Box 6-1 Studying a Marine Ecosystem

Students at Harvey Mudd College monitor a saltwater aquarium to study the chemistry of a marine ecosystem. When fish and food are introduced into the aquarium on day 0 in panel a, organic compounds are metabolized to produce NH3. Anunonia is toxic to marine animals when the level exceeds 1 ppm but, fortunately, it is removed by Nitrosomonas bacteria, which colonize the aquarium filter and oxidize NH3 to nitrite (NO2). Alas, NO2 is also toxic at levels above 1 ppm, but it is further oxidized to nitrate (NO J) by a second colonization of Nitrobacter bacteria. The natural process of oxidation of NH3 to NO2 and NOJ is called nitrification. 

Some actions of nitrogen-metabolizing bacteria. Heterotrophic bacteria require complex organic molecules from the breakdown of other organisms for nourishment. By contrast, autotrophic bacteria can utilize CO2 as their carbon source for biosynthesis. 

and NO3 concentrations in a saltwater aquarium at Harvey Mudd College after fish and food were introduced into the tank on day 0. Concentrations are expressed in parts per million of nitrogen (i.e., pig of N per mL of solution). [Data from Hal Van Ryswyk, Harvery Mudd College.] 

Concentrations of O2 and ammonia and the rate of ammonia consumption in sediment from the Niida River in japan in the dark (b) or exposed to light (c). Ammonia was measured as ammonium ion (NH ), which is the predominant form at pH 7 to 8 in the river. [Data from Y. Nakamura, H. Satoh, T. Kindaichi, and S. Okabe, Environ. Sci. Technol. 2006, 40,1532.) 

For best results, treat the oxalic acid solution at 25°C with 90% to 95% of the expected volume of KMn04. Then heat the solution to 60°C and complete the titration. 

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