Determination of the Total Hardness of Water by EDTA Titration and Flame Photometry and ICP-AES
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To determine calcium and magnesium in a sample of tap water by titration with EDTA and then to determine calcium by flame photometry and hence magnesium by difference.
Apparatus and Reagents
Tap water sample
EEL flame photometer
0.01M EDTA solution; ammonia – ammonium chloride buffer solution (pH – 10); Eriochrome black T indicator, 1% solution (in 3:1 mixture v/v of triethanolamine and ethanol); 0.1M magnesium-EDTA complex solution; hydroxylamine hydrochloride.
Burettes, pipettes, volumetric flasks.
- Acidify a 50 cm3 (duplicate) sample of tap water with diluted HC1, boil for one minute, cool and neutralise with diluted NaOH solution (NB if necessary use a larger or smaller water sample). Add 1 cm3 of the buffer solution and 3-4 drops of the indicator solution . (NB (a) if interfering ions such as Cu, Fe, Co, Ni Mn are suspected to be present add a also a few crystals of hydroxylamine and use more buffer, ie. 4cm3 (b) if no magnesium is thought to be present add 0.1cm3 of the Mg-EDTA complex solution prior to the indicator. Titrate with EDTA to the red to blue end-point.
- Standardise the flame photometer with standard solutions of calcium (maximum 50 ppm of calcium) and prepare a calibration graph (at least four points). The sample concentration of calcium should be adjusted (if necessary) by dilution or evaporation. Determine the calcium from the calibration graph and hence obtain the concentration of magnesium by difference.
Calculate the concentrations of calcium and magnesium in the water sample. Analyse statistically the combined results for several analyses if available.
- What is the role of hydroxylamine.
- In previous years sodium or potassium cyanide have been added to solution with the hydroxylamine, why?
- Why is the addition of Mg-EDTA complex sometimes necessary?
- Suggest an alternative approach to the determination of Ca and Mg in water.