Table II. Titrations at Various Potentials
Moles
PO-
tential, millivolts 75 150
250 400
Mole ratio
LIoles FeFeTI(II1) (CN)6-4 (CN)6-4: x 105 x 105 TI(III) 1.343 1,092 1:1.231 1.343 1.084 1:1.240 1.343 1.074 1:1.250 1.343 1.083 1:1.241
being oxidized at the anode and hexacyanoferrate(II1) is being reduced a t the cathode. Table I summarizes the results of a number of titrations. Calculated mole ratios of thallium(lI1) to hexacyanoferrate(I1) obtained from the molarities of the reagents as prepared above are included. The temperature was maintained a t 26" C. Table I1 presents data for the titration performed at various potentials. From these data and many others, we conclude t h a t it is feasible t o determine thallium routinely by the deadotop titration method. We also con-
clude t h a t the mole ratio of thallium (111) t o hexacyanoferrate(I1) in the compound of the two is 5 to 4. This ratio is in agreement with t h a t of the corresponding indium(II1) salt as reported by Martens and Frye ( 5 ) . A series of check measurements was run making use of a much more sensitive current measuring device. A Heathkit Operational Amplifier System Model EUW-19 was set up in connection with an Eico Vacuum Tube Voltmeter. The combination provided three useful scales of which the 0 t o 3 pa. full scale deflection was the most useful. Although under these conditions there was no longer an abrupt change in current; the end point could be determined by the graphical method using tangents drawn t o the two limbs of the curve. A change of 0.04 pa. on this sensitive device indicated that the equivalence point had been reached. A series of titrations at ambient temperature using an applied potential of 250 mv. yielded a n average mole ratio of 1 : 1.247 in excellent agreement with those data obtained with the less sensitive, more rugged instrument.
Failure to clean the electrodes after each titration resulted in a gradual lowering of current readings after the equivalence point had been reached. The electrodes used in this study were dipped in dichromate-sulfuric acid solution after each use and were then carefully rinsed with deionized distilled water and blotted before use. LITERATURE CITED
(1) Delahay,
P., "New Instrumental Methods in Electrochemistry," pp. 258-64. Interscience. New York. 1954. ( 2 ) Fetter, 3 . R., Swinehart, D. F.,'ANAL. CHEM.28, 122 (1956). (3) Foulk, C. Vi'., Bawden, A. T., J . Am. Chem. SOC.48, 2045 (1926). (4) ~, Gaunin. R.. Charlot. G.. Anal. Chim. ,
I
kcta Acta 8;': 65 (19531. (1953). (5) Mart'ens,' Martens, L. 'S., S., Frye, H., ANAL. CHEM.35, 969 (1963). (6) Reilley, C. N., Cooke, W. D., Furman, X. TI., Zbid., 23, 1226 (1951). (7) (71 Salomon, E.. E., Z. Phusik. Physik. Chem. 24, 55 ~, 8alomon. (1897). ( 8 ) Swinehart, D. F., Anal. Chem. 23, 380 (8) I
,
ilQ.51). \ - - - - I
R. B. WILLIAMS HERSCHEL FRYE Department of Chemistry University of the Pacific Stockton 4, Calif.
Higher Recoveries of Carbonyl Compounds in Flash Exchange Gas Chromatography of 2,4-Dinitrophenylhydrazones SIR: h number of modifications of the original description of flash exchange gas chromatography (3)have appeared in the last several years. This conimunication evaluates these modifications and calls attention to a useful way of increasing the recovery of carbonyl compounds regenerated from 2,4dinitrophenylhydrazones(1) with 2keto-glutaric acid (11). Three interesting modifications of the technique were reported b y Stephens and Teszler (6). The use of a rubber adapter for mounting a glass capillary tube containing the exchange mixture into a hypodermic needle obviates the original requirement of turning off the filament current. Use of a resistance wire heater coiled around the capillary tuhe is a more elegant source of thermal energy than a hot oil bath. A third modification made by these workersusing mixed I1 and formaldehyde 2,4dinitrophenylhydrazone as a chaser in Table 1.
Method
the bottom of the exchange tube-is open t o criticism from the standpoint of formaldehyde determination in unknown mixtures [compare Shepartz and McDoneIl (41. The use of sodium bicarbonate(II1) in the bottom of the exchange tube to increase carbonyl compound recovery [a modification first reported by Ten Hoopen (6) in 19631 appears t o be the preferred method. Several parameters have been explored which are involved in the use of 111. The best results are obtained when 1 mg. of 111 is used in the bottom of an exchange capillary tube (70 mm. from the bottom to the 90" bend) containing 8s an uppcr band 5 mg. of a 1 :3 mixture of I:II. Results of tests using 5 mg. portions of a mixture of propionaldehyde (IV), isobutyraldehyde (V), and kovaleraldehl\.de (VI), 2,4dinitrophenylhydrazones with 3 parts of I1 are tabulated in Table I. Use of I11 with mixtures of I1 contain
Use of NaHC03 in 2-KGA-2,4-DNPH Mixtures for Carbonyl Reg enera tion
Average peak areas in sq. mm. for individual aldehydes I1 7 J. VI 90 f 1
280 f 34
167 f 36
ing benzaldehyde 2,4dinitrophenylhydrazone gave no peak for benzaldehyde. Even in the 111-modified form, the carbonyl flash exchange method has utility only for the analysis of lower molecular weight aldehydes and ketones. No such restriction on molecular weight of aliphatic monocarboxylic acids occurs in the flash esterification method. The work of Hunter ( 2 ) h a s extended the range, of acids determined to CI8and has described techniques for use of the method a t pg. concentration levels. Use of I11 in the mercaptan method ( I ) was not tested; it is probable that improved recoveries of mercaptans would result from a small charge of IT1 in the bottom of the exchange tube. LITERATURE CITED
(1) Carson, J. F., Weston, W. J., Ralls, J. W., *Vatwe 186, 801 (1960). (2) Hunter, I. J., J . Chromatog. 7, 288
(1962). (3) Ralls, J. W., ANAL. CHEM.32, 332 (1960). (4) Shepartz, A. I., blcnowell, P. E., Zbi.d, 32, 723 (1960). (.5) ~, SteDhens. R . L.. Teszler, A. P., Ibid., 1047'( 1960). (6) Ten Hoopen, H. J. G., 2. Lebensm. Untersuch. -Forsch. 119, 478 (1963). JACK W. RALLS
Research Foundation National Canners Association Berkeley, Calif. 946
ANALYTICAL CHEMISTRY
