Suzi - Formal Water Lab

Suzi - Formal Water Lab - 1 Introduction: Why is it...

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Introduction: Why is it important to know the quality and condition of water? Water is a necessity for human life. Billions of liters of water are used daily and it is only natural to ensure its quality to maintain health and prevent sickness. People don’t realize that water contains many different substances that cause it to be “hard” and “soft”. Water (H 2 O) is not solely made up of hydrogen and oxygen. Most water contains many trace elements, specifically divalent cations, which alter its purity. These trace elements contribute to what we call the “hardness” of water. Water obtains elements and minerals such as Ca 2+ and Mg 2+ through different sources of solutes; gases, erosion of rocks, specifically sedimentary rocks, rainwater containing sea particles, and man-made pollutants (12). Hardness of water was originally determined by its ability to precipitate soap (1). Trace elements are involved with precipitating soap, chiefly Mg 2+ and Ca 2+ act as the prime precipitators. Mg 2+ and Ca 2+ are the two main divalent cations that constitute the total hardness of water (1). As quantities of Ca 2+ and Mg 2+ increases, so does the hardness (12). Water hardness can range from zero to hundreds of milligrams per meter (1). In the lab “The Chemistry of Natural Waters”, tests were conducted to selected water samples from State College, PA to calculate the concentrations of trace elements found in thw water. The specific dissolved divalent cations we tested for were Ca 2+ and Mg 2+ . To do this, we used two methods; the use of Atomic Absorption Spectrophotometry (AA) and EDTA titrations. EDTA titration is a form of determining the hardness of water. First a buffer must be added (NH 3 /NH 4 ) to a natural water sample adjusting it to a pH of 10 (12). Eriochrome Black T, an indicator known as EBT, is added to the water sample giving a red wine color (12). Before the addition of the water sample, the indicator is blue. This color change from blue to wine red is due 1
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to the presence of Mg 2+ (12). By adding a chelating substance (EDTA), Mg 2+ and Ca 2+ are removed from the solution (2). EDTA (the chelating agent) binds to Ca 2+ first and once complete, it binds to the Mg 2+ ions (2). When both of these divalent cations bind with the EDTA, they are removed, and the indicator turns back to its blue form (2). This form of removing water hardness expels the permanent hardness ions (2). EDTA can be found in many cleaning agents such as soaps and detergents. The chelating agent removes these cations resulting in softer water. Although EDTA titration is popular to test for hardness because it is easily conducted, it is not the most precise. Atomic Absorption Spectrophotometry is another method used to test the hardness of water (3). An Atomic Absorption Spectrophotometer (AA) is used to quantitatively test for the presence of elements in water. To determine what element is present in a substance, a monochromatic light is emitted from a specific hollow cathode lamp (in our case, a Mg
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Suzi - Formal Water Lab - 1 Introduction: Why is it...

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