To determine the Total Hardness of Water, EDTA is the easiest method. Reason, EDTA reacts with all metal without alkali metal and the proportion is 1:1. The main cause of water hardness is dissolved salts of calcium and magnesium. Moreover the other ions like Strontium, Iron, Barium and Manganese also contribute of water hardness. Traditional it is measured by the amount of soap that is required to produce leather. The EDTA method is more accurate and more rapid.
Mn+ + Na2H2EDTA → [M-EDTA](4-n)- + 2Na+ + 2H+
There are two types of hardness: (1) temporary hardness and (2) permanent hardness.
The temporary hardness is due to the presence of Ca(HCO3) or Mg(HCO3) or both. On the other hand, permanent hardness is due to the presence of sulphates, chlorides and nitrates of calcium and magnesium (as like CaCl2, CaSO4, MgCl2 and MgSO4) in water.
To determine the Total Hardness of Water with EDTA method initially an inorganic acid is added to convert temporary hardness into permanent hardness.
Ca(HCO3)2 + 2HCl → CaCl2 + H2O + 2CO2
At pH 10, EDTA forms colorless, water soluble stable complexes with calcium and magnesium ions. When the indicator Erichrome black T dye is added into the hard water, then the indicator forms unstable complex with calcium and magnesium ions and the solution turn into wine red. If there is no hardness the color becomes blue which is original color of indicator. Now, when this solution is titrated against EDTA, then the calcium and magnesium ions started to form a stable metal-EDTA complex. After all the free calcium and magnesium ions are consumed, the EBT is replaced by EDTA from the unstable complex and liberates the free Eriochrome Black-T. Then the water color change from wine red to blue that indicates the end point.
Apparatus:
- Conical Flask
- Funnel
- Burette
- Sand
- Beaker
- Pipette
- Graduated cylinder
- Hot plate stirrer
- Wash bottle
- Spatula
Chemicals:
- Buffer solution
- Inhibitor
- Eriochrome black T indicator (blue color)
- NaOH
- Standard EDTA Solution 0.01M
- Magnesium sulphate
Procedure:
- Take 50ml of water sample into a conical flask.
- Then add 2-3ml 1:1 HCl and boil for 2 min.
- Cool the solution and neutralized with the dil NaOH.
- Add 1-5ml NH4OH/NH4Cl buffer solution. The pH should be 10. Check the pH with standardize pH meter. (you can also add an Inhibitor, after the buffer solution)
- Add 2-3 drops 0.1M Mg-EDTA solution and 3-4 drops Eriochrome Black T indicator. Then, shake well and the color becomes wine red.
- Fill the burette with standardized 0.01M EDTA solution. Record the initial burette reading and titrate the water sample with this standard solution.
- At the end point the color of the solution turns into blue from wine red. Titrate carefully near the endpoint.
- Take the final burette reading. Let, it is V1 ml.
- You can run a blank titration for more accurate result. Let, it is V2 ml.
Total hardness calculation:
| Serial No | Volume of water sample (ml) | Burette readings (ml) | Volume of EDTA solution (ml) | |
|---|---|---|---|---|
| Initial | Final | |||
| 1 | ||||
| 2 | ||||
| 3 | ||||
In case of blank titration, the calculate volume of EDTA required by sample water, V = (V1-V2)ml
The total hardness (temporary + permanent) can be calculated by using the following formula.
1ml 0.01M EDTA ≡ 0.001001g CaCO3
∴ Vml 0.01M EDTA = V ᵡ 0.001001g CaCO3
Now, 50ml of water sample ≡ V ᵡ 1.001mg CaCO3
∴ 1000ml of water sample ≡ V ᵡ 1.001mg ᵡ 1000/50 CaCO3
≡ V ᵡ 1.001 ᵡ 20 ppm CaCO3