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  • Topic: Complexometric titration, Hard water, Water
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  • Published : February 28, 2013
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Complexometric Determination of Water Hardness
Chris Breault
2/4/2013
Abstract: Unknown sample #94 was titrated using disodium EDTA to standardize, and Erichorchrome Black T as the indicator. Sample 94 was found to have an average water hardness of 268.2 ppm.

Introduction
The experiment uses disodium salt of EDTA to determine the concentration of metal ion impurities in hard water by complexometric titration. Water gathers impurities by dissolving salts in the earth. The amount of impurities in the water determines water hardness. A greater concentration of impurities in a sample of water increases water hardness. A lower concentration of impurities decreases water hardness. By using complexometric titration the hardness of a sample of water can be obtained.

Methods
To begin, a solution of 500 mL of disodium EDTA with a mean M of 0.00412 was prepared. Next, the disodium EDTA solution was standardized using 10.18 mL calcium ion stock solution (1.000g/CaCO3), ammonia/ammonium chloride buffer, and Eriochrome Black T as an indicator. Then, using the standardized disodium EDTA solution, an unknown sample (#94) was titrated. Finally, the results were compared with that of Tempe City’s pulic water supply.

Results
Table 1. | Standardization| | |
Trial| Volume Delivered CaCO3| Volume Delivered Na2EDTA| Molarity| 1| 10.14mL| 24.30mL| 0.00417|
2| 10.29mL| 24.53mL| 0.00419|
3| 10.31mL| 24.43mL| 0.00421|

Calculation for M in trial 1:
10.14mLCaCO3soln24.30mLNa2EDTAsoln∙1.000gCaCO31.00LCaCO3soln∙1molCaCO3100.1gCaCO3∙1molNa2EDTA1molCaCO3=0.0042molNa2EDTA/L

ϵ calculated molarity from eachtrialnumber of titration trials The mean M was found to be 0.0042.

Calculation for precision:
ϵ absolute deviations3Na2EDTAmean∙1000ppt
0.0000430.00419∙1000ppt=3.18ppt

Table 2. | Water Hardness ppm| | |
Trial| Volume CaCO3| Volume Na2EDTA| Hardness|
1| 28.9mL| 19.63mL| 0.3179g/L|
2| 25.0mL| 14.36mL|...
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