DETERMINATION OF CALCIUM WITH EDTA

OBJECTIVES:

1. The student will perform a complexometric titration using EDTA.

2. This method requires pH adjustment for good results.

PROCEDURE:

Solutions needed:

1. 0.05 M EDTA. Prepare 500 mL of 0.05 M EDTA. Either dry the EDTA for 2 hours at 80o C or plan to standardize the EDTA using CaCO3,

2. 6 M HCl

3. 1 M KOH (75 mL of 4 samples)

4. 20 % sucrose (aqueous)

5. 0.2 M potassium carbonate (aqueous)

The analysis:

Dissolve 0.3 gram of unknown in a minimum amount of 6 M HCl in a 400 mL beaker. Ignore silica. Add 10 mL of deionized water. Heat to boiling. Cool and dilute to 100 mL with di. Add about 2.3 of the expected amount of EDTA titrant (assume that the calcium unknown is about 50% calcium oxide). Add 1 M KOH drop-wise until the sample is neutral (1 to 2 mL). Then add 1 mL of 20% sucrose and 2 mL of 0.2 M potassium carbonate solution. Quickly add 1 M KOH to make the pH between 12 and 12.5 (try 15 mL for the first sample). Immediately add about 200 mg (one shot) of Hydroxy-Naphthyl blue indicator and quickly titrate until the last trace of red changes to blue for 30 seconds. The pH adjustment is to remove the magnesium present in the sample.

Standardize the EDTA using CaCO3. Use aliquots for the standardization. Dissolve the CaCO3 by adding a minimum amount of 6 M HCl, add 50 mL of di, heat to boiling. Cool. Treat like the unknown.

CaCO3 + 2 HCl ® Ca2+ + 2 Cl- + CO2­ .

NOTE: The method of aliquots can be used for this method. If this method is to be used, add 2.4 grams of unk. in a 600 mL beaker. Dissolve in a minimum amount of 6 M HCl. Add 50 mL of di. Heat to boiling. Cool and transfer to a 200 mL vol. flask. Dilute with di. Take a 25 mL aliquot and dilute to 100 mL. Follow the above procedure with each sample individually. If you have questions, you should see the instructor.

CALCULATION: Calculate percent calcium oxide (CaO) in the sample. Include a computer print-out for the calculation if the calculation is done on the computer. Check your text for more details.

SAFETY AND DISPOSAL INFORMATION: Dispose of all wash solutions in the "Ca EDTA WASTE" container.

CAUTION: Remember to treat all chemicals with respect.