Determination of Water Hardness Using Standardized Edta Solution

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RESULTS

Sample A

Trial 1

Trial 2

Trial 3

Volume of sample aliquot, mL

25 mL

Average molarity of EDTA used in titration, mmoles/mL

4.039 x10-3 M

Volume of EDTA used in titration, mL

48.8 mL

50 mL

49.7 mL

Total hardness of H2O as ppm CaCO3, mg/mL

0.788 ppm

0.808 ppm

0.800 ppm

Average total hardness of H2O as ppm CaCO3, mg/mL

0.799 ppm

TABLE 1: Sample A

Sample Calculations:

Trial 1: (Average Molarity of EDTA used) x (volume of EDTA used) x (1mmole CaCO3/ 1 mmole EDTA) x (100.0869 mg/ 1 mmole CaCO3) = 19.7 mg CaCo3

Total hardness = 19.7 mg/ 25 mL = 0.788 ppm

*repeat for other trials

Average ppm = (0.788 + 0.808 + 0.800)/ 3 = 0.799 ppm

%Ca = (Average Molarity of EDTA used) x (Average volume of EDTA used) x (1mmole CaCO3/ 1 mmole EDTA) x (1 mmole Ca/1 mmole CaCO3) x (40.078 mg Ca/ 1mmole Ca) x 100% = 63.4 % Calcium

Sample B

Trial 1

Trial 2

Volume of sample aliquot, mL

25 mL

Average molarity of EDTA used in titration, mmoles/mL

4.039 x 10-3 M

Volume of EDTA used in titration, mL

43.5 mL

44.8 mL

Total hardness of H2O as ppm CaCO3, mg/mL

0.704 ppm

0.724 ppm

Average total hardness of H2O as ppm CaCO3

0.714 ppm

TABLE 2: Sample B

Sample Calculations:

Trial 1: (Average Molarity of EDTA used) x (volume of EDTA used) x (1mmole CaCO3/ 1 mmole EDTA) x (100.0869 mg/ 1 mmole CaCO3) = 17.6 mg CaCo3

Total hardness = 17.6 mg/ 25 mL = 0.704 ppm

*repeat for other trials

Average ppm = (0.704 + 0.724)/2 = 0.714 ppm

%Ca = (Average Molarity of EDTA used) x (Average volume of EDTA used) x (1mmole CaCO3/ 1 mmole EDTA) x (1 mmole Ca/1 mmole CaCO3) x (40.078 mg Ca/ 1mmole Ca) x 100% = 55.9 % Calcium

DISCUSSION

The first part of the experiment was to prepare and standardize the EDTA solution. The average molarity was calculated as 4.039x10-3 M. The second part is to compute for the total hardness of water. The students were given two samples, namely A and B. On sample A, the average calcium carbonate content (as ppm) was 0.799 ppm CaCO3. The standard deviation obtained was 0.0101. While on sample B, the average calcium carbonate content (as ppm) was 0.714. And the computed standard deviation was 0.0141. It can be seen that the standard deviation on sample A is smaller than that of sample B because its computed total hardness for the three trials were closer to the mean. The calculated calcium content for sample A was 63.4 %, while the calculated calcium content for sample B was 55.9%. This means that sample A is “harder” than sample B. And based on the data above, higher volume of EDTA was used on sample A than on sample B.

Water Hardness

Calcium carbonate

Category

0-43

soft

43-150

Slightly Hard

150-300

Moderately Hard

300-450

Hard

450

Very Hard

Water Hardness Table

The range of the computed calcium carbonate content for both sample A and B falls on the soft category because they were all less than 1.

The possible errors committed may be from the titration process. The students may not be able to detect the first appearance of the clear blue color. Or that they might have over titrated the sample. Another cause can be from not properly calibrating the equipments used such as leaving an air gap on the burette.

CONCLUSION

In conclusion, the students were able to learn how to prepare and standardize the EDTA solution, and they were able to familiarize themselves with concepts of complexometric titration and they determined the water hardness using EDTA solution. The results obtained from this experiment were reasonable. The hardness of a water sample was successfully discovered by finding the CaCO3 content in the sample. The calcium content of sample A was found to be 63.4 %. The calcium carbonate content (as ppm) of sample A from the three trials performed was 0.788 ppm, 0.808 ppm, and 0.800 ppm. The confidence interval was found as 0.799 ± 0.0129 ppm. The upper limit was 0.812 ppm, while the lower limit was 0.786 ppm. This showed that the water fell into the range of numbers indicating hard water. On sample B, the calcium content was found to be 55.9 %. The calcium carbonate contents (as ppm) were 0.704 ppm and 0.724 ppm. The confidence interval is 0.714 ± 0.0222 ppm. The upper limit