Water Hardness

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Complexometric Determination
Of Water Hardness

Angelica Aquino
CHM 152 LL
January 5, 2013

1. Prepare an approximate 0.004 M disodium EDTA solution. To prepare this solution, weigh about 0.7-0.8 g of Na2EDTA and dissolve in 500 mL deionized water in your plastic bottle. Make to to shake the bottle to dissolve the salt. 2. Obtain a 250-mL Erlenmeyer flask and transfer a 10 mL of CaCO3 using a buret into the flask. 3. Measure 30 mL of deionized water and add it into the titration flask. 4. Make sure to stir the solution

5. Inside a fume hood, add 3mL of ammonia/ammonium chloride buffer (pH10) into the flask and stir for another 30 seconds. 6. Add four drops of Eriorchrome Black T indicator solution. Stir for another 30 seconds 7. Begin titration. Record your initial volume and final volume. At the endpoint, the color will change to pink to violet to blue. 8. Repeat this titration two more times. Now that you recorded the volume, your second and third trial should be a lot easier. 9. Take your water sample that you obtain from the stockroom and record the unknown number. 10. Transfer 25 mL of the unknown water sample to a 250-mL Erlenmeyer flask. 11. Add about 20 mL of deionized water to the titration flask. Stir for 30 seconds 12. Inside a fume hood, add about 3 mL of ammonia/ammonium chloride buffer (pH10) and stir for another 30 seconds. 13. Add four drops of Eriorchrome Black T indicator solution to your flask and stir for 30 seconds. 14. Begin titrating. Record you initial volume and final volume. Repeat this twice more. Calculate the average hardness and your experimental precision from the three trials.


EDTA Solution:
0.7533g of Na2EDTA mixed with 500ml DI water

Table A: Standardization of EDTA
Trial| Start Volume| End Volume| Amount of EDTA|
1| 3.90ml| 29.85ml| 25.95ml|
2| 5.35ml| 28.35ml| 23.00ml|
3| 0.45ml| 23.50ml| 23.05ml|

Moles EDTA...
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