Experiment #6
Complex-Formation Titrations with EDTA

Directions follow for a displacement titration of calcium with the magnesium/EDTA complex.

(1) Preparation of Solutions

A pH-10 buffer and an indicator solution are needed for these titrations.

(a) Buffer solution pH 10. (Sufficient for 80 to 100 titrations.) Dilute 57 mL of concentrated NH3 and 7 g of NH4Cl in sufficient distilled water to give 100 mL of solution.

(b) Eriochrome Black T indicator. (Sufficient for about 100 titrations.) Dissolve 100 my of the solid in a solution containing 15 mL of ethanolamine and 5 mL of absolute ethanol. This solution should be freshly prepared every two weeks; refrigeration slows its deterioration.

(c) Calmagite indicator. (Sufficient for about 200 titrations.) Dissolve 0.05 g of the indicator in sufficient distilled water to give 50 mL of solution.

(2) Preparation of Standard 0.01 M EDTA Solution

Procedure

Dry about 4 g of the purified dihydrate Na2H2Y.2 H20 (Note 1) at 80 oC to remove superficial moisture. Cool to room temperature in a desiccator. Weigh (to the nearest milligram) about 3.8 g into a l-L volumetric flask (Note 2). Add no more than 600 to 800 mL of water (Note 3) and swirl periodically. Dissolution may take 15 min or longer. When all the solid has dissolved dilute to the mark with water and mix well (Note 4). In calculating the molarity of the solution correct the weight of the salt for the 0.3% moisture it ordinarily retains after drying at 80 oC.

               Notes

1.    Directions for the purification of the disodium salt are described by W. J. Blaedel and H. T. Knight, Anal. Chem., 1954, 26(4), 741.

2.    The solution can be prepared from the anhydrous disodium salt, if desired. The weight taken should be about 3.6 g.

3.    Water used in the preparation of standard EDTA solutions must be totally free of polyvalent cations. If any doubt exists concerning its quality, the water should be passed through a cation-exchange resin before use.

4.    As an alternative, an EDTA solution that is approximately 0.01 M can be prepared and standardized by direct titration against primary-standard CaCO3 by displacement titration.

(3) The Determination of Calcium by Displacement Titration

Discussion

A solution of the magnesium/EDTA complex is useful for the titration of cations that form stabler complexes than the magnesium complex but for which no indicator is available. Magnesium ions in the complex are displaced by a chemically equivalent quantity of analyte cations. The remaining uncomplexed analyte and the liberated magnesium ions are then titrated; either Eriochrome Black T or Calmagite can serve as indicator. Note that the concentration of the magnesium solution is not important; all that is necessary is that the molar ratio between Mg2+ and EDTA in the reagent be exactly unity.

Procedure

Preparation of the Magnesium/EDTA Complex, 01 M     (Sufficient for 90 to 100 titrations.) To 3.72 g of Na2H2Y.2 H20 in 50 mL of distilled water, add an equivalent quantity (2.46 g) of MgS04.7 H20. Add a few drops of phenolphthalein, followed by sufficient 0.1 M NaOH turn the solution faintly pink. Dilute to about 100 mL with water The addition of a few drops of Erio T to a portion of this solution buffered to pH 10 should cause development of a dull violet color. Moreover, a single drop of 0.01 M Na2H2Y solution added to the violet solution should cause a color change to blue, and an equal quantity of 0.01 M Mg2+ should cause a change to red. The composition of the original solution should be adjusted with additional Mg2+ or H22Y2- until these criteria are met.

Titration     Weigh a sample of the unknown (to the nearest 0.1 mg) into a 500-mL beaker (Note 1). Cover with a watch glass, and carefully add 5 to 10 mL of 6 M HCI. After the sample has dissolved, remove CO2 by adding about 50 mL of deionized water and boiling gently for a few minutes. Cool, add a drop or two of methyl red, and neutralize with 6 M NaOH until the red color is discharged. Quantitatively transfer the solution to a 500-mL volumetric flask, and dilute to the mark. Take 50.OO-mL aliquots of the diluted solution for titration, treating each as follows: Add about 2 mL of pH- 10 buffer, 1 mL of Mg/EDTA solution, and 3 to 4 drops of Erio T or Calmagite indicator. Titrate (Note 2) with standard 0.01 M Na2H2Y to a color change from red to blue.

Report the number of milligrams of CaO in the sample.

               Notes

1.    The sample taken should contain 150 to 160 mg of Ca2+.

2.    Interferences with this titration are substantially the same as those encountered in the direct titration of Mg2+ and are eliminated in the same way.