The use of aquatic plants to help demonstrate chemical buffering

In a simple but clear class experiment, submerged aquatic plants can be used to demonstrate buffering by calcium hydrogen carbonate in freshwaters. With the plants submerged in an aquarium, measurements were made of aquatic oxygen concentration and pH in either buffered (i.e. hard water with high concentrations of calcium hydrogen carbonate) or buffered (i.e. soft water with low concentrations of calcium hydrogen carbonate) and in illuminated or dark conditions. For the hard water trial where the plants were illuminated, there was an increase in oxygen concentration probably due to photosynthesis. In the dark, the oxygen concentration fell, probably due to respiration. With soft water, the oxygen concentration rose during the course of the experiment but at the end, oxygen concentration was marginally lower than at the start. In general, the soft water trials showed larger relative changes in pH than did the hard water trials. It would appear that without buffering (soft water), carbon dioxide from plant respiration produces decreases in pH. With buffering (hard water), the pH shows comparatively small changes. The possible buffering mechanism (analogous to that in human blood) is explained.