mass spectral data clbtained with the technique described above will aid significantly in the identification or determination of the structure of compounds separated b:7 gas chromatography. ACKNOWLEDGMENT
R e are indebted to the R. & H. Filter Co., P.O. 5048, Wilmington, Del., for constructing the pressure reduction system for us. LITERATURE CITED
(1) Biemann, K., 11th Annual Conference
on Mass Spectrorietry and Allied Topics, San Francisco, May 1963.
(2) Biemann, K., Bommer, P., Burlingame, A. L., McMurray, W. J., Tetrahedron Letters, No. 28, 1969 (1963). (3) Brunnee, C., Jenckel, L., Kronenberger, K., 2. Anal. Chem. 189, 50 (1962); 197,42 (1963).
(4) Dorsey, J. A., Hunt, R. H., O'Neal, M. J., ANAL.CHEM.35, 511 (1963). (5) Ebert, A. A., Ibid., 33,1865 (1961). (6) Gohlke, R. S., Ibid., 34,1332 (1962). (7) Ibid., 31, 535 (1959). (8) Henneberg, D., 2. Anal. Chem. 183,12 (19611. ( 9 j Holmes, J. C., Morrell, F. A., Appl. Spectry. 1 1 , 86 (1957). (101 Lindeman. L. P.. Annis. J. L.. ANAL. ' &EM. 32,1742 (1960). ' (11) McFadden, W. H., Teranishi, R., Black, D. R., Day, J. C., J. Food Science 28,316 (1963).
(12) Miller, D. O., ANAL.CHEM.35, 2033 (1963). (13) Ryhage, R., Ibid., 36, 759 (1964).
J. THROCK WATSON~ K. BIEMANN
Department of Chemistry Massachusetts Institute of Technology Cambridge, Mass. l
64.
M.I.T. DuPont Teaching Fellow 1963-
RECEIVED for review December 23, 1963. Accepted March 3, 1964. Work supported by research grants from the National Science Foundation (G-21037) and the National Institutes of Health, Public Health Service (RG-9352).
Ligand Bridging in the Oxidation of Chromium(II) at Mercury Electrodes SIR: Preliminary ~ r o r kon the effect of halide on the rate of electrolytic oxidation of Cr(I1) a t mercury electrodes has been done by Abubaker and Malik ( I ) , and more extensive citudies have been made by hikens and Ross (2) and Kemula and RakoMska ( 5 ) . These experiments suggested that the electrode reaction mechanism in the presence of chloride involves t k e adsorption of chloride on the mercury electrode followed by electron transfer through the adsorbed chloride to give CrC1+2as the product. However, the observed rate enhancement in the presence of halide does not by itself imply a bridged activated complex. 'Te have studied the reaction by controlled potential electrolysis to obtain direct evidence for this mechanism.
was carried out substantially to completion (98-99%) and the integral of the current-time curve read from the integrator. Cr(I1) solutions were prepared and stored by a modification of the method suggested by Stone (6'). Analysis of the Cr(I1) solutions was accomplished by oxidation with E'e(II1) followed by titration of the resulting Fe(I1) with a standard KRIIn04solution. Chloride and iodide analyses were done by potentiometric titration of the product solution with a standard solution of AgClO4 in 2F HC104using a silver wire indicator electrode and a glass reference electrode. When the ligand is chloride, the system is especially amenable to study because the product, CrC1+2, while thermodynamically unstable, is highly substitution-inert. This makes it powible to titrate a solution containing CrCl+Z and C1- with Agf
without appreciable decomposition of the complex, so that the concentration of free chloride ion in the solution can be readily determined. When iodide is present, any CrI+2 formed is readily decomposed by silver ion, so that the iodide titration does not distinguish between bound and free iodide ( 7 ) . RESULTS
With chloride as the complexing ligand, a series of electro-oxidations was carried out at various initial ratios of chloride to chromium(I1). The results are presented in Figure 1. (C1-)a and [Cr(II)Iorefer to initial concentrations, and (CrC1+2) refers to the final concentration of this species. These data demonstrate that a t least one chloride ion is present in the transition state for each Cr(I1) ion that is oxidized, as long
EXPERIME '4TAL
Controlled potential oxidations of e out a t Cr(I1) in 2F HClO, w ~ carried -0.012 volt vs. SCE in the presence and absence of chloride a t :t stirred-mercurypool electrode with area 10 sq. cm., in a three-compartment cell designed to provide complete exclusion of oxygen during the course of the reaction. When the effect of iodide was being studied the oxidationf, were carried out a t -0.192 volt. An .halytical Instruments, Inc., potentios tat and integrator were employed. A solution (40 ml.) of the complexing ligand a t the desired concentration in 2F HC104 was placed in the main compartment and Cr(1I) solution placed in the separate reference and auxiliary electrode compartments to prevent diffusion of oxygen into the main compartment through the fritted disk connections. The main compartment was thoroughly deaerated with purified Nz, and then 10 ml. of 0.1F Cr(I1) solution in 2F HC104 was added under anaerobic conditions. The electrolysis
0
0
predicted experimental
100
A h
la
Y
L.
2
80
h + 2 0
60
Y
8 40 20
20 Figure 1.
40
60
100 120 140 160 % (Ct-lO/(Cr( ~ 1 ) ~
80
180
200
Results of oxidation or Cr(ll) at mercury pool electrodes VOL. 36, NO. 6, MAY 1964
0
1137
as any free chloride is present in the C1- -+ solution. The reaction Cr+3 CrC1+2 is sufficiently slow that this source of CrCl+* may be neglected. The possibility that two chloride ions are present in the transition state, although not likely, cannot be absolutely eliminated because of the rapid rate of the reaction ( 7 ) :
+
CrClz+
+ C r + 2 e C r + *+CrCl+2 + C1-
When the oxidation is carried out in the presence of both chloride and iodide, the iodide [which is more strongly ad41 sorbed on mercury than is chloride ( can replace chloride in the transition state even when chloride is present in excess of both iodide and chromium. Thus, for example, electrolysis of 50 ml. of a solution containing 1.0 meq. C1-, 0.1 meq. I-, and 0.7 meq. Cr(I1) resulted in the production of only 0.5 meq. CrC1+2 instead of the 0.7 meq. CrC1+2 that would have resulted if no iodide had been present (Figure 1). DISCUSSION
The results of the controlled potential oxidation of Cr(I1) in the presence of chloride require that chloride be bound to chromium in the transition state. Zwickel and Taube (8) have shown that chloride enhances the rate of oxidation of Cr+2 by C O ( S H & + ~and that the product of the reaction by the chloridedependent path is CrC1+2. This indi-
cates that catalysis of the oxidation of Cr+2 by chloride with formation of CrC1+2need not imply an inner-sphere, bridged-complex mechanism. But in the case of oxidation of Cr(I1) a t mercury electrodes in the presence of chloride, a bridged, inner-sphere complex can be formed, and since this is the usual mechanism for oxidation of Cr(I1) in homogeneous solution, it is reasonable to assume that it is favored here. The fact that chloride has little effect on the rate of the homogeneous Cr+2Cr+3 exchange reaction (3) suggests that the rate enhancement observed by Aikens and Ross is due to formation of a bridged, activated complex between Cr+2 and chloride adsorbed on the mercury electrode, rather than reaction a t the electrode of CrCl+ whose equilibrium concentration is extremely small. The analogy between chloride adsorbed on mercury electrodes and the chloride in complex ions-e.g., Co(NH3) & 1 + L in which it is known that chloride serves as a bridge for electron transfer, is sufficiently clear to make this a reasonable and attractive hypothesis. In the chloride-iodide competition experiment described, 91% of the halide present is chloride; yet only 71% of the Cr(I1) oxidized is CrC1+2. According to the above point of view, the distribution of product between C r I t 2 and CrCl*z should be controlled by the surface concentration ratio of chloride to iodide on the mercury electrode. Since iodide is
more strongly adsorbed on mercury than is chloride, this result is a t least qualitatively explained by the proposed mechanism. Chronopotentiometric and oscillopolarographic studies of the oxidation mechanism of Cr(I1) are currently in progress to try to settle this and other related questions. LITERATURE CITED
(1) Abubaker, K. M., Malik, W. U., Indian Chem. SOC.36,459 (1959). ( 2 ) Aikens, D. A., Ross, J. W., Jr., J. Phvs. Chem. 65.1213 (1961). (3) Lnderson, A.; Bonner, S . A,, J . Am. Chem. SOC.76,3826 (1954). (4) Grahame, D. C., Chem. Revs. 41, 441 ' (1947). (5) Kemula, W., Rakowska, E., Roczniki Chem. 36,203 (1962). (6) Stone, H. W., A N ~ LCHEW. . 20, 747 (1948). (7) Taube, H. Meyers, H., J. Am. Chem. SOC.76,210i (1954). ( 8 ) Zwickel, A., Taube, H., Ibid., 83, 793 (1961).
JANET G. JONES FREDC. ANSON
Division of Chemistry and Chemical Engineering California Institute of Technology Pasadena, Calif. RECEIVED for review January 24, 1964. Accepted February 27, 1964. Work supported by the National Science Foundation. Contribution from the Gates and Crellin Laboratories of Chemistry.
Large Volume Activation Analysis of Organically Bound iodine Using Isotopic Neutron Sources SIR: The analysis of organically bound halogens is especially tedious and time consuming. A method for the determination of organically bound iodine by the technique of neutron activation analysis was developed with specific reference to acetrizoic acid. In conventional methods of activation analysis using isotopic neutron sources, a small sample is placed a t a distance from the source, and the efficiency of utilization of neutrons is very low. Thus, a limiting factor in the precision of such techniques is the total number of counts which can be obtained from the activated species to estimate the desired constituent. In the work reported herein, macro volumes and concentrations were used and an activation analysis technique adapted to the sample. An isotopic neutron source was immersed directly in the sample solution to give 4-pi absorption of neutrons. The sample solution served as a moderator for the neutrons. This also allowed 1 138
o
ANALYTICAL CHEMISTRY
for the absorption of neutrons a t the resonance energies of the species to be activated, where neutron absorption cross section increases greatly (10). Subsequently the activity of a liter of activated sample was counted using a large volume liquid scintillation counter. Preliminary calibration curves of induced activity were prepared. This method of irradiation and counting increased the number of counts per unit time per unit concentration over conventional methods, thus improving sensitivity and precision. The complete analysis can be carried out in less than 1hour. Iodine-I27 is the only naturally occurring isotope of iodine. It has a thermal neutron activation cross section of 5.6 barns (12) and reacts to form by the ( n , r ) reaction ( 5 ) . Iodine128 has a half life of 25 minutes and decays through beta, gamma, and electron capture processes. The determination of iodine by
neutron activation analysis has been investigated by several workers (2, 3, 5, 6, 15, 16). In particular, Tsuji et al. determined organically bound iodine by placing a 5-ml. sample externally to a 50-mc. Ra-Be neutron source. In this case, the small sample size and relatively low source activity resulted in activation levels which were too low to give good precision. Another method of analysis has been suggested by Kabasakalian and McGlotten (11),who made a study on the polarographic behavior of ten iodinated x-ray contrast compounds. EXPERIMENTAL
Apparatus and Reagents. An xray contrast compound, acetrizoic acid (Urokon Sodium) (3-acetylamine-2,4,6,-tri-iodo benzoic acid) , was used as the source of organically bound iodine, This compound contains 68.35% iodine by weight. The official method of analysis for iodine
