Calcium oxalate is an ionic compound with the chemical formula CaC2O4 and a salt of oxalic acid. It's highly insoluble and dissolves poorly in water. One method for dissolving calcium oxalate in the lab is the application of a compound called ethylenediaminetetraacetic acid or EDTA. EDTA is highly effective at binding the calcium ions and thereby decreasing the concentration of calcium in solution, shifting the reaction equilibrium so that more calcium oxalate will dissolve. In the procedure outlined below, you'll first make calcium oxalate using common chemicals then dissolve it using EDTA.
- EDTA (available from some science supply stores)
- Calcium chloride (available at some automotive and hardware supply stores)
- Oxalic acid (available from many hardware stores)
- Fume hood
- Safety goggles
- Measuring scale
Dissolving calcium oxalate with EDTA is an application of Le Chatelier's principle. When calcium oxalate dissolves, it dissociates into calcium ions and oxalate ions. By binding the calcium ions with the EDTA, we reduce the concentration of the products, so we shift the process to the right even though the equilibrium constant does not change. Other substances that bind or react with the oxalate ions or the calcium ions should have the same effect as well.
Calcium oxalate and oxalic acid are hazardous if ingested or allowed to come in contact with eyes or skin. Never perform this experiment without gloves, safety goggles and proper safety equipment. Never eat or drink preparations of oxalic acid or calcium oxalate. Never ingest EDTA or bring it in contact with your eyes or skin except as directed by a physician.
Put on the goggles and gloves. Oxalic acid and calcium oxalate are potentially toxic if ingested. Perform this experiment under the fume hood for safety.
Measure out .032 ounces (.9 grams or approximately .01 moles) of oxalic acid into the beaker and add just under .338 fluid ounces (10 milliliters) of water. Swirl it gently in the beaker until the oxalic acid dissolves.
Add .049 ounces (roughly 1.3 grams) of calcium chloride to the solution and swirl gently. As the reaction progresses, solid calcium oxalate will form and precipitate out of the solution. You now have calcium oxalate--the same substance from which kidney stones often form.
Add about .01 ounces (.29 grams) of EDTA to the solution and swirl it gently in the beaker. Some of the calcium oxalate should begin to dissolve.
Things You'll Need
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