Edta

Determination of Mg by Titration with EDTA

INTRODUCTION: Many metal ions react with electron pair donors to form coordination compounds or complex ions. The formation of a particular class of coordination compounds, called chelates, are especially well suited for quantitative methods. A chelate is formed when a metal ion coordinates with two (or more) donor groups of a single ligand. Tertiary amine compounds such as ethylenadiaminetetraacetic acid (EDTA) are widely used for the formation of chelates. Complexometric titrations with EDTA have been reported for the analysis of nearly all metal ions. Because EDTA has four acidic protons, the formation of metal-ion/EDTA complexes is dependent upon the pH. For the titration of Mg2+, one must buffer the solution to a pH of 10 so that complex formation will be quantitative. The reaction of Mg2+ with EDTA may be expressed as: Mg2+ + H2Y2- = MgY-2 + 2H+ The structure of EDTA and the magnesium-EDTA complex (without the hydrogen atoms) is shown below:

The endpoint of the titration is determined by the addition of Eriochrome Black T, which forms a colored chelate with Mg2+ and undergoes a color change when the Mg2+ is released to form a chelate with EDTA. While it is possible to achieve relatively good results by titration with EDTA prepared directly from the solid, better results should be obtained when the EDTA is standardized against a solution containing a known amount of metal ion. You will be provided with a standard solution of Zn2+ which you will use to standardize your EDTA solution.

Truman State University CHEM 222 Lab Manual Revised 01/04/08

REAGENTS AND APPARATUS EDTA (Na2H2Y 2H2O) pH 10 buffer (2000 mL has been prepared by dissolving 140.0 g of NH4Cl in 650 mL of deionized water, adding 1136 mL of conc. ammonia and diluting to 2000 mL) Eriochrome Black T (ground 1:10 with NaCl) pHydrion paper Standard Zn solution: (This has been prepared for you.) An accurate mass (~1.3g) of pure zinc has been dissolved in a...