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UK CHE 226 - Ecperiment 3 - Complexometric Titration of Zn(II) with EDTA
Course Che 226-
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Department of Chemistry University of Kentucky EXPERIMENT 3 Complexometric Titration of Zn(II) with EDTA UNKNOWN Submit a clean, labeled 250-mL volumetric flask to the instructor so that your unknown zinc solution may be issued. Your name, section number, and your locker number should be written legibly on this flask. The flask does not need to be dry on the inside, but needs to have been rinsed with deionized water after it has been washed. Note that the flask must be turned in at least 1 lab period before you plan to do the experiment so that the Teaching Assistants will have time to prepare the unknown. ***USE ONLY DEIONIZED WATER (NOT DISTILLED WATER!) THROUGHOUT THE ENTIRE EXPERIMENT*** BACKGROUND This experiment is an example of a classic titrimetric analysis. Classical methods of analysis such as titrimetric and gravimetric analyses are usually capable of very high precision and accuracy - typically on the order of0.1% or even better if done properly. However, there is always a tradeoff. Usually classical methods are slower and much less sensitive than modern instrumental methods of analysis such as atomic absorption spectroscopy, gas and liquid chromatography, and mass spectrometry. In a titration, an accurately known mass of sample is dissolved in an aqueous solution, often with some sort of chemical treatment such as acid-digestion of solid samples, and diluted with high purity water to an accurately known volume. Then, an accurately known volume of the sample solution, called an aliquot, is pipetted into a titration vessel and the analyte of interest is carefully titrated with a standardized solution of an appropriate titrant to the endpoint or equivalence point of the titration. To do this, you need to know when you reach the endpoint. This is often accomplished by means of an indicator that undergoes a color change at the endpoint. From the volume and molarity of the titrant, one can then calculate the number of mols of titrant used. From the known stoichiometry of the reaction between the titrant and the analyte, one can calculate the mols of the analyte and therefore the mass and/or molarity of the analyte. With appropriate calculations, one can then determine the concentration and/or total mass of the analyte in the original sample to complete the analysis. CHE 226 - Analytical Chemistry Laboratory 17 Zn EDTADepartment of Chemistry University of Kentucky This titration is known as a complexometric or chelometric titration because the titrant, a ligand, reacts with the analyte, a metal ion, to form a complex, more specifically a chelate in this case. A chelate is a ligand that has two or more sites that bind to the central ion. EDTA [ethylenediaminetetraacteic acid, C10H16N2O8, (HOOCCH2)2N-CH2CH2-N(CH2COOH)2, MM = 292.24 g/mol, often symbolized by H4Y] is an excellent chelating agent. It forms very strong 1:1 complexes with almost every divalent and trivalent metal ion depending on solution conditions. Ignoring charges for the moment, EDTA + M  MEDTA Although it is an equilibrium, the reaction lies very far to the right. The equilibrium formational constants, Kf, are on the order of 108 - 1025 depending on the metal and other conditions. EDTA itself is a tetraprotic4-acid; it has 4 ionizable protons with pKa's = 1.99, 2.67, 6.16, 10.26. In its fully ionized form, Y , the four acetate groups and the lone pairs on the two nitrogens make it a hexidentate ligand that wraps itself very tightly around a metal ion. Usually, titrations are performed in basic solution, roughly pH 8-11. The fully protonated form, H4Y, is only sparingly soluble in water, so the standard form of EDTA used analytically is usually the disodium salt Na2H4Y 2H2O (372.24 g/mol), which is much more soluble and available in primary standard purity, except for a small (about 0.3%) amount of adsorbed water. PROCEDURE Preparation of Solutions EDTA, 0.01 M. This solution must be prepared at least one day ahead of time, a week is preferable, to ensure that the solute is completely dissolved. EDTA solutions are prepared at an approximate molarity, and then standardized against a solution of a primary standard such as CaCO3. 1. Dissolve about 3.8 g of the dihydrate of the disodium salt (Na2H2Y 2H2O) and 0.1 g MgCl2 in approximately 1 L of deionized water in a large beaker or a 1-L plastic bottle using a magnetic stirrer. A small amount of sodium hydroxide can be added if there is any difficulty in dissolving the EDTA. Try not to exceed 3.8 g of the disodium salt because much more than this dissolves only with difficulty. 2. Before use, the EDTA solution should be filtered using a Buchner funnel and suction filtration. See a teaching assistant for the apparatus. [NOTE: Break the suction before you turn off the water flow on the vacuum aspirator.] 3. Store the solution in a clean, labeled 1-L plastic bottle that has been rinsed with deionized water. Never store reagent solutions in volumetric flasks. CHE 226 - Analytical Chemistry Laboratory 18 Zn EDTADepartment of Chemistry University of Kentucky Ammonia/Ammonium Chloride Buffer Stock Solution, pH 10. Each titration will require the addition of pH 10 ammonia buffer. The stock buffer solution has been prepared for you, and you should not have to prepare it. The appropriate quantity (7-8 mL) is dispensed directly into your titration flask from the plastic Repipet® repetitive dispenser located in Hood #7. The buffer should only be added immediately before you titrate an individual sample. Recipe: 1. Dissolve 64.0 g of ammonium chloride in 600 mL of concentrated ammonia (14.8 M, 28% NH3). 2. Slowly and carefully add 400 mL deionized water with stirring. This should be sufficient for over 120 titrations. Calcium Standard Solution. A CaCO3 solution is prepared as a primary standard for Ca and used to standardize the 0.01 M EDTA titrant you prepared. 1. Tap out approximately 1 g of predried analytical-reagent-grade CaCO3 in a weighboat. Accurately weigh (to within0.1 mg) approximately a 0.25-g sample by difference into a 150- or 250-mL beaker. NOTE: NEVER transfer chemicals inside an analytical balance. 2. Add about 25 mL deionized water and then slowly add concentrated HCl dropwise with periodic stirring until the sample dissolves completely. Then add 2 drops more. Keep the beaker covered


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UK CHE 226 - Ecperiment 3 - Complexometric Titration of Zn(II) with EDTA

Course: Che 226-
Pages: 7
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