spot with Fe+3. Separation is not impaired by 100yo impurities of Al+3 and Cr+3. Good separations can be obtained with this solvent system a t any pH from 1 to 5 without the use of buffers (Table 111). As can be seen from Table 111, even unbuffered samples separate well. Best separations are obtained a t pH 1 and 4. A close control of pH is not essential for good separations. Further work is in progress and results will be published later.
ACKNOWLEDGMENT
Table 111. Separation of Unbuffered Samples at Different pH Values with Solvent System 1
The authors are grateful to A. R. Kidwai, head of the Department of Chemistry, for research facilities.
Rf
LITERATURE CITED
PH
Fe + 2
Fe +a
1 2 3 4 5
0.17 0.16 0.17 0.15 0.24
0.80 0.62 0.68 0.78 0.69
(1) Stevens, H. M., Anal. Chim. Acta 15, 538-42 (1956).
MOHSIN QURESHI ICJBAL AKHTAR Chemical Laboratories Aligarh Muslim University Aligarh, India
Rapid Potentiometric Determination of Ascorbic Acid SIR: Visual titration with 2,6dichlorophenol-indophenol is the official method for determining ascorbic acid (4), but it cannot be used in colored solutions nor can it be readily automated. Birch, Harris, and Ray ( I ) suggested potentiometric titration with a bright platinum electrode but encountered the following difficulties : high variable initial potential, slow equilibrium, drifting of potential, and indefinite end point. Harris and Olliver (2) improved the end point by using a mercury-platinum electrode. However, the electrode had to be prepared fresh each day and the response was sluggish past the end point. Kontio and Casagrande (3) raised the p H from the usual pH of 2 to 3-4 and obtained a 40-50 mv./ml. change a t the equivalence point. Carbon dioxide was used to prevent oxidation by air, and each type of solution required a different optimum pH. Using an untreated platinum electrode (or one aged in water for one week) eliminates the above difficulties, and ascorbic acid can be titrated rapidly a t pH 2. The method has been applied to an automatic titrator and is recommended even for colorless solutions.
mately 70% of the dye required to reach the end point was added. One-ml. aliquots of dye were then added and the potential was measured about 30 seconds after each addition until the end point was passed. The baseline potential was the lowest mv. reading obtained during the titration. The instrument was set to 35 mv. above the baseline potential. The standard and test solutions were titrated to %IO mv. of this arbitrary end point. RESULTS AND DISCUSSION
Figure 1 is an example of a complete titration curve. The readings before the equivalence point are stable after about 30 seconds of stirring, while past the equivalence point the readings are stable immediately. The slope between 2 5 4 5 mv. above the baseline is 230 % 50 mv./ml. (average of 27 plots by four different workers). Therefore, a t the arbitrary end point of 35 mv. above the baseline, an error of % l o mv. has an effect of only %0.6% for a 15-ml. titer. The baseline potential of a multiple vitamin solution containing ascorbic acid was measured daily. Each day the baseline potential decreased, asymptotically becoming stable after three successive days. This effect occurred
EXPERIMENTAL
Apparatus. Manual potentiometric
measurements using standard calomel and platinum electrodes were made with a Beckman Model G p H meter and automatic titrations were performed on a Fisher Model 36 Automatic Titrimeter. The platinum electrode was used as received from the manufacturer and kept immersed in water between use. Acid-cleaned platinum electrodes become usable only after 1 or 2 weeks of aging in water. Procedure. The U.S.P. method (4) was followed except for the end point. First the approximate baseline potential was determined. Approxi1342
ANALYTICAL CHEMISTRY
290 270
I70
i 1,
even when an electrode was used after a period of storage in water. During a given day, a different type of solution (in this particular case orange juice as compared to a vitamin mixture) exhibited a significantly different baseline potential, and therefore, it is recommended that the baseline be redetermined for each new type of solution. Amounts of thiamine, riboflavin-5phosphate, or nicotinamide equivalent to the ascorbic acid in solution did not interfere with the assay; nor did a mixture of these vitamins with sodium pantothenate and pyridoxine interfere, even when decomposed (120’ C. for 1 hour). Also, no difficulty was encountered in determining the ascorbic acid content of orange juice. The precision of the method calculated from 35 manual titrations on standard solutions, vitamin mixtures, and orange juice was +o.5y0(average deviation) for 15- to 16-ml. titer. The precision of the automatic titration using the Fisher Titrimeter was within *O.ly0. Recovery experiments on vitamin mixtures, both fresh and decomposed, indicated a maximum error of %l.Oa/,. Assays of vitamin mixtures containing known amounts of ascorbic acid also indicated a maximum error of f1.0%. LITERATURE CITED
,
2
,
,
4
,
, , ,
6
,
,
,
,
,
, , ,
8 I O 1 2 1 4 M L OF DYE
,
,
]
1618
Figure 1. Titration curve of ascorbic acid with 2,6-dichlorophenolindophenol
(1) Birch, T. W., Harris, L. J., Ray, S. N., Biochem. J . 27, 590 (1933). (2) Harris, L. J., Olliver, M., Zbid., 36, 155 (1942). (3) Kontio, V. P., Casagrande, V. M., Suomn Kemistilehti 18B. 9 (1945). (4) “Pharmacopoeia of the United States of America,” 16th Rev., p. 66 (1960).
EDMUND E. SPAETH~ VICTORH. BAPTIST MARTINROBERTS
Don Baxter, Inc. Glendale 1, Calif. 1 Present address, California Institute of Technology, Pasadena, Calif.
