Authors: Rachel Casiday and Regina Frey
Key Concepts: Acid-base equilibria; equilibrium constants; buffering; Le Châtelier's Principle.
Many people today are interested in exercise as a way of improving their health and physical abilities. But there is also concern that too much exercise, or exercise that is not appropriate for certain individuals, may actually do more harm than good. Exercise has many short-term (acute) and long-term effects that the body must be capable of handling for the exercise to be beneficial. In particular, exercise initiates chemical changes in the blood which, unless offset by other physiological functions, cause the pH of the blood to drop. If the pH of the body gets too low (below 7.4), a condition known as acidosis results. This can be very serious, because many of the chemical reactions that occur in the body, especially those involving proteins, are pH-dependent. Fortunately, we have buffers in the blood to protect against large changes in pH. This tutorial describes, in detail, the most important pH buffer in the blood, the bicarbonate-buffer system, and briefly tells about some of the other pH buffers in the blood. To explain how buffers work, the tutorial uses both quantitative descriptions (e.g., equilibrium constants and titration curves) and qualitative descriptions (e.g., equilibrium shifts and Le Châtelier's Principle). The tutorial also shows how other organs in the body, such as the kidneys and the lungs, can help to control pH by removing components of the bicarbonate-buffer system that are present in excess. This tutorial is one of four blood-chemistry tutorials available for General Chemistry from Washington University in St. Louis.
At Washington University in St. Louis, this tutorial accompanies a second-semester acid-base titration experiment. In the experiment, students use phenolphthalein and a pH meter to determine the concentration and pKa of an unknown weak acid. Students generate a titration curve and describe acid-base equilibria quantitatively. The tutorial helps students apply this quantitative knowledge to a situation that is very relevant to most students (exercise), and enhances the students' quantitative understanding of equilibria with a qualitative viewpoint.
