Acid Content of Beverages A General Chemistry Experiment Involving the Resolution of Apparent Anomalies William H. Fuchsmanl and Sandhya Garg Oberlin College. Oberlin. OH 44074 For the past five years we have introduced first-semester general chemistry students to the experimental consequences of the differences between strong and weak acids through a comparison of acidities measured by p H with acidities measured by titration with hydroxide ion solutions. We find that despite their being able to explain on demand the differences between strong and weak acids, most students do not exDect different ex~erimentalmethods of measuring aciditiesof weak acids tiprovide strikingly different lahoratorv resiults. Thus we can connect the chemistrv of strong add weak acids not only to the world outside-the laboratow hut also to the kinds of problem solving that engage r e d chemists by asking studedts (1) to measure the aciditv of a sour fruit beverage by two methods, (2) to explain t h e discrepancies they bbtiin, and then (3) to show experimentally by use of known laboratory acids that their explanations are valid. We use apple juice as the sour beverage, but in this report we provide data for other beverages as well. In our curriculum the experiment precedes by a semester thediscussion oftitration curves but follows both thedefinition of p H in terms of [H30+] and also a discussion of the relative reactivities of strong and weak acids. The experiment involves titration to an arbitrary p H 7 e n d p ~ i n t . ~ Experimental Equipment and Reagents reauires The eroeriment meter (standardized at oH 41. a . . ~. a oH .~~~ magnetic stirrer and stir bar, a 50-mL buret, and a 100-mLheaker. In addition to a supply of a beverage suitable for analysis, the experiment requires aqueous solutions of NaOH (standardized at O.IW M and 0.00100 M),acetic arid (approx~mately0.1 MI, and hydrochloric acid (approximately0.001 M)?.$
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Procedure Each student places 25 mL of apple juice in s 100-mL beaker on the magnetic stirrer, stirs briefly, and then measures the initial pH of the sample. With the pH electrode still in place, the student resumes stirring and adds 0.100 M aqueous NaOH from a buret until the samole reaches . oH I .. at which mint the student records the volume oi titrant nmumed. The student repeats both measurements and calculates average experimental values f
