Content area
Full Text
S. K. Crossno and L. H. Kalbus California State University, San Bernardino, CA 92407
G. E. Kalbus California State University, Long Beach, CA 90840
An important laboratory experiment often included in a quantitative analysis course is the titration of mixtures containing various combinations of sodium hydroxide, sodium carbonate, and sodium bicarbonate (I, 2). Such an experiment illustrates the principles of acid-base reactions and presents an interesting problem in stoichiometry for students. A recently developed experiment also provides for a variety of new experiments involving the analysis of commercial products, such as carbonated beverages and antacids, and makes use of rubber balloons in the laboratory apparatus (3).
Any sample capable of releasing carbon dioxide, either by physical agitation or by acid treatment, can be analyzed by this method. Because many real life commercial samples contain dissolved carbon dioxide gas or some type of carbonate, the experimental procedure can be modified to accommodate the analysis of all such samples. The evolved carbon dioxide gas is passed into a balloon containing excess sodium hydroxide solution where it is absorbed and converted into an equivalent amount of sodium carbonate. Containment of the carbon dioxide in such a closed, expandable system provides the necessary contact time with the sodium hydroxide for complete absorption, which is not possible if passed through an open container of excess sodium hydroxide.
The resulting mixture consisting of the excess sodium hydroxide and sodium carbonate is titrated with standard HCl. Titration to the first colorless phenolphthalein endpoint neutralizes the excess sodium hydroxide and converts all of the sodium carbonate into sodium bicarbonate. Continuation of the titration to the second methyl orange endpoint converts the sodium bicarbonate to water and carbon dioxide. The difference in milliliters between the first and second endpoints is used to calculate the carbon dioxide present in the sample or the grams of substance being sought. When this technique was applied to the analysis of carbon dioxide evolved from standard sodium carbonate, recoveries between 98 and 100% were obtained.
The following equation is used and its derivation should also be carried...