heating rate, or to operate the apparatus a t a constant temperature, a t any point in a chromatogram without altering the detector sensitivity. Carrier gas volume flow rate, measured at the column exit, was maintained constant by means of a Moore constant differential flow controller. The apparatus could equally 1%-ellbe used as a lonheat capacity, constant temperature unit. Rapid temperature adjustments were then achieved with the Pyr-OVane acting as a manually set thermostat. The effects of heating rate, flow rate, and pressure drop on retention volume are being investigated, in order to obtain an expression for the retention behavior of solutes. A more complete description of the apparatus and the observed interdependence of the above variables in programmed temperature columns is in preparation.
B
LITERATURE CITED
MINUTES
Figure 2.
G a s chromatograms of alcohols
A.
Programmed temperature chromatogram of nine alcohols. ( 1 ) methanol, ( 2 ) ethanol, (3) 1 -propanol, (4)1 -butanol, ( 5 ) 1 -pentanol, (6) cyclohexanol, (7) 1 -octanol, (8)1 -decanol, and ( 9 ) 1 -dodecanol E. Constant temperature chromatogram a t 165" C. of same alcohol mixture
maintained a t a temperature of 203" C. The detectors were Fenwal lOs-ohm matched, mounted thermistors; the reference thermistor %vasisolated in a helium atmosphere in the detector block. The thermistor detectors could be operated at a rated maximum ambient temperature of 250" C. (bead temperature 300' C.). Variations in column temperature did not significantly affect the detectors until the column temperature approached that of the detector block (Figures 1, A , and 2, A ) . Peak areas for individual solutes were reproducible to rt3% over a sixfold change in heating rate. The stainless steel columns were uni-
formly wrapped ivith Glasohm insulated resistance wire. Po\ver vias supplied from a Pyr-OVane temperature controller (Minneapolis-Honeyn-ell) , whose set point was linearly driven up-scale by means of a 5000 r.p.m. motor and gear reduction train (about 3 X 105 to 1). A thermocouple attached to the column provided the temperature measurement on the Pyr-0-Vane. Variable rates were achieved over the range 3" to 17' C. per minute by means of a Netron variable ratio speed changer. Varying the heating rate corresponds to varying the flow rate during a constant temperature run. It was possible to vary the
(1) Ashbury, G. K., Davies, A. I., Drinkwater, J. W.,ANAL.CHEM.29, 918 (1957). (2) Berridge, N. J., Watts, J. D., J. Sei. Food Agr. 5 , 417 (1954). (3) Evans, J. B., Willard, J. E., J. Am. Chem. SOC.78,2908 (1956). (4) Greene, S. A,, Moberg, hl. L., Wilson, E. >I., .4~'.4~.CHEM. 28, 1369 (1956). ( 5 ) Greene, S. A,, Pust, H., Ibid., 29, I055 (1957). Kamer,' J. H., van de, Gerritsma, K. W., Wansink, E. J., Biochem. J. 61,174 (1955). Keulemans, A. I. V., Verver, C. G., "Gas Chromatography," pp. 60-1, Reinhold, New Tork, 1957. Lichtenfels, D. H., Fleck, S. A., Burom, F. H., As.&. CHEM. 27, 1510 (1955). Patton, H. W.,LeKis, J. S., Kaye, W.I., Ibid., 27, 170 (1955). STEPHEN DALNOGARE C. EUGEXEBENNETT Polychemicals Department E. I. du Pont de Nemours & Co., Inc. Wilmington, Del. RECEIVEDfor review April 28, 1968. .-iccepted -4pril 30, 1958.
Benzene Extraction of Antimony Iodide SIR: The solubility of antimony iodide in benzene was first reported by MacIvor (4). Fauchon (1) found that antimony iodide dissolves in potassium iodide solution to give a yellow color of unknown composition, which AlcChesney ( 3 ) used to develop a method for the colorimetric determination of microgram quantities of antimony. The isolation of antimony by benzene extraction from acid iodide solution was used by Vest and Hamilton ('7) prepara1 158
ANALYTICAL CHEMISTRY
tory to a spot test. I n another paper, West (6) reported that bismuth is the only other element extracted in significant amount. Because of this high selectivity it seemed desirable to make a quantitative study of the extraction to see how efficiently the antimony may be removed. It was hoped that the dependence of the extraction on the iodide concentration in the aqueous phase would throw some light on the inorganic
chemistry of antimony in iodide solutions. EXPERIMENTAL
Reagents. Antimony trioxide (Baker) was purified as directed by Schumann (5). A stock solution of antimony was prepared by dissolving a weighed quantity of the purified oxide in 5M sulfuric acid. Rfallinckrodt analytical reagent grade benzene was redistilled in an all-glass still. For the
perchloric acid experiments, a stock solution of antimony iodide was prepared by dissolving a weighed amount of the Fisher product in benzene. Other chemicals mere of reagent grade. Apparatus. Standard-taper borosilicate glass-stoppered tubes were used to contain the solutions during the shaking period. Absorption measurements were made with a Beckman Model B spectrophotometer, using rectangular borosilicate glass 1-em. cells. Procedure. I n each experiment 20 ml. of aqueous phase was prepared by mixing 10 ml. of l.02.lO-3M antimony(111) in 5M sulfuric acid, 5 ml. of 10M sulfuric acid, z ml. of 1.00M potassium iodide, and ( 5 - 2) ml. of water. The resulting solution was shaken for 10 minutes with a measured volume (0.5, 1.0, or 2.0 ml.) of benzene, and then centrifuged to effect clear separation of phases. I n all cases, extraction was so efficient that it was pointless to analyze the benzene phase. It was satisfactory simply to determine the small amount of antimony remaining in the aqueous phase, using the method of McChesney (S), assuming that the amount in the benzene phase was the difference between this small amount and the known total. I n the cases where extraction was so nearly complete that McChesney's method could not be applied directly t o the aqueous phase, a larger volume of the latter was extracted with a second portion of benzene, which was backextracted into pure water to concentrate the antimony. The extraction was also studied using 5 M perchloric instead of sulfuric acid, with the additional difference that the antimony iodide was all in the benzene phase a t the start. I n this series of experiments the temperature was controlled a t 25' C. A benzene extract containing a known amount of antimony was shaken with a solution of 0.1M potassium nitrate, thus returning all antimony (and iodide) to the aqueous phase. The iodide was determined by amperometric titration with silver nitrate, using the rotated platinum electrode. It was found that three iodide ions were present for each antimony ion, thus establishing that the benzene extraction removes antimony as the triiodide. That such a relatively nonpolar substance should be extracted by a solvent such as benzene is certainly reasonable. This is contradistinctive to the ethyl ether extraction of antimony from 6.9.M hydriodic acid as studied by Kitahara ( 2 ) . His finding that the extraction is quantitative from such a solution would seem to be best explained by considering the extracted compound to be hydrogen tetraiodoantimonate(II1). Further confirmation that the present study involves extraction of antimony triiodide is found in the fact that a benzene extract from sulfuric acidpotassium iodide solution has an ultraviolet absorption spectrum identical to that of a solution of antimony triiodide
plex ion according to the equation: Table I. Extraction Coefficient for Antimony between Benzene and 5M Sulfuric Acid Solutions Containing Potassium Iodide
KI, ill E 0.0035 540 0.0085 2830 0.0110 2960 0.0135 2540 1820 0.0185 860 0.0285 0.0385 345 190 0.0485 5M perchloric acid (1
KI, V 0.0586 0.0600" 0.O70Oa 0.0785 0 .O80Oa
0.0900" 0.0985 0.1000" solutions.
E 138 108 76 62 57 39 34 29
(Fisher) in benzene. A solution of antimony triiodide in benzene reacts with rhodamine B in benzene to give a color which appears identical to that in the spot test described by Kest and Hamilton. Thus, the implication by these authors that the antimony is extracted as a tetraiodoantimonate(II1) species must be regarded as unjustified. RESULTS AND DISCUSSION
The results of the extraction study are given in Table I, in terms of the extraction coefficient. This is defined as: E =
molarity of Sb in benzene phase molarity of Sb in aqueous phase
The extraction is seen to be an extremely efficient means for separating antimony. At the acidity used ( 5 M ) the iodide concentration for optimum extraction is about 0.010M. However, extraction is quantitative (99.9%, for equal volumes of phases) in the range from 0.005 to 0.03M potassium iodide. Vnder optimum conditions, the estraction coefficient is about 3000, so that 9970 of the antimony is extracted even if the ratio of benzene to aqueous volumes is only 1 to 30. If it is desired to concentrate the antimony into an aqueous phase, rather than into a benzene phase, back-extraction of the latter with an aqueous phase containing no iodide will result in a complete transfer. At low acidities there may be some trouble with the formation of yellow antimony oxyiodide if larger amounts of antimony are present. If it is assumed that only Sb13 and Sb14- exist in solutions containing greater than 0.01M iodide-Le., after the extraction coefficient has passed through a maximum-and that only antimony triiodide exists in the benzene phase, the following relation between the extraction coefficient and the iodide concentration can be derived: 1 1 (I-) E=jj+D.K where D is the distribution coefficient of the molecular form, SbI3, between benzene and the aqueous phase, and K is the dissociation constant of the com-
SbId-
SbI,
+ I-
The above simple system, which is probably commonly assumed, would lead one to expect that a plot of 1/E us. iodide concentration would be linear, with an intercept of 1/D and a slope of l / D K . However, the data presented in the table do not yield a straight line when plotted in this way. Rather, a log-log plot s h o w that 1/E varies directly with the iodide concentration raised t o the pom-er 2.6. The experiments using perchloric instead of sulfuric acid, and starting with the antimony iodide in the benzene phase instead of in the aqueous phase, gave results in fairly close agreement with the sulfuric acid series. Therefore, the simple picture of the equilibrium system is not sufficient. The presence of polynuclear species does not seem likely in view of the good agreement between experiments using different concentrations (through different volume ratios) of antimony. The formation of complexes containing more iodide-e.g., Sb16-- and SbIB---is an attractive explanation, but seems somewhat unlikely because the absorp tion spectrum of the yellow color is independent of iodide concentration up to 6 M . Also, consideration of approximate ionic sizes indicates that probably only four iodide ions can combine symmetrically with one antimony ion. For the present then, the chemistry and algebraic theory of the extraction are unknown. However, it is clear that it is a practical method for the highly efficient and nearly specific separation of antimony from other elements. ACKNOWLEDGMENT
The experiments using perchloric acid were performed by Janet Kinslow as part of an independent study project a t Carleton College, and she presented the substance of this paper a t the Eighth Annual Undergraduate Chemistry Symposium, DePaul University. LITERATURE CITED
(1876): ' Schumann, R., J . Am. Chem. SOC.46, 52 (1924). West, P. K,, Proc. Intern. Congr. on Anal. Chem., Oxford, 1952, p. 61. West, P. IT7., Hamilton, W. C., ANAL.CHEW24, 1025 (19S2). RICHARD W.RANETTE Carleton College Korthfield, Minn. RECEIVEDfor review August 1, 1957. Accepted April 3, 1958. VOL. 30, NO. 6, JUNE 1958
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