Determinacion De Acido Citrico Y Ascorbico

I n the Laboratory

Quantitative Determination of Citric and Ascorbic Acid
in Powdered Drink Mixes

W

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

Numerous student experiments have been developed to
determine the quantity of citricacid by sodium 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 + 3 NaOH
citric acid
(1)C3H5O(COO)3Na3 + 3H2O

C6H8O6 + NaOH
ascorbic
acid

C6H7O6Na + H2O

(2)

In these experiments, 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
redoxtitrations (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− + 5 I − + 6 H +
C 6H 8O 6 + I 2
ascorbic
acid

(3)

C6H6O6 + 2H+ + 2I −

I2 + I −

I3− + starch

3I2 + 3 H2O

(4)
−

(5)

I3

starch–I3− complex
(blue)

(6)

Even for low concentrations of ascorbicacid, this method is
extremely accurate. Ascorbic acid has been reported for use
as a primary standard for I2 (12). Citric acid is not oxidized
by KIO3.
This article describes a procedure by which the above
reactions are used to quantitatively determine: first, the
amount of total acid; second, the amount of ascorbic acid;
and lastly, the amount of citric acid in a given sample of powdereddrink mix.
www.JCE.DivCHED.org

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Experimental 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 ascorbicacid determined by titration of the sample with KIO3. The amount of
NaOH neutralized by citric acid is determined by subtraction as shown in eq 7:

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 samplemay be performed 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 ascorbic
acid (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...