Water Hardness Lab

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Next, use a 10 mL transfer pipet to add 10.00 mL of standardized calcium ion stock solution to a 250 mL Erlenmeyer flask. Add 30 mL of DI water to this same flask. Now, place a magnetic stir bar in the flask, set it on top of a magnetic stirrer, and begin stirring. After 30 seconds, remove the mixture and transport it to the nearest fume hood. While inside the fume hood, add about 3 mL of ammonia/ammonium chloride buffer (pH 10). Use added caution at this point for the chloride buffer acts as an inhalation irritant. Bring the flask back over to the magnetic stirrer and let it stir for 30 more seconds. Next, add four drops of Eriochrome Black T indicator solution to the flask and allow it to stir for an additional 30 seconds.

The flask is now ready for titration. Be sure that the burette is securely fastened to a ring stand. Place the titration flask approximately five inches below the opening of the burette. While titrating, remember to slow down near the end of the reaction. The desired color change (from pink to violet to blue) should only take 3-5 seconds. If there is uncertainty as to whether or not the anticipated endpoint has been achieved, stop the burettes flow, record the delivered volume, and then add a few more drops. Observe intently for a more complete color change. Repeat this entire process twice more to gain the data for a total of three titration trials. Before proceeding further, be sure to rinse and clean all equipment, such that unnecessary cross contamination is avoided.

Now, the unknown water sample will be incorporated into the experiment. Obtain an unknown water sample from the chemistry stockroom and record its given number in a lab notebook; this will ensure the accuracy of future data comparisons. This water sample can then be titrated with the standardized disodium EDTA solution. Begin by transferring 25.00 mL of the unknown water sample to a 250 mL Erlenmeyer flask. Add 20 mL of DI water to this same flask. Add the magnetic stir bar and allow the mixture to stir for 30 seconds. Then, take the flask underneath a fume hood and add 3 mL of ammonia/ammonium chloride buffer (pH 10). Return to the magnetic stirrer and continue to stir the mixture for 30 seconds. The flask is now ready for four drops of Eriochrome Black T indicator solution. After this indicator is added, allow the mixture to stir for an additional 30 seconds before you begin titrating. This process will need to be completed twice more, for a total of three titrations. Throughout the experiment, be mindful of the fact that the two flask mixtures must be titrated within the first 15 minutes of their fruition.

The data collection portion of the experiment is now complete. Next, calculate the mean molarity of the disodium EDTA solution for each of the first three titration trials. Then, calculate the estimated relative precision in parts per thousand (ppt). Finally, calculate the water hardness in parts per million (ppm). Results from this experiment may be compared with your city’s municipal water hardness expected ranges to determine accuracy.

Results

Conclusion

The results from this experiment were very practical. The hardness of an unknown water sample was effectively determined by calculating the calcium content within said sample. The calcium content for the three trials performed were XX, XX, and XX. The average of these three trials was XXX ppm. From a comparison of the results of our experimental calculations with the normal ranges of expected water hardness in the city of Mesa, it can be concluded that the ____ of _____ in the sample used in this experiment was about _______.

References

Harvey, D. (2016). Complexation titrations. UC Davis ChemWiki.

Mesa Community College Chemistry Department. (2016). Complexometric Determination of Water Hardness by EDTA Titration. Chm 152 LL Lab Manual.

Perlman, H. (2016) Water hardness. The USGS Water Science School.

Playle, R. C., Gensemer, R. W. and Dixon, D. G. (1992), Copper accumulation on gills of fathead minnows: Influence of water hardness, complexation and pH of the gill micro-environment. Environmental Toxicology and Chemistry.

Purdue University. (2016). What is titration? Found on June 5, 2016, from http://chemed.chem.purdue.edu/genchem/lab/techniques/titration/

Sengupta, P. (2013). Potential health impacts of hard water. International journal of preventive medicine, 4(8).

WFH. (2016). What is hard water? Reviews of the Best Water Filtration Systems. Found on June 7, 2016 from http://www.waterfiltrationhq.com/what-is-hard-water/

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