David N. Bailey Lebonon Volley College Annville, Pennsylvonio 17003
The Determination of Ascorbic Acid A quantitative analysis experiment
Since the advent of the demand for "relevance," the teaching of quantitative analysis laboratory has been hampered by a lack of "relevant" experiments which will keep the students interested while, at the same time, teach the important techniques involved. One answer to the dilemma of the instructor faced with this situation has been to use commercially available foodstuffs, patent medicines, or natural products. The disadvantage of this approach is that only rarely are the samples of sufficient size, uniformity, and stability that the instructor can spend the time and effort necessary to obtain an "accepted value" to be used for grading purposes. The experiment must then he graded on the basis of something other than the accuracy of the student's results. The present paper will describe an experimental determination of ascorbic acid (Vitamin C ) using a sample which is easily prepared, analyzed, and sufficiently stable to be suitable for use as an unknown in the quantitative analysis laboratory. Ascorbic acid is rapidly and quantitatively oxidized by iodine in acid solution. As shown in eqn. (1)
the standard method for the determination of ascorbic acid involves the direct titration of an acidified sample with a standard iodine solution1 and this method has been suggested for use in teaching laboratories.z The difficulties inherent in handling standard iodine solutions makes this procedure somewhat undesirable for use by beginning quantitative analysis students. The proposed experiment avoids the difficulties by the in situ generation of a known excess quantity of iodine followed by back titration of the excess with standard thiosulfate. The procedure therefore also illustrates the application of the back titration technique. Because of the nature of the sample, the problem of large excesses of iodine causing nonspecific further oxidation of other substances is eliminated.' Experimental Directions Each student will need approximately 500 ml of 0.01 M KIOJ solution, 500 ml of 0.07 M NazSIOa solution, 100 ml of 0.5 M HzS04 solution, 300 ml of 0.3 M H2S04 solution, 20 ml of a starch indicator solution, and 10 g solid KI. The concentration of the KI0- solution should be known to at least 1 oart in 500: the concentrations of the other solutionsneed only be approximate. The NazSz03solution is first standardized by pipetting 50.04
ml of the KIO, solution into an Erlenmeyer flask, adding 2.0 g solid KI and 10 ml of 0.5 M H2SOI and immediately titrating with the NazSz03 solution. Two milliliters of starch solution should he added just prior (i.e., at the point at which the solution turns pale yellow color) to the endpoint. The unknown, after drying for one week over CaCI2, is analyzed by accurately weighing an appropriate quantity (about 0.4 g, if the sample is approximately 50% ascorbic acid) and dissolving it in about 60 ml of 0.3 M sulfuric acid. Two grams of solid KI and 50.00 ml of the standard KI03 solution are then added and the excess iodine is immediately hack-titratedwith the standard Na~S203solution. Unknowns
Unknowns for this experiment can be conveniently made by thoroughly mixing reagent grade ascorbic acid with a suitable diluent. The diluent should have essentially the same texture and density as the ascorbic acid powder and should be acidic since basic materials tend to hasten decomposition of the ascorbic acid. Boric acid provides a suitable material for use as a diluent.3 A convenient and extremely satisfactory method of assuring complete mixing is to first ball-mill the materials, followed by sieving the mixture and finally using a V-blender. When samples were prepared in this manner, a junior-level quantitative analysis class of 48 students achieved the results shown in the table. (The results of five of the students were so divergent that they were discarded before means and standard deviations were calculated.) As may be seen, the students' agreement with one another is approximately 1%on samples designed to be so closely similar as to make it very difficult for the student to know which samole he had and thus alter his data to conform to other students who received the same sample. The standard deviation reported here is a measure of both the ho. mogeneity of the sample as well as of the analytical preci. sionof the method, If the specialized equipment mentioned above is not available, the ascorbic acid and diluent may be hand mixed, then put into a large bottle, and the bottle rotated on a set of motor-driven rollers for 24-36 hours.3 This type of mixing results in a somewhat higher standard deviation ave,ging approximately 4% relative, ~ l ~ it hhas ~not ~ been attempted, the inclusion of several stainless steel halls or glass marbles in the mixing jar and a longer bling time should improve the homogeneity of the sample Samples prepared and analyzed in late summer and then used during the fall showed no significant change in ascorbic acid content, thus indicating a stability of at least 5-6 months. Typical Student Results Mean result
(%
Sample
Students
'Stevens, J. W., Ind. Eng. Chem., (Anal. Ed.), 10, 269 (1938 2Moore, C. E.,J. CHEM. EDUC., 25,671 (1948). SShesller, J. C., and Klatt, L. N., "Changing Student Atti-
1 2
8 10
tudes about Quantitative Analysis Laboratories," J. CHEM. EDUC.. 51.239 (1974).
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488
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3 4
5
10 10
Ascorbic Acid)
Absolute Std Dev
Relative Std Dev (ppt)
90.68 90.82 91.29 91.04 90.97
0.80 0.65 0.97 0.84
8.9 7.1 10.6 9.2
1.31
144
~
h
This method of analysis has been found to be suitable for use with many other commercially available products such as various flavors of Hi-C,grapefruit flavored Tang, and Vitamin C tablets. Although such samples cannot easily he used for quantitative analysis unknowns since the composition varies and/or the ascorbic acid rapidly
air-oxidizes in solution, these samples may be useful in other courses where accuracy is not the primary concern. Replicate analyses on all these materials showed standard deviations of 1%relative when the sample size was chosen to contain 100 mgof ascorbic acid.
Volume51, Number 7. July 1974
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