Reactivity: Synthesis of Kidney Stones

• To learn techniques of synthesis and separation
• To explore some possible chemistry of kidney stones

Kidney stones are crystals of calcium oxalate, CaC₂O₄, that form in the kidneys when oxalic acid is present. Calcium is always present in extracellular fluids at a fairly high concentration. Calcium oxalate is extremely insoluble in water: only about 1*10^-5 g of calcium oxalate will dissolve in 100 mL of water at room temperature. Consequently, once kidney stones form, they will not readily redissolve.

In this experiment, you will synthesize calcium oxalate starting from calcium carbonate and oxalic acid, H₂C₂O₄ according to equations (1) and (2).

Several types of reactions are involved in the synthesis, including Bronsted acid-base, precipitation, and Lewis acid-base reactions.

Bronsted acid base reactions involve the transfer of hydrogen ions, H⁺, from one substance to another. The substance donating the hydrogen ion is called an acid, and the substance accepting it is called a base. Reaction (1) is a Bronsted-Lowry acid base reaction. Some other examples of acid-base reactions are given in Table 1.

Lewis acid base reactions involve the donation of an electron pair from one species to another. The recipient of the pair of electrons is called the acid; the donor of the pair of electrons is called the base. The union of acid and base is called the adduct. Some examples of Lewis acid-base reactions are given in Table 2. Note that Bronsted-Lowry acid base reactions may be interpreted in terms of the more fundamental Lewis ideas.

Precipitation Reactions result in the formation of a solid when two solutions are mixed. The solid is called a precipitate. Reaction (2) is a precipitation reaction. Some other examples of precipitation reactions are given in Table 3.

After you synthesize and isolate kidney stones (calcium oxalate), you will attempt to dissolve them by treating them with reagents that have high affinity for either the Ca²⁺ ion or the oxalate anion, C₂O₄^2-. To be effective, these substances much have such high affinity for the respective ions that they overcome the high affinity of the calcium and oxalate ions for one another. EDTA^4- has affinity for Ca²⁺, and may react with CaC₂O₄ according to equation (3).

Fe³⁺ has affinity for C₂O₄^2-, and may react with CaC₂O₄ according to equation (4).

Focus Questions

As you proceed through the experiment, keep the following questions in mind. When you have finished the experiment, provide answers to them.

1. In Part 1, what is the significance of having unreacted CaCO₃ present at the end of reaction?
4. What type of reaction is occurring in equation 1, BL acid-base, precipitation, or Lewis acid-base?
5. What type of reaction is occurring in equation 2, BL acid-base, precipitation, or Lewis acid-base?
6. What type of reaction is occurring in equation 3, BL acid-base, precipitation, or Lewis acid-base?
7. What type of reaction is occurring in equation 4, BL acid-base, precipitation, or Lewis acid-base?

Equipment and Materials

• assorted beakers and flasks
• Pasteur pipets
• aspirator trap
• filter paper
• calcium carbonate (CaCO₃)
• HCl (6 M)
• oxalic acid
• Na₄EDTA
• Ferric nitrate hexahydrate (Fe(NO₃)₃.6H₂O)

Safety goggles must be worn at all times in the laboratory. Hydrochloric acid is corrosive. Wear gloves when handling it. In the event of skin contact, flush the affected area with copious quantities of cold water.

Record all data and observations in your notebook.

Part 1: Making Kidney Stones Weigh 1.05 g of calcium carbonate into a small beaker, and add 4-5 mL of water. Swirl to achieve a slurry. Calculate the volume of 6 M HCl needed to react with 1.00 g of calcium carbonate according to equation (1). With continuous stirring or swirling, add this volume of 6 M HCl to the calcium carbonate beaker using a Pasteur pipet. When reaction is complete (how will you know?), suction filter the mixture to remove unreacted calcium carbonate. Transfer the filtrate to a clean beaker.

Weigh 0.90 g of oxalic acid into a small beaker, and add about 5 mL of water. Warm gently to dissolve the oxalic acid. Using a Pasteur pipet, add the solution of Ca²⁺ to the oxalic acid solution, with swirling. When addition is complete, let the beaker stand for a few minutes so the solid can settle. Suction filter the mixture. Wash the solid twice with small (3-mL) portions of acetone, then suction the product dry. Weigh the dried calcium oxalate and calculate percent yield.

Part 2: Dissolving Kidney Stones Weigh about 0.128 g (1 mmole) of the product from Part 1 into a small beaker, and add 10 mL of water. Add 0.380 g Na₄EDTA. Warm the mixture gently, swirling occasionally. What happens?

Weigh about 0.128 g of product from Part 1 into a small beaker, and add 5 mL of water. Weigh 0.133 g of Fe(NO₃)₃.6H₂O and dissolve it in about 5 mL of water. Add the resulting solution to the beaker containing the calcium oxalate. Warm the mixture gently over a burner flame. What happens?

Clean-up. When you have finished all of your work:

• Clean glassware by recommended procedures, shake off excess water, and place in the drying oven.
• Return all borrowed equipment to the instructor before leaving lab.
• Clean up your work area before leaving lab.

Place all solutions in the appropriately marked waste bottles. After weighing the product, place it in the appropriate waste bottle.

Preparation Questions