Experiment 6: EDTA Titration of the Hardness of Water
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Vincent Kurp Quantitative Analysis Experiment 6: EDTA Titration of the Hardness of Water Purpose: For this experiment we created our own procedure. The main purpose of this lab is to determine the amount of the calcium ion Ca2+ in tap water. We used a solution of EDTA as the titrant for a complexation titration to find the amount of the ion Ca2+ in tap water, as well as the amount of calcium carbonate in an antacid tablet. Procedure: First a pH meter was calibrated. Then 0.1M HCl was added to an ammonium buffer until a pH of 10 was reached. An EDTA solution was prepared with 1.0 g EDTA, 5 mL of buffer, and diluted to 250 mL with distilled water. To standardize the EDTA solution 0.5 g CaCO3, 5 mL buffer, and a small spoonful of Hydroxynapththol Blue Indicator were added to an Erlenmeyer flask and titrated with EDTA. This was repeated three times. 100 mL tap water, 5 mL of buffer, and indicator were mixed in an Erlenmeyer flask and titrated with the standardized EDTA. This titration was also repeated three times. A massed antacid tab was crushed. Half of the tablet was weighed out and dissolved in 100 mL 0.1 M HCl in a volumetric flask. A 10 mL aliquot of the antacid solution, 2 mL buffer, and indicator were mixed in an Erlenmeyer and titrated with standardized EDTA. This process was also repeated three times. Data: Mass EDTA: 0.9390 g Mass CaCO3: 0.5013 g Mass Antacid: 1.2891 g Standardization of EDTA Trial 1 2 3 Average EDTA Added (ml): Standardization 11.60 8.38 8.51 EDTA Concentration (M) 0.02159 0.02989 0.02943 0.02697 Hardness of Water Titration Trial EDTA Added (ml): Tap Water 5.00 4.79 5.21 1 2 3 Average Ca2+ Concentration (M) 0.0013 0.0013 0.0014 0.0013 Ca2+ Concentration (Ppm) 52.10 52.10 56.10 53.43 Antacid Titration Mass Antacid Tablet Used: 0.6498 Trial EDTA Added (ml): Antacid 18.84 18.92 19.21 1 2 3 Average Ca2+ Mass (g) 0.02036 0.02045 0.02076 0.02053 Calculations: Concentration of EDTA (Trial 1 ex): π πΆππΆπ3 × 0.5013 π × 1 πππ πΆππΆπ3 1 0.005 πΏ 1 πππ πΈπ·ππ΄ 1 × × × × → 2+ 100.085 π πΆππΆπ3 0.100 πΏ π»πΆπ 1 1 πππ πΆπ πΏ πΈπ·ππ΄ 1 πππ πΆππΆπ3 1 0.005 πΏ 1 πππ πΈπ·ππ΄ 1 × × × × = 0.02159 π 100.085 π πΆππΆπ3 0.100 πΏ π»πΆπ 1 1 πππ πΆπ2+ 0.0116 πΏ Ca2+ Concentration in Tap Water (Trial 1 ex): π πΈπ·ππ΄ × πΏ πΈπ·ππ΄ 1 πππ πΆπ2+ 1 × × → 1 1 πππ πΈπ·ππ΄ 0.100 πΏ πππ πππ‘ππ 0.005 πΏ 1 πππ πΆπ2+ 1 0.02697 π × × × = 0.0013 π 1 1 πππ πΈπ·ππ΄ 0.100 πΏ πππ πππ‘ππ Ppm of Ca2+ in Tap Water (Trial 1 ex): π πΆπ 0.0013 π × 2+ 40.078 π πΆπ2+ 1000 ππ × × → 1 πππ πΆπ2+ 1π 40.078 π πΆπ2+ 1000 ππ × = 54.05 πππ 1 πππ πΆπ2+ 1π Mass of Ca2+ in Antacid (Trial 1 ex): π πΈπ·ππ΄ × 0.02697 π × πΏ πΈπ·ππ΄ 1 πππ πΆπ2+ 40.078 π πΆπ2+ × × → 1 1 πππ πΈπ·ππ΄ 1 πππ πΆπ2+ 0.01884 πΏ 1 πππ πΆπ2+ 40.078 π πΆπ2+ × × = 0.02036 π 1 1 πππ πΈπ·ππ΄ 1 πππ πΆπ2+ Mass Percent of Ca2+ in Antacid Tablet: π πΆπ2+ πππππ’πππ‘ππ 0.02053 π × 100% = × 100% = 3.16% π π‘πππππ‘ π’π ππ 0.6498 π Discussion: At a high pH, EDTA is reduced and no longer has any electrons on its bonding group to give away and form a complex. This is why the ammonium buffer was added, to raise the pH and ensure that the EDTA being added was in its non-acidic form. Some common metallochromatic indicators are Murexide, Eriochrome Black T, and Calmagite which is what we used for this experiment but any of the three would have worked. We calculated the weight percent of calcium carbonate in an antacid tablet to be only 3.16% which is much lower than 15.8% that is reported on the label. There is no way that the manufacturers made an error this large so we can assume that there was a human error that occurred somewhere in our experiment. Conclusion: This error could have occurred when our tablet was being dissolved. The tablet was not ground up fine enough and there were larger chunks of tablet floating around in the solution that wouldn’t dissolve. Its possible that deposits of calcium carbonate were trapped inside of these and were unable to react with the EDTA. To improve this procedure I would make sure to grind up the tablet as fine as possible. I would also recommend using a different indicator. The endpoint was difficult to determine because it was hard to distinguish one color from the other. Perhaps one of the other metallochromatic indicators would have worked better.
