Química

In the Laboratory

Quantitative Determination of Citric and Ascorbic Acid in Powdered Drink Mixes A High School or General Chemistry Experiment
Samuella B. Sigmann* and Dale E. Wheeler A. R. Smith Department of Chemistry, Appalachian State University, Boone, NC 28608; *sigmannsb@appstate.edu

W

Numerous student experiments have been developed to determine the quantity of citric acid bysodium hydroxide titration in a sample of food or drink (1–7). Most food samples that contain citric acid also contain ascorbic acid. Since both citric and ascorbic acid react with sodium hydroxide during titration, the total acid content (both citric and ascorbic acid) of the sample is actually being determined as seen in eqs 1 and 2:
C3H5O(COOH)3 + 3NaOH citric acid (1) C3H5O(COO)3Na3 + 3H2OExperimental Over view Common, commercially available powdered drink mixes (e.g., Kool-Aid), contain citric and ascorbic acid in an approximately 100:1 (w w) ratio. Titration with standardized NaOH gives the amount of NaOH neutralized by the total acid in the sample. The amount of NaOH neutralized by ascorbic acid is equal to the amount of ascorbic acid determined by titration of the sample withKIO3. The amount of NaOH neutralized by citric acid is determined by subtraction as shown in eq 7:

C6H8O6 + NaOH ascorbic acid

C6H7O6Na + H2O

amount NaOH amount NaOH amount NaOH neutralized − neutralized = neutralized (7) (total acid) (ascorbic acid) (citric acid)
The amount of citric acid is determined according to eq 1. This article describes the development of the experiment andpresents data obtained by the authors. Student-generated data are also included and compared to that generated by the authors. Students gain practical experience in titration techniques. Concepts of acid–base chemistry as well as oxidation–reduction reactions can be demonstrated. Calculations to determine the mass of each of the two acids present and percent by mass of each acid in the sample may beperformed and enables students to relate learned information to “real-world” commercially-available products. For ascorbic acid, the percent RDA (Recommended Dietary Allowances) may be calculated and compared to the value listed on the package label (5). Experimental Development by the Authors A standardized 0.09740 M solution of NaOH was prepared. The reaction ratios of citric acid (1:3) and ascorbicacid (1:1) to NaOH were verified by titration using thymol blue as the indicator according to eqs 1 and 2. The citric acid was Certified ACS grade (Fisher Scientific). Pure L-ascorbic acid was purchased at a local health food store. The ratio of ascorbic acid to KIO3 (3:1) was verified by titration according to eqs 3–6. Thymol blue was selected as the best indicator for the student procedure. Inthe titration with KIO3, the endpoint was marked by the appearance of the blue starch–I3− complex. These indicators worked well in all but the darkest powdered drink mixes. Blue and purple mixes were avoided for this procedure. The precision and accuracy of the method was tested. One liter of a solution with known amounts of citric acid
• Journal of Chemical Education 1479

(2)

In theseexperiments, the amount of ascorbic acid is generally assumed to be insignificant with respect to the amount of citric acid present and the total acid amount is equated to the citric acid alone. There are also numerous student experiments that determine the amount of ascorbic acid in a sample by various redox titrations (8–10). One example is oxidation of ascorbic acid by potassium iodate, KIO3 (11).Ascorbic acid is readily oxidized by KIO3 in a 3:1 ratio according to eqs 3–6: IO3− + 5I − + 6H + C 6H 8O 6 + I 2 ascorbic acid 3I2 + 3H2O (3)

C6H6O6 + 2H+ + 2I − (4)
−

I2 + I −

I3

(5)

I3− + starch

starch–I3− complex (blue)

(6)

Even for low concentrations of ascorbic acid, this method is extremely accurate. Ascorbic acid has been reported for use as a primary standard...