom walls, clings to hair, clogs skin pores, and make house cleaning much more difficult. Hardness also clogs and obstructs water pipes, drains and faucets.

Application of soap often indicates the hardness of a given sample of water. If proper foam and lather is produced with the application of soap, then the water may be called soft. However, there are more precise methods to determine the hardness of water, viz. by EDTA titration. Here this procedure is applied.


To determine the hardness of water with the help of EDTA titration.

Materials / Equipment Required

  1. You would require some reagents, which you can procure and prepare at your school chemistry laboratory. These are:
    1. EDTA solution (0.01 M): To make this solution, weigh 3.723 g of disodium salt of EDTA. Dissolve it in distilled water and make up the solution to one litre.
    2. Buffer solution: This is prepared by dissolving 17.4 g of ammonium chloride in 142 ml of concentrated ammonia. Distilled water is then added to it to make up a required volume of 250 ml.
    3. Erichrome black-T indicator: You can prepare this by mixing and grinding together 0.40 g of erichrome black-T and 100 g of sodium chloride.
  2. Conical fllask
  3. Pipette
  4. Note book and pen


Take 50 ml of hard water sample in a dry conical flask with the help of a pipette. Add one ml of the buffer solution to the sample in the flask. Add a pinch of Erichrome black-T indicator and pour the standard 0.01 M EDTA solution that you have prepared from the burette into the water sample. Observe how the colour of the solution changes from red to blue. Record in your notebook, the volume of EDTA solution consumed to bring the change in colour.

Calculation of hardness

Let the volume of EDTA solution = v ml.

Now, 1 ml of 0.01M EDTA = 1 mg of CaCO3

So, v ml of 0.01M EDTA = v mg of CaCO3

Total hardness of the water sample =V * 1000mg/l (asCaCO3)

= V*20 mg/l (as CaCO3)

The total hardness of all the water samples should be estimated as per the above equation.

Limit of permissible Total Hardness of water in Indian Standards, expressed in mg per litre of CaCO3 is 1500. If no alternate source is available, this limit is extended up to 3000 mg of CaCO3 per litre.

You have just learnt how to find out the hardness in water! How nice it would be if there was a method to remove or reduce the hardness in water? Did you know that Alum can also help decrease the hardness in water? Try and experiment with alum treatment to bring down the hardness index within permissible limits. Record your findings.

Source: Harness water resources for a better future - Activity Guide for the 13th National Children's Science Congress 

This project can also be adapted to test the following hypothesis: Hardness of water from deep borewells is always greater than the hardness of municipal supply water’