Ruth Pontoriero 1 Ruth Pontoriero CHM 337: Quantitative Analytical Chemistry Lab #6: EDTA Titration of the Hardness of Water Introduction: This lab used a complexation titration, involving a metal-ligand complexation, to determine overall hardness and calcium concentration of tap water. EDTA was used as a titrant. EDTA is a weak acid with 6 binding sites: four carboxylate groups and two amino groups. Methods: A pH meter was calibrated. In this case, the pH meter was calibrated such that actual pH 10 yielded a reading of pH 9.15. 1M HCl was added to an ammonium buffer until the pH reached the calibrated pH of 10. A 0.1 M EDTA solution was made by adding 1.4781g of EDTA to 490mL distilled water and 10mL of pH adjusted ammonium buffer. A 0.1 M HCl was made and 0.5089g of CaCO3 was added. A titration was performed using 3mL CaCO3 solution and 5mL buffer using Calmagite as the indicator and EDTA as the titrant. This was repeated for a total of four trials. This entire process was repeated using 3mL of tapwater instead of 3mL CaCO3 for a total of three trials. A solution was made using a 1.9828g antacid tablet which, according to the manufacturer, contained 0.500g CaCO3, and 10mL 1M HCl. A titration was performed using 3mL of the antacid solution and 5mL buffer with Calmagite as the indicator using EDTA as the titrant. This was repeated for a total of three trials. Ruth Pontoriero 2 Results and Calculations: Titration CaCO3 EDTA (mL) Molarity EDTA Mean molarity SD molarity Trial #1 25.0 0.00610 0.00732 0.000814 Trial #2 19.8 0.00770 Titration Tapwater EDTA (mL) [Ca2+] Ca2+ (ppm) Average [Ca2+] SD [Ca2+] Average Ca2+ ppm SD Ca2+ ppm 4.0 0.00976 391 0.00976 0.00024 391 9.50 Titration Antacid EDTA (mL) Average EDTA (mL) SD EDTA (mL) Mass Ca2+ (g) Percent mass Ca2+ (%) Mean % mass Ca2+ SD % mass Ca2+ 20.1 19.9 0.208 0.120 6.05 5.97 0.0764 Trial #3 Trial #4 19.8 0.00770 Trial #1 19.6 0.00778 Trial #2 4.1 0.0100 401 Trial #1 Trial #3 3.9 0.00952 382 Trial #2 Trial #3 19.8 19.7 0.118 5.95 0.117 5.90 Mass of EDTA needed for solution: 0.500L ∗ 0.01mol EDTA 292.14g EDTA ∗ = 1.4607g EDTA 1L 1mol EDTA Molarity of CaCO3 solution: 0.5089g CaCO3 1000mL 1mol EDTA ∗ ∗ = 0.0508M EDTA 100mL 1L 100.0869g EDTA Molarity of EDTA: 0.0508M CaCO3 ∗ 0.003mL CaCO3 ∗ 1 = 0.00610M EDTA 0.0250mL EDTA Ruth Pontoriero 3 Concentration of Calcium in the tap water sample: [Ca2+ ] = V EDTA ∗ M EDTA ∗ 1 mol Ca2+ 1 mol ∗ 1 mol EDTA V water = 0.004L EDTA ∗ 0.00732mol EDTA 1 mol Ca2+ 1 ∗ ∗ 1L 1 mol EDTA 0.003L water = 0.00976M Hardness of water in ppm: Hardness = [Ca2+ ] Molar mass 1000mg 40.087g Ca2+ 1000mg ∗ ∗ = 0.00976M ∗ ∗ 1 mol 1g 1mol Ca2+ 1g = 391ppm Expected mass of Ca2+ in antacid tablet: Mass CaCO3 ∗ 1mol CaCO3 1mol Ca2+ molar mass Ca2+ ∗ ∗ molar mass CaCO3 1mol CaCO3 1mol Ca2+ = 0.500g CaCO3 ∗ 1mol CaCO3 1mol Ca2+ 40.087g Ca2+ ∗ ∗ 100.0869g CaCO3 1mol CaCO3 1mol Ca2+ = 0.2003g Ca2+ Expected mass percent of Ca2+ in tablet: mass Ca2+ 0.2003g Ca2+ ∗ 100% = ∗ 100% = 10.1% Ca2+ mass tablet 1.9828g tablet Parts per solution: 0.2032g Ca2+ = 20.32 parts per solution 0.010L Ruth Pontoriero 4 Mass of Ca2+ in tablet: V EDTA ∗ M EDTA ∗ 1mol Ca2+ molar mass Ca2+ 20.32 parts ∗ ∗ 1mol EDTA 1mol Ca2+ solution = 0.0201L EDTA ∗ 0.00732M EDTA ∗ ∗ 1mol Ca2+ 40.087g Ca2+ ∗ 1mol EDTA 1mol Ca2+ 20.32 parts = 0.120g Ca2+ solution Percent Ca2+ in sample: mass Ca2+ 0.120g Ca2+ ∗ 100% = ∗ 100% = 6.05% mass of tablet 1.9828g Discussion: As the pH lowers, the form of EDTA changes such that it has fewer binding sites. As the pH is raised, more of the EDTA will be in the form which has six binding sites. The ammonia buffer is used to maintain a constant pH during the course of the titration. Calcein is a metallochromic indicator used to detect calcium. It is only useful at a high pH and therefore could probably have been used in this experiment. The York Water Company lists the Calcium content as being 23.6 ppm. The difference between this value and our calculated value may be because of the place from which the water was collected or from the method of collection, or it could be from experimental error. Soft water contains only sodium ions, therefore, according to the values by the York Water Company, the water in York is somewhat hard. Our experimental values show it to be considerably harder than reported, but those values may be inaccurate. The calculated percentage of calcium in the Tums tablet was 10.1%. All three of our experimental values (6.05%, 5.95%, and 5.90%) were less than this value. Ruth Pontoriero 5 Conclusion: Our experimental value of calcium concentration in the tapwater was considerably higher than the reported value. Our experimental value of calcium concentration for the Tums tablet was lower than the calculated value. These differences may be due to experimental error or they may be accurate. One method to possibly improve accuracy would be the use of a different indicator as the color change was a little ambiguous.