Experiment 10 - Experiment 10 The Chemistry of Natural...

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Experiment 10: The Chemistry of Natural Waters By: Dan Morgenstern Partners: Mark Moore, Andy Mackowski, Dan Mendenhall November 13, 2006 TA: Dan Mao Room 105C Monday, 1:25-5:30 pm 1
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Introduction For the experiment, The Chemistry of Natural Waters , the goal was to obtain a water sample from a source such as a stream, sink in a building, water fountain, or anywhere that water was accessible. After this sample was taken, various tests to find the hardness of the sample were taken. The hardness of a water sample is defined as the presence of Ca 2+ (Calcium 2+) and Mg 2+ (Calcium 2+), which are the dissolved divalent (valence of 2) cations. Dissolved polyvalent cations (valence of more than 2) also exist in the water sample. For example, water that has a large concentration of Ca 2+ and Mg 2+ would be considered hard, but if the sample had a low concentration of these divalent cations, it would be considered soft. Hardness can be helpful in everyday life duties, like washing things with soaps and detergents. Four students each took a water sample from their respective dormitory building and used various tests for their respective sample’s hardness for comparative reasons (1). A very easy way to find what solutes are present in water samples is evaporating the sample, or solvent, and then observing the residue of the nonvolatile (unable to evaporate) solids that is left. The residue that remains is after the evaporation of the water sample is called the total dissolved solids (TDS). Divalent cation analysis by EDTA Titration was one of the tests used for hardness determination. EDTA is an abbreviation for ethylenediaminetetraacetic acid, C 10 H 16 N 2 O 8 . This is the chelating agent in the complexation titration. In a complexation titration, a quantitative analysis of a water sample or residue from the TDS experiment, for total divalent cation is carried out for the process. Because of solubility, the reactant used in laboratories is EDTA’s disodium salt (C 10 H 14 N 2 O 8 Na 2 ●2H 2 O). An indicator, usually eriochrome black T (EBT), 2
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usually used in hardness determination. An EDTA titration is probably the most common way to determine the hardness of a water sample (1). Atomic Absorption spectrophotometry (AA) is another way to determine the hardness of a water sample. AA is a sensitive, element specific method when analyzing metals, which can be alkalis alkaline earths, or transition metals. Monochromatic light that has energy equivalent to the electronic energy separation of the atoms of interest (Ca 2+ and Mg 2+ ) is projected through the water sample to be analyzed. Atoms in the sample that have electronic energy separation will absorb the light. The amount of absorbance is proportional to the concentration of the atoms of metal in the water sample.
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