Determining The Concentration Of Water Hardness

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Water hardness is defined as the sum of the concentrations of Ca2+ and Mg2+ in a water sample which is measured by amount of CaCO3. Typically, water hardness indicates the water quality, as hard water has greater concentrations of Ca2+ and Mg2+ and soft water has proportionally less concentrations of the aforementioned ions.1 Water that contains Ca2+ and Mg2+ concentrations above 100 milligrams per liter (mg/L) is categorized as hard while soft water contains ion concentrations below that threshold.2 Hard water has not been found to cause any negative health effects and can even contribute to a person’s necessary intake of calcium and magnesium. To prevent excess calcium and magnesium, the body self-regulates via an intestinal
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In this case, Ethylenediaminetetraacetic acid (EDTA) is combined with eriochrome black (EBT) to test for the presence of Ca2+ and Mg2+. Once the indicator is added to the water sample, the Mg2+ reacts with it to turn the sample a red wine color. If no Mg2+ is present in the sample, the solution will be blue for the entirety of the titration and a buffer is added. The next step in the process requires the addition of the EDTA which reacts with the Ca2+ and forms CaEDTA immediately. After all of the Ca2+ reacts, the EDTA begins to bond with the Mg2+ creating MgEDTA chelate, and the solution begins to turn blue because the Mg2+ is being taken out of solution. Once the number of drops required to turn the solution blue are counted, the equation MEDTAVEDTA=MCa2+VCa2+ for which the volume is the number of drops and the molarity is the Ca2+ concentration. Using the calculated molarity, the hardness of the water is calculated in parts per million (ppm).1 The reaction of EDTA Titration can be modeled by the following equation: HD2- + Mg2+ + Ca2+ MgD- + H+ + Ca2+ EDTA CaEDTA + MgEDTA + HD2-Though EDTA and AA provide information regarding water hardness, they are fundamentally different tests. AA utilizes absorbance from the change in energy of the water sample to quantify the concentration of metal ions. EDTA Titration requires an actual reaction between the water sample and the EDTA which is measured by the number of EDTA drops necessary to turn the solution blue; the quantification of drops is used to calculate the concentrations of Mg2+ and Ca2+. From the test results, different concentrations are calculated and subsequently different hardness values. Though the two tests should result in a similar hardness, error accounts for a considerable variance in the values.The water samples for the lab include tap water from McDonald’s on College Avenue, the Atherton

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