Complexometric Determination of Water

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*Repeat for trial 2 and 3

Average molarity: sum of molarity/3

(2.07x10^-3)+(2.02x10^-3)+(2.2x10^-3)/3 = 2.09 x10^-3

Absolute deviation and Estimated precision

(2.09 x10^-3)- (2.03x10^-3) = 2.0 x 10^-5 —repeat for trial 2 and 3

Average molarity- molarity for trial n = absolute deviation

(sum of absolute deviations/3)/Average EDTA molarity x 1000 ppt

[(2.0x10^-5) + (7x10^-5) + (1.1x10^-4)/3]/ 2.09x10^-3 x1000 ppt = 31.9 ppt

Data 2

Water Sample Titration-Unknown Code 3

Average water hardness: 156.16 ppm

Unknown Sample #3

Trial 1

Trial 2

Trial 3

Initial buret reading

0 ml

12 ml

13.5 ml

Final buret reading

22.6 ml

29.6 ml

29.3 ml

Volume delivered

22.6 ml

17.6 ml

15.8 ml

Water Hardness per trial

189 ppm

147.3 ppm

132.2 ppm

Calculating Water Hardness

[V mL Na2EDTA solution/0.02500 L CaCO3 solution] x [M mol Na2EDTA/1L Na2EDTA] x [1 mol CaCO3/1 mol Na2EDTA] x [100.1 g CaCO3/1 mol CaCO3]

[22.6 ml/.02500 L CaCo3] x [2.09 x 10^-3 M Na2EDTA/1 L Na2EDTA] x [1 mol CaCO3/1 mol Na2EDTA] x [100.1 g CaCO3/1 mol CaCO3] = 189 ppm. Repeat for trial 2 and 3

Average Water hardness: Sum of water hardness/3

189+147.3+132.2 = 156.16 ppm

Discussion

During the standardizing disodium EDTA solution portion of this experiment, I found the average molarity to be .00209 M while the actual molarity that needed to be utilized was .004 M. This is most likely as a result of my color blindness to violet-blue. I really have a hard time differentiating these two colors and during the experiment, there were times when the blue indicator was reached, but I saw it as purple which skewed my data of volume of solution delivered, which then rendered my molarity so far from the .004 M solution that was needed to effectively complete this experiment.

In the second portion of the experiment however, all three trials yielded dissimilar results as my first trial, again, because of my color blindness to violet and blue. The average Calcium ion found in my sample however was 156.16 ppm. The trials rendered results of 189 ppm, 147.3 ppm, and 132.2 ppm. Hard water makes up about 85% of the nation’s drinking water, according to the U.S Geologic survey. Water hardness in Mesa, Arizona, ranges from 12 gpg (grain per gallon) to 22 gpg, depending on the water source that servers a specific area—this corresponds to over 180 ppm.[2] Soft water has a ppm range from 0 to 60 (0-3.5 gpg). Moderately hard water has a range of 60-120 (3.5-7.0 gpg) whereas hard water ranges from 120-180 (7.0-10.5 gpg); very hard water goes over 180 ppm (over 10.5 gpg).[3]

If I were to redo this experiment, I would put more indicator before each trial so that I could more easily detect the color change to have more accurate results. My very first trial was still a more violet hue before I stopped titrating to write down the volumetric value for that trial and my endpoint had already passed. It was extremely difficult pinpointing the true endpoint for each trial. An abrupt color change would have been more helpful for me; for example, going from a neutral hue to a more vibrant hue like from white to pink.

Conclusion

The results from this experiment was quite reasonable, notwithstanding errors made by passing the endpoint. The titration proved to be an effective manner to detect water hardness by finding the calcium content in the samples given. The water hardness calculations were 189 ppm, 147.3 ppm, and 132.2 ppm, the average being 156 ppm, indicating moderately hard water.

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