Nitrogen Excretion

In this exercise, students compare the relative "merits" of ammonia, urea, and uric acid as excretory forms of nitrogen. The features they consider are solubility, toxicity, nitrogen content, and oxidation state of associated carbons. The only feature they actually examine is solubility.

In the experimental part of the exercise, students determine the solubility of uric acid and urea in water. To a preweighed test tube, students add about 2 mL of water and reweigh the tube to determine the amount of water accurately. They then add uric acid in (very) small amounts until no more dissolves; typically, no one adds more than the first small amount because uric acid is very insoluble. They reweigh the tube to determine the amount of uric acid added and calculate the (maximum) solubility of uric acid in water on a g/g basis. Then they repeat the process with urea. They typically find that 2 mL of water will dissolve almost 2 g of urea.

The exercise continues with paper work: calculation of the weight percent nitrogen in ammonia, urea, and uric acid; determination of excess electrons on carbon (none for urea, six for uric acid) compared with another typical 5-carbon compound, valine (24 electrons); and calculation of weight percent nitrogen in saturated urea solution. Students complete the exercise by filling out a table that compares ammonia, urea, and uric acid in terms of toxicity, solubility, nitrogen content, oxidizable carbons, and types of animal that might use each.