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result is expected only if the local electronic environment about each element in the alloy is very similar to that encountered in the pure metal, Le., the so-called “alloying effects” (10) are minimal. Indeed, theoretical studies (17) of the electronic structure of CuNi alloys using the coherent potential approximation (18)have found that charge exchange between the two constituents of the alloy is very small, the constituent atoms remaining essentially neutral as they are in the pure metals. This suggests that SIMS is a powerful tool for investigating alloying effects and electronic structure changes between pure metals and their alloys; the deviation in the ion yield ratio S M , + / S N , + of the alloy from that of the metals is a measure of the electronic perturbation of the alloy matrix. Ln a previous SIMS investigation of solid solutions of transition elements including Cu in Ni, Blaise and Slodyian (19) observed a “reinforcement phenomenon” in the ion production from solute elements on the basis of knowledge of the bulk composition. Our results show that, at least for the CuNi case, this “reinforcement phenomenon” is most likely due to differences in the surface and bulk concentrations. We conclude that SIMS is a powerful technique for determining the surface concentrations of alloys and studying local atomic electronic changes between alloys and their metal constituents. LITERATURE C I T E D Figure 1. The Cu+/Ni+ secondary ion ratio vs. surface and bulk composition (circles) for a series of CuNi alloys. The curve represents a plot of e q 3.
established by Auger electron spectroscopy. (ii) SIMS can be employed for quantitative surface analysis if the ion yields SM,+(-) for elements in specific environments are known. (iii) The empirical calculation of Zcu+/ZNi+ from eq 2 by using the ratio of ion yields SCu+/SNif from the pure Cu and Ni metals, and its agreement with the alloy data indicates that the ion yield ratio for the CuNi alloys is unchanged from that of the pure metals. The first result shows that the SIMS and Auger spectral information result from the same surface region of the sample. The sampling depth of SIMS is only about two to three atomic layers due to the small escape depth of secondary ions. The Auger determinations (16) of the surface compositions were carried out with the Auger peaks near 100 eV for which the electron escape depths are less than 10 A. The second result illustrates the necessity for developing a method of determining and tabulating absolute secondary ion yields for elements in a variety of different chemical environments, i.e., different compounds or matrixes, before SIMS can be used as a general quantitative surface analysis tool. Although Benninghoven (15)has determined such yields for a variety of metals, much work remains to be done before a general tabulation for elements in various compounds can be compiled. The third, and probably most significant, result shows that the ratio of absolute ion sputtering yields from the pure metals Cu and Ni is transferable to the alloys of those metals. This
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(1) Day, R. J.; Unger, S. E.; Cooks, R. G. Anal. Chem. 1980, 52, 557A. (2) Hewitt, R. W.; Shepard, A. T.; Baltinger, W. E.; Winograd, N. Anal. Chem. 1978, 50, 1286. (3) Honda, F.; Fukuda, Y.; Rabalais, J. W. Chem. Phys., 1980, 47, 59. (4) Gardella, J. A., Jr.; Hercules, D. M. Anal. Chem. 1980, 52, 226. (5) Dawson, P. H.; Tam, W. C. Surf. Sci. 1979, 87, 464. (6) Hopster, H.; Brundle, C. R. J. Vac. Sci. Techno/. 1979, 76, 548. (7) Day, R. J.; Unger, S. E.; Cooks, R. 0.Anal. Chem. 1980, 52, 353. (8) Benninghoven, A.; Sichterman, S. Org. Mass Spectrom. 1978, 12, 1180. (9) Jonkmn, H. T.; Michl, J.; King, R. N.; Andrade, J. D. Anal. Chem. 1978, 50, 2078. (10) Deline, V. R.; Katz. W.; Evans, C. A., Jr. Appl. Phys. Lett. 1978, 33, 832. (11) G i m a a c k , K. I n “Inelastic Ion-Surface Collisions”; Toik, N. H., Tully, J. C., Heihnd, W., Whtte, C. W., Eds.; Academic Press: New York. 1977; p 153. (12) Honda, F.; Lancaster, G. M.; Fukuda, Y.; Rababis, J. W. J. Chem. Phys. 1978, 69, 4931. (13) Lancaster, G. M.; Honda, F.; Fukuda, Y.; Rabalais, J. W. J. Am. Chem. Soc. 1979, 107, 1951. (14) Honda, F.; Fukuda, Y.; Rabahis, J. W. J. Chem. Phys. 1979, 70, 4834. (15) Benninghoven, A. Surf. Sci. 1975, 53, 596. (16) Watanabe, K.; Hashiba. M.; Yamashina. T. Surf. Sci. 1977, 69,721. (17) Seib, D. H.; Spicer, W. E. Phys. Rev. 6: Solid State 1970, 2, 1676. (18) Soven, P. Phys. Rev. 1969, 778, 1136. (19) Blaise. G.; Soldylan, G. J. Phys. (Orsay, F r . ) 1974, 35, 237.
Yasuo F u k u d a F u m i h i r o Honda J. Wayne Rabalais* Department of Chemistry University of Houston Houston, Texas 77004
RECEIVED for review June 9, 1980. Accepted August 4,1980. This material is based upon work supported by the National Science Foundation under Grant No. CHE-7915177.
Kinetics of Back-Extraction of Nickel Dithizonate Sir: It has already been demonstrated that for many metal chelate systems the rate of extraction as well as the yield increases in inverse proportion to the hydrogen ion concentration. This can be attributed to a mechanism in which the rate-determining step is the reaction of the hydrated metal 0O03-2700/80/0352-2214$01 .OO/O
ion with the ligand anion, whose aqueous phase concentration naturally increases as the [H30f] decreases. By the same token one might expect the back-extraction rates to increase with increasing [H,O+]. If this is the case, as was indicated in preliminary experiments, it becomes interesting to explore 0 1980 American Chemical Society
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the comparative effects of other acids, most notably Lewis acids such as strongly complexing metal ions. Does the back-extraction proceed via acid attack of the chelate or does it first undergo dissociation? Further, can Lewis bases, Le., other ligands, catalyze back-extraction, particularly if the chelate is coordinatively unsaturated? An interesting series of chelates to use for such a study is the nickel(I1) dithizonates. Not only have they long been known to exhibit slow rates of extraction ( I ) but the pH range required for their back-extraction is 3-4 units below that needed for extraction, exhibiting a kind of “hysteresis”. Back-extraction kinetics can be followed readily by removing samples a t predetermined intervals from a highly rapidly stirred mixture of the chelate in chloroform and an appropriate aqueous phase. The procedure used closely followed that used in this laboratory for extraction kinetics ( 2 , 3 ) . The rates of back-extraction from chloroform were readily followed by monitoring the concentration of the nickel chelate in the chloroform phase spectrophotometrically inasmuch as its spectrum is significantly different from those of other dithizonates. In every case, the back-extractions were found to be first order in chelate. Elaboration of experimental rate expressions was accomplished by obtaining the dependencies of the pseudo-first-order constants upon the relevant concentration variables. Thus, the back-extraction rate was found to exhibit first-order dependence on [H30+],[Hg2+],and [EDTA] and second-order dependence of [Ag+] and [CN-I. Among the acids, Ag+ and Hg2+enhance back-extraction rates much more than does H30+. Interestingly, CU*+,a strong Lewis acid, does not affect back-extraction. Among the bases, cyanide is much more effective than ethylenediaminetetraacetate (EDTA). Because of the great variation in stripping rates observed among the various reagents, none of which are soluble in CHC13,it appears that a common point in the reactions is the prior, rapid transfer of the nickel chelate to the aqueous phase. This is further corroborated by the qualitative observation that acetic acid, a weak but chloroform-soluble acid, was a better stripping catalyst than H30+. Once in the aqueous phase, the chelate might either form an intermediate complex with reagents like Ag+, Hg2+,or CN- or undergo an inherently slow dissociation with reagents like H,O+ or EDTA. Inasmuch as reactions in the intermediate complex forms do not require the nickel dithizonate to dissociate first, these are more rapid.
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For example, when phenanthroline or dipyridyl form mixed ligand chelates with nickel dithizonate (4),both the extraction and the back-extraction rates of nickel are considerably enhanced (5). In the systems studied here a suitable intermediate complex can be formed by a reagent which either bonds to the central metal ion, provided its coordination number is capable of increasing, or to one of the bonding atoms of the ligand, provided it has unused bonding capability. Cyanide ion, a softer ligand than EDTA, probably adds to the nickel ion in much the same fashion as do the neutral heterocyclic nitrogen ligands (4), whereas Ag+ and Hg2+,which can form complexes with bound sulfur (as in thioethers), probably bond to the sulfur atoms in the chelated dithizone. Thus, nickel dithizonate may be viewed as an “amphoteric Lewis salt”, capable of interacting with both Lewis bases and acids. These findings suggest a novel approach to the rapid decomposition, and possibly rapid formation as well, of complexes of metal ions such as Co, Ir, Rh, and Cr, whose usual substitution reactions are inherently slow. This approach would provide the bases for improved analytical procedures for such metal ions. Further, because of the restricted list of back-extraction catalysts (relatively), inert complexes, such as nickel dithizonate, can be used in the development of highly selective analytical methods for catalytically active species (e.g., for Hg2+in the presence of C:u2+). Studies along the various lines suggested by this work are under way.
LITERATURE CITED (1) Sandell, E. 8 . “Colwmetric Determination of Traces of Metals”,3rd ed.; Interscience: New York, 1959; p 147. (2) Honaker, C.; Freiser, H. J. Phys. Chem. 1962, 66, 127-130. (3) Carter, S.; Freiser, H. Anal. Chem. 1979, 51, 1100-1101. (4) Math, K. S.; Freiser, H. Anal. Chem. 1969, 4 7 , 1682-1685. (5) Freiser, B. S.; Freiser, H. Taknta 1970, 77. 540-543.
Kousaburo Ohashi Henry Freiser* Department of Chemistry University of Arizona Tucson, Arizona 85721 RECEIVED for review May 23,1980. Accepted August 19,1980. This work was supported by research grant from National Science Foundation.
Rapid Scan Square Wave Voltammetric Detector for High-Performance Liquid Chromatography Sir: Voltammetric detectors for high-pressure liquid chromatography (HPLC) are widely used because of their good sensitivity for many electroactive compounds (1). In the most commonly employed constant potential mode, this technique has only modest selectivity. Although electroanalytical techniques are inherently unselective, the selectivity should be improved dramatically by using a scanning technique which provides potential as well as time resolution. As pointed out by Johnson ( 2 ) ,despite pessimistic attitudes toward pulse techniques in this application (11, square wave voltammetry ( 3 , 4 )should provide the full resolution obtainable with respect to potential without degradation of sensitivity. In principle square voltammetry should have the following attributes: (1) discrimination against charging currents and against “background” currents because of the current measurement scheme, (2) little electrode fouling because of the small amount 0003-2700/80/0352-22 15$01.OO/O
of material converted, and (3) good potential resolution on a time scale short in comparison with that required for chromatographic resolution because high scan rates can be employed. We describe here the use of rapid scan square wave voltammetry for HPLC detection. The model compounds chosen are N-nitrosodiethanolaniine (NDELA) and N nitrosoproline (NPRO), and the chromatographic separation is based on work reported elsewhere (5,6). The advantages and possibilities of the technique are discussed, and the selectivity and sensitivity of the detector are compared with others. The chromatographic system, which is described in detail elsewhere ( 5 ) ,employed a detector based on the PARC 310 polarographic detector (Princeton Applied Research, Princeton, NJ). The excitation wave form was generated by D/A converters 0 1980 American Chemical Society
