Table 1. Colorimetric Analysis of Vanadium in EPR Solutions
v
Added, fig.”
126 126 63 63 25 25
v
found,
ug.
V recovered, %
125 126 60 60 26 28
99 100 95 95 104 112
a I’anadium added 2 ~ ammonium 8 metavanadate t o 10 grams of a 37, elastomer solution in toluene.
There was, however, some uncertainty as to whether an aralytical procedure for vanadium preqenf in this form could apply to samples whose vanadium content originated from catalytic quantities of trialkll vanadates. To verify our results, a number of samples were submitted for neutron activation analysis. Data in Table I1 showing agreement between the two methods give a good indication that this colorimetric procedure c etects the total vanadium content. For the colorimetric analyses in Table 11, 10 grams of sample were analyzed. Two-gram portions of samples that contained over 10 p.p.m. vanadium yielded results of almost equal accuracy. Results of 10
replicate analyses of sample A gave a standard deviation of 0.91. The occasional substantial variance between the two procedures is most likely the result of mechanical loss during ashing. The neutron activation data wherein this error is obviated are probably the more accurate. No attempt was made to attain agreeable results for samples that contained less than 1 pap.m. vanadium. Since no difficulty was encountered in analyzing amounts of vanadium ranging from 10 to 20 pg. in standard solutions of ammonium vanadate, high results of samples D and F are attributable to substances that do not interfere with vanadium a t higher levels. Cheng’s suggestion for the removal of some interfering ions might well increase the accuracy of analyses of vanadium below 1 p.p.m. For the analyses in Table 11, no attempt was made to remove any ions that might interfere. Aluminum, iron, and chloride were present in varying amounts ranging from 20 to 100 p.p.m. Acetone used for rinsing glassware interferes because it reacts with DAB in the presence of vanadium to form a blue colored solution. The use of dichromate cleaning solution should be avoided since Cheng points out that chromium(V1) oxidizes DAB. Glassware rinsed well with distilled water is all that is needed.
Table 11. Determination of Vanadium in Toluene Solutions of EPR Vanadium. u.u.m.
Sample
DABa
Neutron activation
A B C D E F G H
10.2 14.0 14.6 0.5 13 0 0 9 15 2 18 2
9.9 & 0.2 14.1 & 0 . 1 15.6 f 0 . 3 0.14 f 0 . 0 1 1 4 8 f O 2 0 27 f 0 01 188*03 179*03
~
Average of three determinations.
ACKNOWLEDGMENT
General Atomics Division of the General Dynamics Corp. furnished the neutron activation analyses. LITERATURE CITED
(1) Cheng, K. L., Talanta 8, 658 (1961). (2) McCoy, J. W., “The Inorganic Analvsis of Petroleum,” Chemical Publishing, 1962, p. 236. (3) Roza, J. T., Zeeb, L. E., Petrol. Process 8, 1708 (3953).
(4) Snell, C., Snell, F. D., “Colorimetric Methods of Analysis,” Vol. 11.4, p. 347, Van Yostrand, S e w York (1959).
Sun Oil Co. Marcus Hook, Pa.
A . J. SMITH
The Dead-Stop Titration of Thallium SIR: The dead-stop titration method originated by Salomon (7) and developed by Foulk and Bawden (3) has been applied to the analysis of many metals including zinc by Swinehart (8), gallium by Fetter and Swinehart ( 2 ) , and indium by Martens and Frye (5). An apparatus similar to the simple, rugged devics used by these workers has been employed in the thallium determination. The galvanometer employed had a sensitivity of 1 X ampere pcsr millimeter. A number of presentations of the theory have been made, induding those of Reilley, Cooke, and Furman ( 6 ) ,Gaugin and Charlot (4),and 1)elahay ( I ) . EXPERIMENTAL
Standard solutions of thallium(II1) and hexacyanoferrate(I1) were prepared. The thallium(II1) solution was prepared by first removing the oxide from Fisher reagent grade thallium metal with nitric acid. then washing in distilled water and drying thoroughly under nitrogen, and h a l l y dissolving in aqua regia to ensure quantitative oxidation of the thalliuni(1) to thallium
(111). The excess oxidant was removed by evaporation of the solution to dryness. Baker and Adamson reagent grade potassium ferrocyanide was pulverized and dried to constant weight a t 100’ C. The anhydrous salt was then weighed and dissolved in doubly distilled water; 2 grams per liter of reagent grade sodium carbonate was added to stabilize the solution. Volumes of the standard thallium(II1) solution were dispensed from a calibrated buret into the titration vessel, the mechanical stirrer was started, and current readings were taken a t constant time intervals after each addition of the hexacyanoferrate( 11) solution; the latter solution was added by means of a precision calibrated buret. No supporting electrolyte, in the polarographic sense, was used. DISCUSSION
The reduction of thallium(II1) is irreversible a t the potentials used in these determinations, whereas that of hexacyanoferrate( 111) is reversible. The hexacyanoferrate(I1) solution doubtless contains a little hexacyanoferrate(”, and current will flow
when the hexacyanoferrate(I1) is in excess. The sharpness of the end point depends on the low solubility of the thallium( HI) - hexacyanoferrate(I1) and the consequent low concentration of hexacyanoferrate(I1). As soon as the concentration of thallium(II1) has been reduced sufficiently, current will begin to flow because hexacyanoferrate(I1) is
Table 1.
Moles T1( x 111 106:I
Results of Titrations“ &fOle5 l f d e ratio Fe(x 105 --4 Fe(Tl(II1) CN)s- 4 :
1.343 1,075 1:1.249 1.343 I . 083 1:I ,240 1.343 1.067 1:1.258 1.343 1.083 1:1.240 1.343 1.070 1:1.255 1.343 1.074 1:1.250 1 ,078 1:1.246 1.343 1.343 1.070 1 : l 255 1.343 1.071 1:1.254 Av. mole ratio: 1: 1,250. Std. dev.: 0.0066. 5 Data in Table I are all at a potential of 250 mv.
VOL. 36, NO. 4, APRIL 1964
e
945
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 that it is feasible t o determine thallium routinely by the deadotop titration method. We also con-
clude that 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 that 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 by 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.
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 Regenera tion Average peak areas in sq. mm. for individual aldehydes Method 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 ) has 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
