Determination of Water Hardness by EDTA Titration

 

Introduction

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.

 

Procedure

Note: Use deionized water throughout.

  1. Using a pipet or buret add 50mL of 0.01M CaCO3 standard solution to a 250mL Erlenmeyer flask
  2. Add 2mL of pH 10 buffer (buffer must be added before the indicator)
  3. Add five drops of 0.5% Eriochrome Black T indicator
  4. Titrate with 0.01M EDTA until the color changes from red wine through purple to pure blue
  5. Repeat to achieve 3 similar results
  6. Obtain approximately 500mL of tap water (allow the tap to run for several minutes before collecting)
  7. Titrate a 50mL aliquot of the tap water using the procedure described above
  8. If the end point is reached in less than 10mL, double the sample volume to 100mL for future trials
  9. Repeat to achieve 4 results
  10. To determine calcium concentration, pipet 4, 50 mL samples of tap water into clean flasks.
  11. Add 30 drops of 50% w/v NaOH to each solution, swirl and leave for a couple of minutes to completely precipitate the magnesium.
  12. Add about 0.1g of hydroxynaphthol blue to each flask.
  13. Titrate each sample with EDTA to a violet/blue endpoint, allow to sit for 5 minutes, add more EDTA if necessary.

 

Calculations

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.

Questions

  1. Why is it important to control the pH?
  2. Water softeners remove hardness by exchanging calcium and magnesium ions with another cation. Which cation might you expect to be used?
  3. Draw the structure of the CaEDTA2- complex. Which atoms on the EDTA molecule form bonds to the calcium?
  4. Water from an area of southeast England was found to contain 550ppm water hardness. What might you predict about the geology of this area?
  5. The equilibrium constant for the following reaction is 6.92 x 10-7. If 0.1M H2EDTA2- is initially present in solution what is the pH at equilibrium?

H2EDTA2- + H2O = H3O+ + HEDTA3-

 

References:

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.