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1 EXPERIMENT 7 ANALYSIS OF VINEGARS 07F Quality control is important for any commercial product. However, it is not likely that two batches of a product are exactly the same. The analyses of solutions are easier than the analyses of solids because a solution is homogeneous in composition. Analytical procedures to determine the concentrations of essential ingredients must be fast, reliable, and cheap. Commercial vinegars are essentially dilute solutions of acetic acid in water; consequently, the analysis is straightforward. You will work in groups of three or four during this experiment: performing the analyses, analyzing the data, and preparing a report of your results. There will be three clearly labeled bottles of vinegar in the lab for your analyses. One has not been adulterated; one has been adulterated by the addition of water; the third has been adulterated by the addition of concentrated acetic acid. You should be able to identify which is which from your analyses. The concentrations are clearly, but slightly, different. 1. Each group is to analyze one bottle by two methods to compare the precisions of the two methods. 2. Each group will also analyze all three bottles of vinegar using one method to see if the three samples are different. 3. Multiple titrations of each dilution must be made and different dilutions of each sample must be used. Triplicate analyses are not sufficient. Why is it necessary to analyze different dilutions of each sample? A. Method 1 One method of analysis is the standard indicator titration that was used in Exp. 2. Review this experiment for the procedures for the determination of acetic acid in the commercial vinegars. B . Method 2 The other method that you will use is a potentiometric (or pH) titration with the Vernier interface and a laptop computer for data acquisition and analysis. In this technique, you will measure the pH of the solution as a function of base, added drop wise. You know the general shape of a pH titration curve; however, this experiment will show you that the theoretical curves are real. The equivalence point volume is determined from the steepest part of the curve; hence, you need to plot the first or second derivative vs. V(base) as well as pH vs. V(base) {Harris, pp. 210 – 211}. Alternatively, you may use a Gran plot, V B *10 -pH vs. V B to determine the equivalence point {Harris, pp. 211 – 212}. There were “occasional” problems with the Vernier drop counters last year, which may or may not occur this fall. If you have difficulty obtaining data with the Vernier system, you may use a pH meter and record the pH of the solution after different additions of base during the titration. A manual pH titration like this is somewhat more tedious, but the second and third titrations can be done with fewer points because you know the region in which you need many points.
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