In the Laboratory

Ascorbic Acid as a Standard for Iodometric Titrations

An Analytical Experiment for General Chemistry
Cesar R. Silva, Jose A. Simoni, Carol H. Collins, and Pedro L. O. Volpe*
Institute of Chemistry, Unicamp, P.O. Box 6154, 13083-970 Campinas, So Paulo, Brazil; *volpe@iqm.unicamp.br
 Vitamin C is a familiar name to chemistry students, Experimental Procedure
and simple experiments that determine the amount of
vitamin C in pharmaceutical prescriptions are important Glassware and Reagents
in the general chemistry laboratory (1, 2). This procedure All reagents were of analytical grade or better. A buret
involves an ascorbic acid redox titration with iodine (50 mL) and a microburet (5 mL) were used without cali-
solution using starch as an indicator: bration. A pipet (10 mL) was calibrated with water (5
repli-cates), giving (9.970 0.004) mL.
 H
O O OH
CH CH OH(aq) Preparation of
2 + I2(aq) Solutions
 HO OH Iodine (3.8 g) and 20 g of potassium iodide (KI) were
completely dissolved in 20 mL of distilled water and the
Ascorbic acid (C6H8O6) mix-ture was diluted to 1 L. This iodine solution (0.015 mol
{1
L ) was stored in a dark flask for further standardization.
Twenty-five grams of sodium thiosulfate (Na 2S2O3?5H2O)
H
O O OH and 0.1 g of sodium carbonate (Na 2CO3) were dissolved in
CH CH OH(aq) distilled water and diluted to 1 L in a volumetric flask. The
2 + 2 H+(aq) + 2 I(aq)
function of carbonate in the 0.1 mol L {1 thiosulfate solution is to
O O
prevent decomposition by light and bacterial action.
Dehydroascorbic Starch (0.1 g) was mixed with 100 mL of distilled
acid (C6H6O6) water and stirred until the almost complete dissolution of
the starch. The mixture was then heated to boiling for 2
The end point of this titration is determined by the first ex- minutes to obtain total dissolution. The starch indicator
cess of iodine in the reaction medium that reacts with starch, solution was left to cool and stored in a plastic bottle.
forming a complex with an intense dark blue-violet color (3). Potassium dichromate (for 2 h at 393 K before in the
In conventional analytical iodometric experiments, most reaction was titrated with the sodium thiosulfate solution to the
of the experimental work is involved with the standardization appearance of a pale yellow coloration. At this point,
of the 3 mL of the starch solution was added, and the titration
, the amount of vitamin C in some pharmaceutical was continued until the disappearance of the blue color of
tablets. the iodinestarch complex.
Standardization of the Iodine Solution
with the Standardized Na2S2O3 Solution
A 10-mL aliquot of the iodine solution was transferred to a
conical flask, diluted with 40 mL of distilled water, and titrated
with the previously standardized sodium thiosulfate solution
until the brown color changed to pale yellow. Immedi-ately, 2
mL of the starch solution was added and the titration was
continued until the intense blue color disappeared.
Standardization of Iodine Solution with Ascorbic Acid
Approximately 70 to 90 mg of analytical grade ascorbic
acid (Merck), dried at 393 K for 2 h and weighed to within
0.00001 g, was quantitatively transferred to a conical flask
and dissolved in 50 mL of distilled water. Two milliliters of
starch solution was added, and the colorless ascorbic acid
JChemEd.chem.wisc.edu Vol. 76 No. 10 October 1999 Journal of Chemical Education 1421
 In the Laboratory

solution was immediately titrated with the iodine solution. The titration must be performed
immediately to prevent air-oxidation of the ascorbic acid (6 ). The end point was deter-mined when a
pale blue color remained with the first excess of iodine.
Determination of Vitamin C in Pharmaceutical
Preparations
A pharmaceutical tablet of 2 g of vitamin C (Redoxon, manufactured by Roche) was ground in a
beaker and then quantitatively transferred to a 250-mL volumetric flask and diluted to the mark with
distilled water. Aliquots of the solution were taken using a calibrated pipet and titrated with the
standardized iodine solution as described above.

Results and Discussion

The conventional chemical procedure to standardize iodine solutions has the disadvantage of being very
time consuming, because of the two chemical reactions shown below:
2{ + { 3+
Cr2O7 (aq) + 14H (aq) + 6I (aq) 3I2(aq) + 2Cr (aq) + 7H2O
2{ 2{ {
6S2O3 (aq) + 3I2(aq) 3S4O6 (aq) + 6I (aq)
According to these reactions, the concentration of the
Na2S2O3 solution is calculated by taking into account that each mole of K 2Cr2O7 reacts with 6 moles of
Na2S2O3. Using this conventional procedure the average value (3 replicates) of the concentration was (0.1078
0.0001) mol L{1, with a standard deviation of less than 0.2 parts per thousand.
The next step was the standardization of the iodine solu-tion with the standardized thiosulfate
solution. The average value (6 replicates) of the iodine concentration found was
{1
(0.0152 0.0001) mol L , with a standard deviation of 7 parts per thousand.
The results for the standardization of the same iodine solution using untreated ascorbic acid as a
{1
standard gave an average value (7 replicates) of (0.0152 0.0001) mol L for the concentration of the
iodine solution, corresponding to an error of 3 parts per thousand. This value is the same as that obtained
from the standardization of the iodine solution with standardized thiosulfate, within experimental error.
Using ascorbic acid, previously dried at 393 K for 2 h, as the standard, the average value (3 replicates) of
the iodine concentration was (0.0152 0.0001) mol L{1, the same as used without any prior treatment.
Thermogravimetric analysis of this ascorbic acid showed that its water content was very low (about 0.5%) even
before drying, and so the standard-ization of the iodine solution is not affected when this pro-
cedure is carried out with undried ascorbic acid.
As recommended in the literature (6 ), a standard for titra-tion has to be readily available, of high
purity, stable in air, of relatively high molecular mass, soluble in titration solvent, and not hygroscopic so
that it can be weighed accurately. As can be seen from the experimental results in comparison with the
conventional procedure for iodometric titrations, the use of ascorbic acid as a standard can be suggested for
this purpose. In addition, since there are fewer steps in the overall procedure, the cumulative experimental
errors are smaller with ascorbic acid than with thiosulfate.
However, there is the disadvantage that ascorbic acid is not very stable when dissolved in water,
because it scavenges oxygen. Thus, care should be taken in the laboratory work, and each titration must be
performed individually immediately after the dissolution of this standard.

1422 Journal of Chemical Education Vol. 76 No. 10 October 1999 JChemEd.chem.wisc.edu