Complexometric Determination of Water Hardness
September 9th 2013
Dr.David Nachman, CHM152LL
The water hardness for unknown water sample number 40 was determined. From the results of the two complexometric titrations, the water hardness of the unknown sample was calculated to be 250.9 ppm CaCO3, which agrees with the ranges of acceptable water hardness in the city of Phoenix and Tempe Arizona. Introduction
When rainfall picks up impurities from the soil, ions of sodium, magnesium, calcium, iron, and other metals are dissolved into the water. These impurities are what cause residues left on glassware from the reaction with the soap used for cleaning. Water hardness is from the metal ions with a +2 charge or higher being dissolved into the rainfall. When reporting water hardness, it is reported in units of mg CaCo3/L of solution, or in ppm due to one mg of solute having one millionth of the mass of a liter of water or dilute aqueous solution. It is reported using only CaCo3 because typically, Calcium is the largest contributor to the water hardness. The purpose of this experiment was to determine the concentration of metal ion impurities in an unknown sample of hard water by performing a complexometric titration with EDTA. The results were then compared to the expected range for municipal water hardness from multiple city websites. Methods
All procedures from the CHM 152 Lab, Complexometric Determination of Water Hardness (1) were followed precisely. Unknown water sample #40 was mixed with approximately 20 ml of DI water, 3.0 ml of Ammonia/Ammonium Chloride (NH(aq) Buffer pH=10), and 4 drops of Eriochrome Black T indicator solution. It was then titrated with .004197 M EDTA, which was also made in the lab from 0.7582 g of , added to 500 mL of DI water, that was then standardized with a stock Calcium Ion Solution (CaCO(aq) 1.00g/1.00L) by titration. A total of 3 titration trials were completed to find the average molarity...
References: 1. Complexometric Determination of Water Hardness, Procedures, Mesa Community College CHM152LL website, http://www.physci.mc.maricopa.edu/Chemistry/CHM152/index.html, accessed 9/7/2013.
2. City of Phoenix website. http://phoenix.gov/waterservices/quality/index.html, accessed 9/8/2013. Copyright 2013
3. City of Tempe Az website. https://www.tempe.gov/index.aspx?page=1289#Hardness, accessed 9/8/2013. Copyright
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