Complexometry

Date of Revision: 9/6/2003

DETERMINATION OF COPPER AND NICKEL BY COMPLEXOMETRIC TITRATION

INTRODUCTION: Monel metal is an alloy of nickel and copper which is highly resistant to corrosion; it is commonly used in applications involving exposure to acids. Depending on the specific application, small quantities (less than 1% by mass) of other metals, like iron and manganese, may also beincluded in the alloy. You will be given a quantity of Monel metal as a powder and asked to determine the mass percentage of nickel and copper present in the powder. In this experiment, you will dissolve the powder sample and a portion of this solution will be analyzed by complexometric titration. The remaining portion of the solution will be saved for Experiment #4; in this experiment, the nickel andcopper present in small replicate volumes are separated using ion-exchange chromatography, then the amount of nickel is determined based on a gravimetric analysis. The copper recovered from the separation will be analyzed by iodometric titration in Experiment #5.

PRINCIPLE: The use of ethylenediaminetetraacetic acid (EDTA) as a titrant for the determination of metal ion concentrations is acommon analytical method. 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 resulting product of this reaction is a metal-chelate complex with one-to-onestoichiometry; the reaction is described as: Y4- + Mn+ ----> MYn-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 solutioncontaining 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 theintrinsic 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 formation constants see your text book.) Another condition which influences the formation constant is thepresence of species (e.g,

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ligands) in solution which can form complexes with the analyte ions. If the concentration of a ligand is large or the formation constant for its reaction with the analyte is large, the titration reaction will no longer be quantitative. This phenomenon is often exploited by analysts to determine the concentration of two or more metal ions in the same solution. As anexample, consider a solution which contains both Cu2+ and Ni2+, both of which you wish to quantify. The first step in this determination is to titrate the solution with a standardized solution of EDTA; this result will give you the combined concentrations of the Cu and Ni ([Cu2+] + [Ni2+]). The next step is to determine the Ni concentration only; this can be performed by adding thiosulfate (S2O32-)to the solution prior to titration with EDTA. The thiosulfate will selectively bind Cu and prevent it from reacting with EDTA. This titration will then give the concentration of Ni only. Lastly, the concentration of Cu can be calculated by subtracting the concentration of Ni from the combined concentration obtained in the first step. In this experiment, you will be determining the concentration...