Analyticalchemlab 8 - Lab report PDF

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Experiment 8 Determination of Total Hardness of Water by Complexometric Titration Lab instructor-Andrew Cho William Parnell 4/11/2017

Abstract: The hypothesis for this experiment was to determine if complexometric titration technique can identify the total “hardness” of water through mathematical means. The issue with identifying the “hardness” of water is that the method is subject to both random and systematic error. The significance of this lab is that hard water can shorten the life of machinery that uses water as a part of the operation. By determining the hardness of water, adjustments can be made to the water to extend the life of the machinery. This type of experiment is used in some professional settings such as detergent companies to test the solubility of their soap and industrial companies test the “hardness” of the water to protect against mineral deposits in the water (source). This lab used the complexometric titration to determine the amount of calcium ions in a solution and in the tap water. The indicator can detect both calcium ions and magnesium ions in a solution. Both solutions from procedure one and procedure two were titrated with EDTA. The two solutions turned colors from red wine to pure blue. Introduction: The hypothesis for this experiment was to determine if complexometric titration technique can identify the total “hardness” of water through mathematical means. This titration technique is

used by soap companies to test the effectiveness of their soap as if the water is too “hard” the soap will not work effectively. To further extrapolate on the definition of “hardness” of water. Hardness is defined as the terms of the capacity of the cations in the water to replace the sodium or potassium cations in soaps. Since calcium and magnesium ions are most present in water the “hardness” is expressed in the concentration of calcium carbonate. Furthermore, companies use this type of titration to see how readily corrosion and build up will occur in machinery and equipment that operate using water. Complexometric titrations are also useful for testing the quality of water for household appliances. This sort of titration is used to determine the amount of metal cations with anions or neutral molecules. The metal ion in the complex is called the central atoms, and the group attached to the central atom is called the ligand. Complexometric titrations are used to detect ions in the water that are deemed as undesirable. This test is important in many industries that use water for various applications such as the effectiveness of soap in certain conditions and in water treatment facilities to test the hardness of the water. The water “hardness” is determined by an EDTA titration after the solution is buffered to pH 10. Magnesium forms the least stable EDTA complex is not titrated until an adequate amount of reagent is added to the complex. The indicator that is used is called Eriochrome Black T. This indicator can detect magnesium ions in a solution. A few drops of EDTA are sufficient enough to detect magnesium ions in a solution. Since this experiment involves the detection of calcium ions in a solution. It can be concluded that calcium ions are analogous to magnesium ions with a few exceptions. The first exception is that calcium ions are more stable complex than magnesium complex. Thus, it is possible that more solution must be used or that the pH has to be higher. The second exception is

that tap water is not a homogenous mixture and that magnesium ions may form a complex more readily than the calcium complex thus giving inaccurate results. The results from each experiment were calculated. Methods: The method used in experiment involved a titration technique called complexometric titration. This titration is used to determine the “hardness” of water. This experiment was split into two separate procedures. The first procedure was to determine the amount of calcium ions in water and in the prepared solution, three separate 250mL Erlenmeyer flasks containing 50mL of the calcium carbonate solution. Five (5) mL of ammonia-ammonium chloride was added separately to each of the three 250mL Erlenmeyer flasks. Finally, fifteen drops of Eriochrome Black T indicator were added separately to the three 250mL Erlenmeyer flasks. An apparatus was set-up with a 50mL buret containing EDTA. The EDTA was allowed to drip into the solution in the three separate 250mL Erlenmeyer flasks until the solution changed color for wine red to pure blue. The second procedure involved the same concepts as the first procedure. However, the second procedure involves the use of tap water. Three separate Erlenmeyer flasks were filled with 50mL of tap water. Then, 5mL of ammonia-ammonium chloride added to each of the three 250mL Erlenmeyer flask, separately. Each of the three Erlenmeyer flasks were titrated with EDTA separately until the solution in the flasks changed colors from red wine to pure blue. The measurements from both procedures were recorded and calculated. Discussion: The experiment was separated into two separate procedures. The titration technique called complexometric titration was used. The first step involved the set-up of a 50mL buret and

three separate 250mL Erlenmeyer flasks. Then use the pre-made solution that consisted of disodium dihydrogen EDTA dihydrate, magnesium chloride with water in the precipitate and, deionized water. This solution was mixed by the teaching assistant (TA). The TA mixed the solution and labeled the solution for later use. Since the solution was prepared before class these steps were not performed by this student. The next step of the procedure one involved two lab groups that shared the prepared solution. A lab student weighed accurately 0.4g of primary standard of calcium carbonate in a 200mL beaker. The calcium carbonate that weighed out in the beaker was transferred into a 500mL volumetric flask and 100mL of deionized water was added to the flask. A 1:1 hydrochloric acid was added dropwise to the volumetric flask until the solution in the flask stopped bubbling. Deionized water was added again till the line on the volumetric flask. The second procedure differed from the first since it involved the use of 50mL of tap water instead of calcium carbonate solution. The set-up of the second procedure was identical to the first procedure since the analyte for procedure two involves an unknown concentration of metal ions in the tap water. A 50mL buret that was filled with EDTA titrated the analyte in the 250mL Erlenmeyer flasks. This step was repeated twice more. The data were gathered and calculated from both procedures. Calculation: Conclusion: The hypothesis for this experiment was to determine if complexometric titration technique can identify the total “hardness” of water through mathematical means. The outcome of this hypothesis is confirmed by the data in the calculations section of this lab report. This lab experiment demonstrated the successful implementation of complexometric titration to test the

“hardness” of tap water. Possible sources of errors in this lab may have resulted from two known systematic errors and random errors. The two known systematic errors that came were weighing the calcium carbonate as the scale was not properly balanced or from measurement error of the titrant as too much may have been added. Possible sources for random error may have been from not measuring correctly. The lab could have been improved by …...