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Anal. Chem. 1982, 5 4 , 1362-1367
vious section, the adsorbed dithizone forms were not stable to X P S analysis. This instability is indicated by the calculated atom ratios described above at all potentials. Decomposition is also evidenced at both Ag and Cu in a rather unique manner at very negative potentials. For adsorbed dithizone forms at both Ag and Cu, a low binding energy N 1s signal is observed at negative potentials. This signal appears at 393.7 (f0.4)eV at Ag and at 395.8 (f0.4)eV at Cu. The signal is larger at Ag than at Cu, and the value of the binding energy is metal dependent, suggesting that this N-containing species involves the metal surface in some manner. The values of the binding energies observed for these systems suggest the presence of silver nitride (Ag3N) in the case of the Ag electrode and copper nitride (Cu,N) in the case of the Cu electrode. Although the XPS spectra of Ag3N and Cu3N have not been reported, N 1s binding energy values for other nitrides that are in the literature are also relatively low. For example, the N 1s signal for vanadium nitride, VN, appears at 397.2 eV and that for chromium nitride, CrN, appears at 396.6 eV (15). Nitrides can be formed with many elements simply by heating in the presence of ammonia or by heating in the presence of a decomposable N-containing species (16). Everhart and Reilley have previously noted decomposition of organic species in the DuPont 650B spectrometer. They theorize that decomposition is initiated by what resembles a low-pressureplasma established above the sample which is a result of the high secondary electron flux characteristic of the DuPont spectrometer (10). Not only would such a lowpressure plasma provide the necessary perturbation for decomposition of surface dithizone species in these systems, but it would also provide the energy necessary for the formation of the respective nitrides from the metals and the decomposition products. However, the absence of nitride signals at certain potentials suggests that the presence of a low-pressure plasma is not the only requirement for dithizone decomposition and/or nitride formation. Nitrides are apparently formed on Ag and Cu only a t electrode surfaces which had been held at negative potentials.
The work of Everhart and Reilley (10) and that of others (I7) further suggests that degradation of organic species in the spectrometer is catalyzed by the presence of sodium. In the electrochemical systems described here, adsorption of the various dithizone forms takes place from sodium hydroxide based pH 12 buffer solutions. At potentials negative of the potential of zero charge on Ag and Cu, specific adsorption of Na' is expected to occur. Furthermore, XPS evidence for the existence of electrodepositedNaO on Ag electrodes at negative potentials was presented previously (1). Hence, the catalytic sodium presumably necessary for significant decomposition of surface dithizone species is probably present on both Ag and Cu electrodes, but only at negative potentials. This fact explains the appearance of the nitride signals only at negative potentials.
LITERATURE CITED (1) (2) (3) (4) (5) (6) (7) (8)
(9) (10) (11) (12) (13) (14) (15) (16) (17)
Pemberton, J. E.; Buck, R. P. J . Elecfroanal. Chem. 1982, 132,291. Pemberton, J. E.; Buck, R. P. J . Phys. Chem. 1981, 85, 248. Pemberton, J. E.;Buck, R. P. Anal. Chem. 1981, 53, 2263. Pemberton, J. E.; Buck, R. P. J . Am. Chem. Soc., in press. Pemberton, J. E.; Buck, R. P., submltted for publication. Katrib, A.; Kasslm, A. Y. Anal. Chem. 1980, 52, 1546. Carlson, T. A.; McGulre, G. E. J . Electron Smxtrosc. Relat. Phenom. 1972-1973, 1 , 161. Fischer, A. 5.; Wrighton, M. S.; Umana, M.; Murray, R. W. J . Am. Chem. SOC. 1979, 101, 3442. Penn, D. R. J . Electron Spectrosc. Relat. Phenom. 1978, 9 , 29. Everhart, D. S.; Reilley, C. N. Anal. Chem. 1981, 53,665. Scofield, J. H. Report No. UCRL-51326; Lawrence Livermore Laboratory, Jan 1973. Reilman, R. F.; Msezane, A.; Manson, S. T. J . Electron Spectrosc. Relat. Phenom. 1976, 8 , 389. Wagner, C. D. Anal. Chem. 1977, 4 9 , 1282. Moses, P. R.; Wier, L. M.; Lennox, J. C.; Finklea, H. 0.;Lenhard, J. R.; Murray, R. W. Anal. Chem. 1978, 5 0 , 576. Baker, A. D.; Betteridge, D. "Photoelectron Spectroscopy: Chemical and Analytlcal Aspects"; Pergamon Press: New York, 1972; p 166. Maxted, E. 6. "Ammonia and the Nitrides"; J. & A. Churchlll: London, 1921; Chapters 3 and 4. Wendt, R. C.; Gibbs, H. H.; Wilson, F. C. Polym. Eng. Sc;. 1979, 19, 342.
RECEIVED for review January 28,1982. Accepted April 1,1982. This paper is part 6 in the series Dithizone Adsorption at Metal Electrodes.
Determination of Chemical Reaction Rate Constants Preceding or Following Electron Transfer by Mechanical Square Wave Polarography Lin Sin-rhu and Feng Qiang-sheng" Shanghai Institute of Metallurgy, Academia Sinica, Shanghal200050, The People's Republic of China
A simple and convenlent method for determining the chemlcal
reactlon rate constants of chemical reaction followed by electron transfer (CE) and electron transfer followed by chemical reaction (EC) processes has been developed by means of mechanlcal square wave polarography. To test the theory, we studied the dlssoclation of Sb'I'EDTA complex and monochloroacetlc acid and the benrldlne rearrangement of hydrarobenrene produced by the electrode reduction of arobenrene. The results were shown to be compatlble with data of other Investigators.
In the kinetic study of fast electrode processes by potential
step chronoamperometry, the measurable apparent standard rate constant k" is limited to the order of 0.2 cm/s ( 1 ) by the RC time constant of the whole determining system. In square wave polarography the recorded current is the summation of the responses of a few hundred or thousand of successive potential steps. If these responses are sampled at the same time interval, then the frequency response of the current recorder may be longer than a few seconds. Therefore, a high sensitivity current recorder or high sensitivity galvanometer (for mechanical square wave polarography) can be used. However, the decay process of charging current is still dependent on RC time constant of the whole determining system, so the measurable upper limit of k" in square wave polarography is in the same order of magnitude as compared
0 1982 American Chemical Society 0003-2700/82/0354-1362$01.25/0
ANALYTICAL CHEMISTRY, VOL. 54, NO. 8, JULY 1982
with a single or double potential step technique. One of the advantages for mechanical square wave polarography (2) is that the resistance of the whole instrument is close to zero while eliminating the charging current in a half-period of each square wave. If this instrument is fitted with a low resistance capillary (3),the RC time constant of the whole determining system will be quite small ( 0.5 M). In HClO., + NaC104 solution, the rate constant scarcely changes with increase in the concentration of HC104. The origin of these phenomenon will be researched further. It may be known from the derivation of eq 14 that kinetic fador is considered in each half cycle. In addition, azobenzene w i l l be adsorbed on the electrode surface, so its electrochemical behavior at the electrode surface in its lower concentration solution will differ from its higher concentration solution.
+
+ k)lJ2 S1J2p S 1 J 2+ (S + k ) l l 2
1 =0-
(S
(20)
where
0 = nFqD1J2Co;
P = exp[(E - Ea)nF/RT] rearrangement of eq 20 yields
Equation 21 is a linear one. From the intercept and slope of the plot [ L ( i ) S / ( O- L(i)S1i2)]2against S, the k can be calculated without the Eo value. The above handling is more simple in mathematics than the approach of Marcoux and others (16, 17). It follows from the previous paper (3) and this paper that analysis in Laplace space is especially valuable for the study of the transient electrode process. ACKNOWLEDGMENT The authors thank Tai Jia-Xing (East China Institute of Chemical Engineering) for his assistance.
Anal. Chem. 1982, 5 4 , 1367-1371
LITEXATURE CITED (1) Delahay, P. "Advances in Electrochemlstry and Electrochemical Englneering"; Interscionce: New York, 1961; Vol. I , pp 254. (2) Feng, Q. S.;Liu, G. L. MuaXueXueSao 1965, 3 1 , 291. Feng, Q. S. HuaXue Xue6ao 1966, 32, 7. (3) Feng, Q. S.; Lln. S. R. Anal. Chem. 1961, 5 3 , 1006. (4) O'Dea, J. J.; Osteryoung, J.; Osteryoung, R. A. Anal. Chem. 1961, 5 3 , 695. (5) Koutecky, J. Collect. Czech. Chem. Commun. 1958, 18, 597. (6) Galus, 2. "Fundamentals of Electrochemlcal Analysls"; Ellis Horwood: 1976. (7) Koutecky, J. Collect. Czech. Chem. Commun. 1955, 2 0 , 116. (8) Bhat, T. R.; Iyer, R. K. Z . Anorg. Chem. 1965, 335, 331.
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(9) Wang, E. K.; Chang, R. 6 . HuaXue XueSao 1965, 3 1 , 18. (10) Moussa, A. A.; Abou-Romla, M. M.; Ghaiy, H. A. Nectrochim. Acta 1974, 19, 957. (11) Gottschalk, G. Z . Anal. Chem. 1956, 759, 257. (12) Hanus, V. Chem. Zvesti 1954, 8 , 702. (13) Delahay, P.; Oka, S.J . Am. Chem. Soc. 1960, 82, 329. (14) Schwarz, W. M.; Shain, I. J . Phys. Chem. 1965, 6 9 , 30. (15) Kern, D. M. H. J . Am. Chem. Soc. 1954, 76, 1011. (16) Marcoux, L.; O'Brien, T. J. P. J . Phys. Chem. 1972, 7 6 , 1666. (17) Cheng, H. Y.; MrCreery, R. L. J . Electroanal. Chem. 1977, 8 5 , 361.
RECEIVED for review September ,14, 1981. Accepted March 9, 1982.
Anodic Stripping Voltammetry for Evaluation of Organic-Metal Interactions; in Seawater Stephen R. Piotrowicx," M. Springer-Young, Jorge A. Pulg, and Mary Jo Spencer' National Oceanic and A tniospheric Administration, Atlantic Oceanographic and Meteoroiogical Laboratories, Ocean Chemistry and Biology Laboratory, 430 1 Rickentlacker Causeway, Miami, Florida 33 149
Dlfferentlal pulse anodlc strlpplng voltammetry (DPASV) is used to study aspects of the speciation of Cd, Cu, and Zn In seawater at natural levels of these metals. The speclatlon at natural pH of these metals appears to be a dynamic process changing wlth tlme constants on the order of hours to days. The technlque Is used to study the interactlon of these metals with marlne fulvlc acids. Results suggest that marlne fulvlc acids Interact wlth these metals to varying degrees and in a manner similar to that observed In natural samples. There Is llttle or no lnteractlon of Cd wlth the marlne fulvlc acids tested, strong lnteractllon wlth Zn, and varying degrees of lnteractlons with Cu. Ttie extent of these lnteractlons wlth Cu appears to be related to dlfferlng structural features of the fulvlc acids.
Developing an understanding of the biogeochemical cycles of trace metals in the sea has been difficult because seawater is not a simple electrolyte and the concentrations of most metals are in the nanomolar and below range. Inorganic speciation has been ashiessed with equilibrium models using known stability constartis and making assumptions regarding activity coefficients at extreme dilutions (1-5). The presence of relatively high concentrationsof organic matter in seawater, however, considerably complicates any analysis of speciation. Organic matter in seawater has been found to complex trace metals, especially copper (6-8). Equilibrium models including organic ligands in fresh and salt water generally employ known complexing ligands (9),make assumptions on the range that the metal-organic ligand stability constant might be, or infer what the stability constant would have to be in order for it to successfully compete with inorganic ligands in a speciation model (10). Determinations of stability constants in fresh and coastal waters have generally concluded that metal-organic ligand stability constants are in the range of lo6and 1O1O (11) and, therefore, that built organic matter is not important in the speciation of trace metals in these waters. Present address: Department of E a r t h Sciences, University of N e w Hampshire, Durham, NH 03824.
Several studies, however, have found that a large portion of the Cu and Zn present in seawater appears to be organically complexed (12,13). Recent evidence suggests that most of the copper present in seawater is complexed or otherwise coordinated with organic ligands with apparent stability constants greater than 1O1O (14). Microbiological techniques have indicated that Cu-organic ligand stability constants in seawater may be in the range of 10l2(15). Stability constants of this magnitude are sufficient such that thermodynamic considerations alone would predict that some metals in seawater should be present as an organic complex. Determining which metals are bound, to what extent they are bound, and the kinetic stability of the complexes are some of the questions that must be investigated prior to developing a complete understanding of the chemistry and cycling of these metals in seawater. A variety of methods have been employed to assess complexation in seawater, with electrochemical methods among the most popular. The main advantages in using electrochemical methods are that the sensitivities are such that minimal sample handling of the sample is necessary, thus, minimum potential for alteration of the sample. Anodic stripping voltammetry (ASV) is the most widely used of these techniques (e.g., ref 16-20). ASV labile metal determinations at natural pH measure that metal present as free ion, simple inorganic complexes, and possibly weak organic complexes (21, 22). Subsequent acidification should release those metals tied up in stronger organic complexes and organic and inorganic colloids and that portion absorbed on particulate matter. Even with the best equipment available the technique had been limited to use where concentrations were in the range of 50 nM and greater for Cu and several nanomolar for Cd and Zn. This has been caused as much by contamination problems as by technology. The development and refinement of the thin Hg-film/glassy carbon electrode (23,24)provide a technique with sufficient sensitivity to be used for direct determinations on surface, open-ocean seawaters (25). The concentrations of Cu and Zn in these waters are tenths of nanomoles per liter and hundreths nanomolar for Cd. A combination of electrode technology and ultraclean handling techniques has provided us with a system capable of evaluating
Thls article not subJectto U S . Copyright. Published 1982 by the Amerlcan Chemlcal Society
