Complexometric Determination of Water Hardness

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Absolute deviation for trial n = | [EDTA]mean – [EDTA]n |

- Trial 1: | [.004447] – [.004028] | ≈ 4.19 E-4

- Trial 2: | [.004447] – [.00458] | ≈ 1.33 E-4

- Trial 3: | [.004447] – [.004734] |≈ 2.87 E-4

To calculate the mean of the above trials, the following formula is used where the sum of the previously calculated absolute deviation is divided by the number of trials. Then that sum is divided by the previously calculated mean. After calculating the absolute values for n trials, the mean is calculated from the sum of the trials’ calculated absolute deviation and divided by the number of trials. This value is then divided by the previously derived [EDTAmean]. This value is multiplied by 1000 to reach an estimated precision in parts per thousand. Below are the data calculations: [pic 6]

Estimated Precision (ppt) =

[pic 7]

Calculating water hardness (ppm)

After solving for the EDTA mean molarity, the next step involved calculating the calcium carbonate present in an unknown sample (No. 73). This method consists of the volume titrated in three trials using the previous approach using the Eriochrome Black T indicator. When all three trials concluded, the volumes recorded are evaluated using the formula below:

[pic 8]

Trial 1: [pic 9]

Calculated Water hardness (parts per million)

Sample 73

DI H2O

NH4CL

EDTA soln.

(ppm)

Trial 1

25.0 mL

19.5 mL

3.0 mL

15.4 mL

275.63

Trial 2

25.0 mL

20.0 mL

3.05 mL

13.9 mL

245. 78

Trial 3

25.0 mL

20.0 mL

2.99 mL

14.7 mL

263.10

Conclusion

The hardness of the unknown water sample was determined using the titration method to determine the endpoint of the reaction of CaCO3 and Na2 EDTA. The estimated precision for the trials with a standardized calcium ion solution was 62.89 ppt. The first trial of the known molarity CaCO3 was the furthest deviation for the other trials. The subjective element of determining the precise point of the color change and hence end point was a factor. This subjective element compounded on the calculation of the final water hardness of the unknown sample, because the average molarity was used in the final calculation. If only the first two trials were used in calculating the average molarity the estimated precision would have been 16.64 ppt as opposed to 62.89 ppt.

The three trails in the unknown water sample titration yielded somewhat different results. This is due to the subjective nature of determining the endpoint as well as the learning curve of the skills needed in the titration method. The average water hardness from the unknown sample was calculated to be 262.029 ppm. This is within the published range from http://www.tempe.gov/city-hall/public-works/water/water-quality/typical-water-quality-values#Hardness which was 220 mg/L to 420 mg/L with a typical reported value being 244 mg/L.

Works Cited

- Brown, Theodore L., Kenneth C. Kemp, Matthew Stoltzfus, and John H. Nelson.Chemistry: The Central Science. Boston: Prentice Hall, 2012. Print.

- Martell, A. E., and Stanley. Chaberek. "Use of Chelating Agents as Reagents in Titrimetric Analysis." Analytical Chemistry 26.11 (1954): 1692-696. Web.

- "Typical Water Quality Values." City of Tempe, AZ : Typical Water Quality Values. N.p., n.d. Web. 11 June 2017.

- Perlman, USGS Howard. "Water Hardness." Hardness in Water, USGS Water Science School. N.p., n.d. Web. 11 June 2017.

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