Determination of hardness of water can be completed by titration with an EDTA solution and Eriochrome Black-T indicator in a basic environmental condition.
Water Hardness Definition
Water hardness definition relates to the presence of soluble bicarbonates, chlorides, and sulfates of calcium and magnesium in the water. The concentration of the calcium and magnesium ions in the hard water sample is expressed in mg/L.
Water Hardness Scale
Water hardness scale ranges from 0 to 65 mg/L (milligrams per liter) as calcium carbonate is classified as soft; 65 to 125 mg/L as moderately hard; 126 to 185 mg/L as hard; and more than 185 mg/L as very hard.
Water hardness unit
The water hardness unit is generally expressed as the equivalent quantity of calcium carbonate (CaCO3) in milligrams per liter or parts per million.
Theory of the determination of hardness of water
Water hardness can be determined by titration with an EDTA solution. It involves adding small amounts of EDTA (Ethylene diamine tetra acetic acid) solution to a water sample until the sample changes color.
EDTA forms colorless stable complexes in hard water with Ca2+ and Mg2+ ions at pH = 9-10. A buffer solution (NH4Cl + NH4OH) is used to control the value of this pH. EBT (Eriochrome Black-T) indicator is used during this determination.
The hard water sample is treated with buffer solution and EBT indicator which forms unstable, wine-red colored complexes with Ca2+ and Mg2+ present in water.
Apparatus
There are need for some apparatus for the determination of hardness of water which is given below:
1. Burette
2. Pipette
3. Conical flask
4. Bottle etc.
Reagents
There are some reagents needed for the determination of hardness of water which is given below:
1. Standard EDTA titrant (0.01 M)
2. EBT (Eriochrome black-T) indicator and
3. Ammonia buffer solution
Procedure for the determination of hardness of water
First, take a 50 ml water sample into a conical flask. Then add 1ml of standard buffer solution to control the pH at 9-10. Then one spoonful of EBT (Eriochrome black-T) indicator is added to the conical flask. The water sample is then turned into wine-red. Now, fill the burette with standard EDTA solution. Then add the EDTA titrant slowly with continuous stirring until the wine-red color of the sample changes to blue. It’s the end point of this determination. Now, note down the volume of EDTA added during the titration.
Table for the determination of hardness of water
The table for the determination of hardness of water is shown below: (N.B.: All of the following values can be changed during your experiments based on requirements)
| Observation Number | Volume of supplies water sample (ml) | Initial Burette Reading (ml) (Vol. of EDTA) | Final Burette Reading (ml) (Vol. of EDTA) | Volume of Consumed EDTA (ml) | Average Volume of Consumed EDTA (ml) |
| 1. | 50 | 40 | 34.4 | 5.6 | |
| 2. | 50 | 34.4 | 29.1 | 5.3 | 5.4333 |
| 3. | 50 | 29.1 | 23.7 | 5.4 |
Calculation of the determination of hardness of water:
Number of moles of consumed EDTA= Strength of Standard EDTA solution x average volume of consumed EDTA solution
= 0.01 x 5.433 x10-5 mol
Number of moles of consumed EDTA= number of moles of CaCO3
Thus, 1 mole CaCO3= (100 x 5.433 x10-5) g CaCO3
= 5.433 x10-3 g CaCO3
Total hardness of supplied water sample is= (5.433 x10-3 x1000)/(50 x10-3)
= 108.66 mg/L
=108.66 ppm
Result: The hardness of the supplied water sample is 108.66 ppm.
Important questions related to the determination of hardness of water
What is the hardness of water in chemistry?
Water hardness is defined as the presence of calcium (Ca2+) and magnesium (Mg2+) ions of bicarbonate, sulfate, or chloride in the water sample.
What is the unit of water hardness?
The water hardness unit is milligrams per liter or parts per million.
What is the formula for the hardness of water?
The formula for hardness of water is the sum of the molar concentrations of Ca2+ and Mg2+.
What are the types of water hardness?
Water hardness types are two namely permanent hardness and temporary hardness.
What is the permanent hardness of water?
Permanent hardness of water due to the presence of soluble salts of calcium and magnesium that is present in the form of sulfides, nitrates, or chlorides.
How permanent hardness of water can be removed?
Permanent hardness of water is removed by treating it with washing soda and some other methods like the ion exchange method, synthetic resins method, and Calgon’s method.
What is the temporary hardness of water?
Temporary hardness of water is due to the presence of dissolved bicarbonate minerals (calcium bicarbonate and magnesium bicarbonate).
How temporary hardness of water removed by?
Temporary hardness of water is removed by a popular method of boiling. In this method, the sample of water can be refined at its boiling temperature.
Temporary hardness of water can be removed by adding Slaked lime or Lime water Ca(OH)2 to remove this hardness.
What is the formula of temporary hardness?
Temporary hardness formula is Mg (HCO3)2 and Ca(HCO3)2.
What is EDTA’s full form?
The EDTA full form is Ethylenediaminetetraacetic acid which acts a a chelating agent.
What is EDTA molecular weight?
EDTA molecular weight is 292.244 g/mol.
What is EDTA structure?
EDTA structure is given below:
What is the Eriochrome black-T formula?
Eriochrome black-T formula is C20H12N3O7SNa.
