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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