Chem 113 Natural Waters bad.docx - The Truth About Tap...

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The Truth About Tap Water: Hazards and Practicality Introduction: Water is a very important part of life on Earth. It covers nearly 70% of the planet’s surface and serves as the most abundant liquid. 1 It has a number of physical and chemical properties that make it especially important for all living things. One interesting quality about water is that it has different levels of hardness associated with it that is based on the amount of dissolved calcium and magnesium ions located within. Hard water systems have a large abundance of these ions, while soft water has relatively minimal concentrations of ions. Ground water systems are a good example of hard water classifications due to the fact that a small number of minerals are carried into the supply and delivered to homes in the surrounding area. 2 Water hardness can be associated with both positive and negative implications. One negative affect that hard water is often characterized with is the formation of scale. This process is defined as an accumulation of the insoluble precipitate, CaCO 3 , on the surface of pipes and other vessels. When hard water is heated, Ca 2+ ions react with bicarbonate ions to form this solid residue. 3 While this scale formation does not implement health in any way, it obstructs water flow, can be expensive to remove, and is often associated with having an unpleasant taste. Hard water can be beneficial as well by providing the body with numerous minerals, including calcium and magnesium, that satisfy dietary needs. 4 Water hardness is typically measured in ppm as mg/L CaCO 3 . This is due to the fact that calcium carbonate indicates the amount of total salts present. The specific salts are not identified and so hardness is typically measured as the total mixture of salts. Due to this reasoning, calcium carbonate is important in this measurement. 6
There are a number of methods that can be used to measure water hardness. The first way to measure water hardness is by performing an EDTA titration. In this type of test, a known volume of the water sample is taken and the pH is diluted using an NH 3 /NH 4 buffer. This buffer adjusts the pH to 10. Next, an EBT indicator is added and if the pH is high, then the HD 2- of the indicator will turn blue. A red color will form as the indicator reacts with the Mg 2+ present in the initial sample. After this step, the EDTA solution is added and it reacts with Ca 2+ to form a colorless chelate. Soon after, the EDTA reacts with the Mg 2+ to form a colorless MgEDTA and the indicator then returns to its original blue color. The start of the titration can be used to determine the concentration of both cations, Magnesium and Calcium. 5 This process has a flaw in that the titration of EDTA is not possible if there is no Mg2+ in the water to react with the indicator to give it the red wine color at the beginning. 5 Another method for measuring water hardness is by Atomic Absorption Spectrophotometry (AA). This technique uses monochromatic light that corresponds to the E of

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