Hardness of Water
Hardness of Water by EDTA Titration
Introduction
Water hardness is caused by natural minerals that are dissolved into the water, predominantly calcium and magnesium, and is measured as the equivalent concentration of calcium carbonate (in milligrams per litre).
Water hardness is usually noticed because of difficulty in lathering soap and the formation of a scum in the bathtub. Ca2+ and Mg2+ form insoluble salts with soaps causing precipitation of the soap scum. Another effect of hard water is “boiler scale”. When hard water comes into contact with dissolved carbonates, a precipitate of insoluble calcium carbonate forms. This “scale” can build up on the inside of water pipes to such a degree that the pipes become almost completely blocked. Water hardness can be readily determined by titration with the chelating agent EDTA.
Equipment
50% w/v NaOH solution (50 g in 100 cm3 solution)
Eriochrome Black T indicator pH 10 NH3 - NH4Cl buffer
Hydroxynaphthol blue indicator
0.01 M EDTA (disodium salt)
Safety Equipment
Enclosed footwear
Laboratory coat
Safety glasses
Rubber gloves
Method
Part A: Determination of total hardness
1. Pipette 50 cm3 mineral water into a conical flask.
2. Add 2 cm3 buffer solution followed by 3 drops of Eriochrome Black T indicator solution.
3. Titrate with 0.01 M EDTA until the solution turns from wine red to sky blue with no hint of red (save the solution for colour comparison).
4. Repeat the titration to obtain two concordant results.
Part B: Determination of concentration of Ca2+(aq) ions
1. Pipette 50 cm3 of mineral water into a conical flask.
2. Add 30 drops of 50% w/v NaOH solution, swirl the solution and wait for a couple of minutes to completely precipitate the magnesium ions as Mg(OH)2(s)
3. Add a pinch of hydroxynaphthol blue (exact amount to be decided by the intensity of the resulting coloured solution) and titrate with 0.01 M EDTA until changes to sky blue (save the solution for colour comparison).
4.
Introduction
Water hardness is caused by natural minerals that are dissolved into the water, predominantly calcium and magnesium, and is measured as the equivalent concentration of calcium carbonate (in milligrams per litre).
Water hardness is usually noticed because of difficulty in lathering soap and the formation of a scum in the bathtub. Ca2+ and Mg2+ form insoluble salts with soaps causing precipitation of the soap scum. Another effect of hard water is “boiler scale”. When hard water comes into contact with dissolved carbonates, a precipitate of insoluble calcium carbonate forms. This “scale” can build up on the inside of water pipes to such a degree that the pipes become almost completely blocked. Water hardness can be readily determined by titration with the chelating agent EDTA.
Equipment
50% w/v NaOH solution (50 g in 100 cm3 solution)
Eriochrome Black T indicator pH 10 NH3 - NH4Cl buffer
Hydroxynaphthol blue indicator
0.01 M EDTA (disodium salt)
Safety Equipment
Enclosed footwear
Laboratory coat
Safety glasses
Rubber gloves
Method
Part A: Determination of total hardness
1. Pipette 50 cm3 mineral water into a conical flask.
2. Add 2 cm3 buffer solution followed by 3 drops of Eriochrome Black T indicator solution.
3. Titrate with 0.01 M EDTA until the solution turns from wine red to sky blue with no hint of red (save the solution for colour comparison).
4. Repeat the titration to obtain two concordant results.
Part B: Determination of concentration of Ca2+(aq) ions
1. Pipette 50 cm3 of mineral water into a conical flask.
2. Add 30 drops of 50% w/v NaOH solution, swirl the solution and wait for a couple of minutes to completely precipitate the magnesium ions as Mg(OH)2(s)
3. Add a pinch of hydroxynaphthol blue (exact amount to be decided by the intensity of the resulting coloured solution) and titrate with 0.01 M EDTA until changes to sky blue (save the solution for colour comparison).
4.
References: Various chemistry notes Chemistry – Human Activity Chemical Reactivity, authors – Mahaffy, Bucat, Tasker, Kotz, Treichel, Weaver, Mcmurray