An Alkalinity Experiment for Nonscience Majors

Abstract

his article describes a simple experiment that is designed to illustrate the concepts of buffer capacity and alkalinity, while also focusing on the geological factors that influence the susceptibility of a body of water to the effects of acid rain. The students perform a semiquantitative titration of four different water samples with a dilute solution of sulfuric acid. The titrations are monitored with a pH meter. Streams flowing through regions rich in limestone have naturally high alkalinities due to the formation of bicarbonate ions from the dissolution of calcium carbonate. The focus of the experiment is to compare the buffer capacities of water samples taken from a stream that flows through a limestone-rich region with those obtained from a stream near its source at a limestone-deficient location. Students plot the pH vs. the number of drops of dilute acid added to each of the samples. The relative buffer capacities are determined from the number of drops required to reach a pH of about 4.4. Tap water and water taken from a major river that flows through central Pennsylvania were found to have fairly large buffer capacities, while deionized water and water taken from a smaller stream near its source were found to have very low buffer capacities. The results show that different bodies of water can have vastly different susceptibilities to the effects of acid rain, depending on the concentration of the bicarbonate ion. The examination of the titration curves enable students to appreciate the sudden drop in pH that occurs once all of the bicarbonate ions have been consumed.