In this article, we are going to talk about the determination of hardness of water by EDTA titration method.The causes of hardness of water is by multivalent metallic cations, which react with soap to form precipitates and with certain anions present in water to form scale.
The primary cations causing hardness are calcium, magnesium, strontium, ferrous ion, and manganous ions.The common anions that combine with these cations to cause hardness are bicarbonates, sulphates, chloride, nitrate and silicate.
As Ca++ and Mg++ ions are present in significant qualities in water hardness is generally attributed to their presence.
When the hardness is numerically greater than the sum of carbonate and bicarbonate alkalinity the amount of hardness that is equivalent to total alkalinity is called the carbonate hardness and the balance as noncarbonated hardness.
When the total hardness is less than the sum of carbonate and bicarbonate alkalinity, the hardness is due to carbonate hardness only.
Hardness may be present from zero to several hundred mg/L as CaCO3. Sodium in very large quantities may behave like a hardness producing ion due to common ion effect. Hardness due to such causes is called pseudo hardness.
Total hardness of water
The calculation of hardness in water by EDTA titration can be found by adding a small amount of a dye such as Erichrome Black T is added to an aqueous solution containing Ca++ and Mg++ ions at a pH of 10 ± 0.1, Ca++ and Mg++ form chelated complexes of wine red colour with EBT.
But EDTA has a stronger affinity towards Ca++ and Mg++. Hence, if EDTA is added the former complex (Ca-EBT and Mg-EBT)is broken and new complex (Ca-EDTA and Mg-EDTA) of blue colour is formed. When all the ions are complexed the solution will turn blue. This is the endpoint of the titration. The higher the pH, the sharper the endpoint.
However, above pH 10 there is a danger of precipitation of CaCO3 and Mg(OH)2. Hence the pH is fixed to 10 ± 0.1. The sample is diluted with distilled water to reduce the concentration of Ca++ and Mg++ ions.
Apparatus for hardness of water experiment
- Erlenmeyer Flask
Reagents for hardness of water experiment
- Ammonia buffer solution
- Erichrome Black T indicator
- Standard EDTA titrant = 0.01M
Procedure for calculation of hardness of water by EDTA titration
- Take a sample volume of 20ml (V ml).
- Dilute 20ml of the sample in Erlenmeyer flask to 40ml by adding 20ml of distilled water.
- Add 1 mL of ammonia buffer to bring the pH to 10±0.1.
- Add 1 or 2 drops of the indicator solution. If there is Ca or Mg hardness the solution turns wine red.
- Add EDTA titrant to the sample with vigorous shaking till the wine red colour just turns blue.
- Note the volume of titrant added (V1 ml).
The video for the calculation of total hardness of water is given below.