3-lab3-volumetric detn of NiAS - Chem 223: Experiment 3...

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Unformatted text preview: Chem 223: Experiment 3 Complexometric Determination of Nickel (This experimental procedure of this experiment is mainly based on the lab. “Volumetric determination of Nickel”. Chem 223. UIUC) References : 1. C. Chaps 7, 9 (223-235), 3 (pp. 79-86) 2. Harris, 7th ed. Chap. 12 3. S&W Chaps 6, 10, pp581-583 4. SWH, Chaps 4, 11, pp. 771-773 I. Purpose of the experiment A volumetric titration is one in which the volume measurement capabilities of the burette are exploited to determine a highly precise value of the mass of one reagent needed to react stoichiometrically and completely with an unknown. The particular type of titration being examined in this lab is a complexometric titration in which the titrant, EDTA, is a ligand that can react completely with the analyte, Ni(II), with a well-defined stoichimetry. This lab also introduces the concept of a secondary standard, since the EDTA is standardized against CaCO 3 to allow calculation of the amount of nickel in the solution II. Introduction The use of ethylenediaminetetraacetic acid (EDTA) as a titrant for the determination of metal ion concentrations is a common analytical method. HOOC - CH 2 CH 2- COOH >N - CH 2- CH 2- N< HOOC - CH 2 CH 2- COOH EDTA EDTA is a hexaprotic acid (pK 1 = 0.0, pK 2 =1.5, pK 3 = 2.0, pK 4 = 2.8, pK 5 = 6.2, pK 6 = 10.3), since each of the carboxyl groups and H + on the amines is ionizable. The completely deprotonated (dissociated) EDTA molecule is capable of forming up to 6 separate coordination bonds with a single metal ion; this is accomplished by donation of six separate lone pairs of electrons on the dissociated EDTA molecule to empty orbitals existing on the metal ion. The structure of EDTA and the magnesium-EDTA complex (without the hydrogen atoms) is shown below: -- Converted from Word to PDF for free by Fast PDF -- www.fastpdf.com -- The resulting product of this reaction is a metal-chelate complex with one-to-one stoichiometry; the reaction is described as: Y 4- + M n+ ----> MY n-4 where Y represents the completely dissociated EDTA molecule and M represents the metal ion. The reaction is usually rapid and, under the correct conditions, quantitative (conversion of 99.9% of the metal ions into a metal-chelate complex is achieved at the equivalence point). The condition of greatest concern is the pH of the solution containing the metal ion. Since EDTA is a weak, polyprotic acid and can only react effectively with a metal ion when it is completely dissociated, the pH of the metal ion solution directly affects whether the titration can be performed quantitatively. This is reflected in the conditional formation constant for the titration; a conditional formation constant for an EDTA titration is the product of the intrinsic formation constant (the equilibrium constant for the condition where all EDTA is completely dissociated) and the fraction of EDTA which is completely dissociated. This fraction is determined by the pH of the solution containing the metal ion. (For a more detailed of conditional determined by the pH of the solution containing the metal ion....
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3-lab3-volumetric detn of NiAS - Chem 223: Experiment 3...

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