Equipment:
- Pipette and filler
- Burette
- Conical Flask
- Edta solution
- pH buffer 10
- Deionised water
- Hard water sample
- Eriochrome Black T Indicator
- Retort stand and clamp
Experiment:
- Rinse pipette, burette and flask with deionised water. Rinse pipette with hard water and burette with edta solution.
- Fill burette to 0 mark with edta, making sure below tap is filled and bottom of meniscus lies on the 0 mark.
- Pipette 50 cm³ hard water into conical flask with 2-3 cm³ pH buffer 10.
- Add minimal amount of solid indicator to the flask until a wine red colour is achieved. Do not use excess indicator.
- Carry out one rough and two accurate titrations. End point at colour change from wine red to blue.
- Calculate total hardness.
Calculating Total Hardness
Lets take concentration of edta as 0.01 M and the titre as 22.15 cm³
50 x M = 22.15 x 0.01
1 1
M = 0.00443 moles/litre Mg+ and Ca+
Total Hardness = 0.00443 x 100g/l CaCO3
= 0.443g/l CaCO3
= 0.443 x 1000 p.p.m
= 443 p.p.m. CaCO3
Possible Questions
1. Why is pH buffer 10 used?
The experiment requires a pH > 9 to occur
2. Why is it important to avoid using excess indicator?
Excess indicator will make it very difficult to detect an accurate endpoint
3. Why is total hardness expressed as CaCO3?
This is for convenience. Total hardness is actually a measure of Ca+ ions and Mg+ ions but the edta solution does not allow for differentiation between these ions.
