Determination of Water Hardness by EDTA Titration
Calcium and magnesium salts dissolving from rocks and minerals cause water hardness. In areas of high water hardness, larger quantities of soap are required for washing and scale forms in boilers, kettles and hot water pipes. The effect on health, however, may be beneficial. Evidence suggests that incidences of cardiovascular disease decrease in hard water areas. The measurement of water hardness is important in water quality monitoring and is usually performed by means of an ethylenediaminetetraacetic acid (EDTA) complexation titration. By convention, the total hardness of water is quoted in terms of the parts per million of calcium carbonate, ignoring the contribution of magnesium salts. In this method a sample of tap water will be analyzed. Calcium and magnesium ions form 1:1 complexes with EDTA according to the equations:
Ca2+ + H2EDTA2- à CaEDTA2- + 2H+
Mg2+ + H2EDTA2- à MgEDTA2- + 2H+
The indicators used are Eriochrome Black T which changes from red to blue in the presence of excess EDTA.
MgIn-(red) + H2EDTA2- à MgEDTA2- + HIn2-(blue) + H+
And hydroxynaphthol blue which changes from
The separate calcium and magnesium concentrations can then be found by a separate titration at higher pH. By adding NaOH, the magnesium precipitate as Mg(OH)2 and the calcium can be determined separately. Simple subtraction from the total concentration of magnesium + calcium, the magnesium concentration can also be found.
Note: Use deionized water throughout.
Determine the mean concentration of the EDTA solution. Using this concentration, calculate the total calcium and magnesium concentration in ppm. Calculate the separate magnesium and calcium concentrations in ppm. Use your total calcium and magnesium value to calculate the hardness of the tap water in ppm CaCO3 assuming that the hardness in due to calcium carbonate only. State the mean and 95% confidence interval for your results.
H2EDTA2- + H2O = H3O+ + HEDTA3-
Standard Methods for the Examination of Water and Wastewater,16th Edition, APHA, AWWA, WPCF.
Christian, G. D., Analytical Chemistry, 4th Edition, J. Wiley and Sons.
Harris, D. C., Quantitative Chemical Analysis, 5th Edition, W. H. Freeman.