Complexometric Determination of Water Hardness
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Complexometric Determination of Water HardnessBy: Daniel Morgan and Melinda AllredAbstract The hardness of both a known and an unknown sample of water were determined using a titration method with a cheatling agent and Calcium Carbonate. The calculated average hardness of the unknown sample was 262.029 ppm.Introduction Water hardness is defined as “the amount of dissolved calcium and magnesium in the water. Hard water is high in dissolved minerals, both calcium and magnesium.” (
Take unknown water sample # 73 add 25.0 mL to the Erlenmeyer flask. Add 20mL of deionized water to the Erlenmeyer flask. Stir solution using a magnetic stirrer and stir bar. Add 3 mL ammonia as a chloride buffer. Stir for 30 seconds. Add 4 drops of Eriochrome Black T indicator solution. Stir an additional 30 seconds. Using a titration flask, titrate the Na2EDTA into the Erlenmeyer flask. Note the starting volume on the titration flask of the Na2EDTA. Watch for a color change from bright pink, to violet, and finally to a blue color. The blue color is the end point of the titration. Note the end volume of the Na2EDTA in the titration flask. Repeat the titration process twice more and record both starting and ending volumes of Na2EDTA for each trial. Results and Discussion*Data Collection from Standardization (Trials 1-3)Calculating Molarity of Na2 EDTA from solution:Standardizing the disodium EDTA SolutionNa2 EDTA DI H2O NH4ClNa2 EDTA [M]Mean MolarityTrial 1 24.8 mL 29.9 mL3.1 mL0.0040280.004447Trial 221.8 mL 30.0 mL3.0 mL 0.004580Trial 3 21.1 mL30.0 mL3.0 mL0.004734Trial 1:[pic 1]Trial 2:[pic 2]Trial 3: [pic 3]Calculating the mean molarity:[pic 4][pic 5] [Na2 EDTA]mean ≈ .004447 (mean molarity) Calculating Relative Precision: Performing these calculations will represent the reproducibility for this set of experiments and the degree of accuracy on each trial. The first calculation is the relative average absolute deviation which will be reported in parts per thousand (ppt) with the expectation of achieving five parts per thousand in this experiment. In this calculation, the absolute value is required for each trial’s deviation from the mean: