Calcium Determination Using EDTA THEORY AND INTRODUCTION The theory for this determination is Chapter 12 of your textbook, pages 255-272. Normal blood serum contains 4.5 to 5.5 mmol/L Ca 2+ . Calcium is the most abundant mineral in the body with 98% present in bones, 1% in teeth and the remainder in solution as a component in blood serum. When an individual is found to be calcium deficient (hypocalcemia) , calcium supplements may be prescribed. These calcium supplements are generally sold as calcium carbonate, calcium lactate, or calcium gluconate. The amount of calcium may be determined by titration with standard ethylenediaminetetraacetic acid (EDTA). This titration uses the metal ion indicator reaction to signal the end point; you can read more about this type of indicator system in your textbook, pages 261-263.

PROCEDURES A. Preparation of 0.05 M Standard Calcium Solution. - Primary standard calcium carbonate (CaCO3) is dried at 100 C for 1 hr and allowed to cool to room temperature in the desiccator. Approximately 2.5 g ( 0.1 mg) of the previously dried CaCO3 is weighted out by difference into a 400-mL beaker. The CaCO3 is dissolved in a minimum amount of 6 M HCl, transferred quantitatively to a 500-mL (+ 0.20 mL, for class A) volumetric flask, and diluted to exact volume with deionized water. The exact [Ca2+ ] may be calculated from the exact mass of CaCO3 used.

PROCEDURES B. Preparation and Standardization of 0.05 M EDTA. - Approximately 18.5 g ( .5 g) of Na2C10H14N2O8  2 H2O (disodium EDTA) and 3 or 4 drops of 3.0 M MgCl2 are placed in a clean 400-mL beaker. The masses here need not be analytical balance accuracy since the resulting solution will be standardized. (The purpose of the small amount of Mg2+ is to cause the indicator to make the best color transition - see textbook, p 261-263.) About 200 mL of deionized water are added to dissolve the edta salt. The solution process will be slow; if after reasonable time complete solution has not occurred, carefully add NaOH pellets one at a time with stirring until all solids have dissolved. Transfer this solution to a clean 1 L bottle and dilute to approximately 1 L. Be sure that the solution is well mixed. Cap and store; do not use a glass stopper because the added NaOH may cause it to “freeze” to the bottle.

PROCEDURES Use a 25.00 mL (+ 0.03 mL, for class A) volumetric pipet (TD) to transfer a sample of the standard calcium solution into a clean 250-mL Erlenmeyer flask. Neutralize the HCl added earlier with ammonia until the solution is basic to litmus. Add 15 mL (graduated cylinder) of the ammonia-ammonium chloride buffer solution and ~ .5 g of the solid preparation of Eriochrome Black T or 3 or 4 drops of calmagite indicator. Carefully titrate with the EDTA solution to the end point, which is the color change from wine-red to a pure blue; no shade of red should remain in the solution at the end point. (Both indicators are available and have the same color transitions at the endpoint; you may use either indicator, which ever one you prefer.) Repeat at least three times. Calculate the concentration of the EDTA solution; the precision ought to be 3 parts per thousand (ppt) or better.

The Unknown Carefully weigh (to the nearest 0.1mg) individually ten of the calcium tablets from the your sample. Record the mass of each tablet; calculate the mean mass and the standard deviation of mass for the tablets. Use a clean and dry mortar and pestle to grind the ten tablets used above to a fine powder. Your tablets may be coated; if they are, use the wire strainer present in the lab to remove the coating material that has not been pulverized with the mortar and pestle. Transfer the powder to a clean weighing bottle and dry at 100 C for 1 hr.

The Unknown Accurately weigh about 0.5 g ( 0.1 mg) of the powdered sample and transfer to a clean 250-mL Erlenmeyer flask. Dissolve the sample in ~50 mL of deionized water. Your tablets may not dissolve completely in water; if they are not completely soluble, add 5 mL of 6 M HCl and carefully warm the mixture to assist in the solution process. Allow the mixture to cool back to room temperature before proceeding.

The Unknown Neutralize the HCl added earlier with ammonia until basic to litmus paper. Add 15 mL of the ammonia-ammonium chloride buffer solution and ~ 0.5 g of the solid preparation of Eriochrome Black T or 3 or 4 drops of calmagite indicator. Titrate with the previously standardized EDTA solution as described above in the standardization of the EDTA. Repeat at least three times; more if results (% calcium) do not agree within about 3 ppt relative error.

The Report Your written report should give each of the following: Your unknown number. 1. The average mass per tablet and the standard deviation in mass for the set of measurements you made. 2. The average mass % Ca, its standard deviation, and the 95% confidence level interval. 3. The average mass (mg) of Ca per average tablet. Note that this value is calculated from the above two results.

REFERENCES 1. R.A. Day, Jr and A.L. Underwood, “Quantitative Analysis: Laboratory Manual, 5th Ed.”, Prentice-Hall, c1986, pages 106 - 109. 2. D.A. Skoog and D.M. West and F.J. Holler, “Fundamentals of Analytical Chemistry, 7th Edition, ” c1996, Saunders College Publishing Co., Chapter 14 of the textbook, pages 278-302. 3. Damiel C. Harris, “Exploring Chemical Analysis, 2e,” c2001, W. H. Freeman & Co, Chapter 12, pages 255-272. Also note a similar experiment, # 23-11, page 507.