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Chemical Reactions of 2,5-Dimercapto-1,3,4-thiadiazole (DMTD) with Metallic Copper, Silver, and Mercury Ling Huang,† Feng Tang,† Baixing Hu,† Jian Shen,*,†,‡ Tsing Yu,§ and Qingjin Meng‡ Research Center of Surface and Interface Chemistry and Engineering Technology, Nanjing UniVersity, Nanjing 210093, P. R. China, Department of Chemistry, Nanjing UniVersity, Nanjing 210093, P. R. China, and State Key Laboratory of Coordination Chemistry, Coordination Chemistry Institute, Nanjing UniVersity, Nanjing 210093, P. R. China ReceiVed: December 6, 2000; In Final Form: May 20, 2001
Chemical reactions of DMTD with metallic copper, silver, and mercury have been studied by infrared (IR), surface-enhanced Raman scattering (SERS), UV-vis, and X-ray photoelectron spectrum (XPS) techniques. It is found that DMTD can react readily with metallic copper, silver, and mercury under mild conditions. The reaction product of DMTD with metallic copper shows an IR spectrum slightly different from that of Cu(DMTD)2, suggesting a Cu+ component in the product, which is further confirmed by the XPS result. The reaction products of DMTD with metallic silver and liquid mercury are Ag(DMTD) and Hg(DMTD)2, respectively. A two-step mechanism is proposed for these surface reactions: first DMTD is chemisorbed on the metal surfaces with the formation of the disulfide salt of DMTD as the intermediate product, and then the unstable disulfide salt reacts further with the excessive DMTD in solution, forming the monosulfide salt of DMTD which covers the metal surfaces in the form of a polymer layer or even a monolayer.
Introduction Some of the nitrogen and/or sulfur containing organic compounds, such as imidazole,1 thiazole,2 and thiadiazole3 are well-known ligands for transition metal ions. They are also surface-active agents and are widely used in the practice although their surface chemistry is frequently not well understood. 4-6 2,5-Dimercapto-1,3,4-thiadiazole (DMTD), known as bismuth I, is also of considerable interest as a ligand for transition metal ions due to its special structure. The compound can exist both in the thiol and the thione forms. In the solid state as well as in solution, it exists as tautomers, namely, the dithiol form, the thiol-thione form, and the dithione form. So DMTD has four donor sites and can coordinate as bidentate ligand with (a) both the nitrogen atoms, (b) both the thiocarbonyl sulfur atoms, or (c) one nitrogen atom and one sulfur atom on either the same side or different sides of the molecule. The π-electron in the aromatic heterocyclic ring can also coordinate with metal ions potentially. Up to now, a large amount of work has been published on compounds containing DMTD and different kinds of metal ions and their complex properties. 7-16 The wide applications in industry are another notable aspects of DMTD. For example, because of the bactericidal activity of the Cu2+ compound of DMTD, it may be used as an intermediate in the synthesis of bioactive agents.10 The tributyl chloride compound of DMTD has been widely used as a protective against marine organisms on the bottom of ships.14 A substantial amount of work has also been carried out on the application of bismuth I and II in quantitative analysis.17-21 Besides, it can * To whom correspondence should be addressed. † Research Center of Surface and Interface Chemistry and Engineering Technology. ‡ State Key Laboratory of Coordination Chemistry. § Department of Chemistry.
be applied as additives in the fields of antioxidation,22 anticorrion,23,24 and antifriction,25-27 as metal deactivators28,29 in motor oils, and also as cross-linkers in polymers.30,31 Recently, Oyama found that DMTD could be used as the cathode material in rechargeable lithium batteries.32-35 This compound is identified to be an attractive cathode material because of its good chemical stability, fast redox kinetics, and low activation energy for charge transfer relative to other organosulfur compounds.36 It combines high theoretical energy storage capability with low weight and good mechanical strength. The applications of DMTD and its derivatives in the energy resources have become a new important trend and validate again the research of DMTD for a prosperous future. Although DMTD is well-known for its ability to form stable complexes with heavy- and transition-metal ions,37 no systematic studies have been performed on the direct chemical reactions of DMTD with zero oxidation state metals, such as copper, silver, and mercury. Additionally, in many industrial uses of DMTD, whether as additives in lubricant oils or as the cathode materials in second batteries, DMTD always contacts directly with zero oxidation state metals. It can be the reactions between DMTD and the surfaces of the zero oxidation state metals that are essential to endow DMTD some special properties and to make DMTD be well used in the aforementioned fields. Thus, the studies of this kind of reactions will not only widen the researches of DMTD, but they will also provide a useful theoretical basis for a better use of this type of compounds and their derivatives in industry. In this study, the reactions between DMTD and metallic copper, silver, and mercury under mild conditions have been studied. Infrared (IR), surface-enhanced Raman scattering (SERS), UV-vis, and X-ray photoelectron spectrum (XPS) techniques were used to characterize the structures of the reaction products, and a possible reaction mechanism is proposed.
10.1021/jp004385j CCC: $20.00 © 2001 American Chemical Society Published on Web 07/27/2001
2,5-Dimercapto-1,3,4-thiadiazole (DMTD)
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Experimental Section Materials and Sample Preparation Methods. DMTD was synthesized according to a previous method.38 The complexes Cu(DMTD)2, Ag(DMTD) and Hg(DMTD)2 were prepared following the known procedures.10,11,12 The ethanol used was of analytical grade. Water was doubly distilled. A 2% HNO3 etched copper powder (0.25 g) was mixed with a 100 mL alcoholic solution of DMTD (1.18 g). The mixture was stirred vigorously at room temperature for 4 days in air. After separation, the gray green product (Cu-A) was obtained, which was used for the IR measurement. The solution left (CuB) was ready for the UV-vis measurement. A silver powder (0.14 g) was immersed in a 50 mL alcoholic solution of DMTD (0.5 g). The mixture was stirred vigorously at room temperature for 20 days. Gray product (Ag-A) was collected, which was ultimately used for the IR measurement. The solution left (Ag-B) was ready for the UV-vis measurement. The liquid mercury (0.296 g) was added into a 50 mL alcoholic solution of DMTD (0.75 g). After 3 days of vigorous stirring, a yellow precipitate formed in the bottom of the flask. The obtained solid (Hg-A) was used for the IR measurements. Instrumental and Experimental Configurations IR Study. The IR spectra of the above-mentioned products were measured with a Bruker IF-66V Fourier transform infrared (FT-IR) spectrometer, equipped with a diode-pumped Nd:YAG laser as the light source. The spectral resolution was 4 cm-1. The samples were measured in the KBr pallets. SERS Study. Electrochemical roughening of the copper surface was carried out in a typical three-electrode potential static system. A platinum wire was used as the counter electrode and a saturated calomel electrode (SCE) as the reference electrode. All potentials are reported with respect to SCE. The copper foils were first polished mechanically and then electrochemically roughened in 0.1 M KCl solution by 5 oxidation-reduction cycles (ORC) between -0.55 and 0.0 V. The foils were washed repeatedly with distilled water after withdrawing. Finally, they were immersed into the alcoholic solution of DMTD for 5 min, rinsed in ethanol to remove the physisorbed substance, and dried in air for lateral measurements. The SERS spectra of the samples were obtained with a Bruker RFS-100 Fourier transform Raman (FT-Raman) spectrometer equipped with a diode-pumped Nd:YAG laser emitting at a wavelength of 1064 nm as the near-infrared excitation source and a liquid nitrogen-cooled germanium detector. The scattered light was at an angle of 180 °. Spectra of the samples were collected at 4 cm-1 resolution by accumulating 200 scans with a laser power at the samples of 150 mw. All data were analyzed using the OPUS 2.0 software systems. UV-Vis Study. The UV-vis absorption spectra were recorded on a Shimadzu model 3100 UV-vis-NIR spectrometer in the range of 190-700 nm. The ethanol was used as the reference background. XPS Study. A piece of copper disk was etched with 10% HNO3 to remove the oxide layer, repeatedly washed by distilled water, and dried in a vacuum. Then the cleaned copper disk was immersed in the alcoholic solution of DMTD for 1 day. The copper disk was rinsed in ethanol to remove the physisorbed substance after withdrawing. XPS spectrum was obtained by means of the VG ESCALABII Model x-ray photoelectron spectrometer, with Al KR-ray (12.5 kV, 20 mA) as the exciting source. The C1s line (BE ) 285.0 eV) from the residual pump-line oil contamination was
Figure 1. IR spectrum of the reaction products of DMTD with (a) metallic copper, (b) cupric ions, and (c) DMTD in solid state.
TABLE 1: Major IR Bands (cm-1) and Their Assignments of DMTD and Its Complexesa DMTD 2480 m 1505 s 1452 s 1265 s 1050 m 940 m 920w 715 m
Cu0 + DMTD
Ag0 + DMTD
Hg0 + DMTD
1468 m 1405vw 1257 m 1042 s
1470 w 1402 vw 1272m 1037 s
1469 m 1412 w 1272 w 1040 s
722 m
721 m
720 w
assignments11 υ(S-H) thioamide bands Ib thioamide bands Ib thioamide bands IIc thioamide bands IIId thioamide bands IVe thioamide bands IVe υas(C-S-C endocyclic)
υ ) stretch; δ ) deformation; ip ) in-plane; s ) symmetric; as ) asymmetric. b Due to δ(N-H) (major contribution) + υ(CdN) (minor contribution). c Due to δ(N-H) (minor contribution) + υ(CdN) (major contribution). d Due to υ(CdS) + υ(CsN). e Due mainly to υ(CdS).
used as the internal standard for spectral calibration. The vacuum extent was higher than 2 × 10-6 Pa. Results and Discussion 1. Surface Reactions of DMTD with Zero Oxidation Metals. DMTD is a strong proton donor with two pKa values at -1.36 and 7.5.39-41 It has been known that the thiolcontaining compounds can interact easily with noble metals and can be adsorbed strongly on the metal surfaces.42-45 Considering that DMTD may react with and be adsorbed strongly on transition-metal surfaces, the surface reactions between DMTD and metallic copper, silver, and mercury in ethanol solution have been carried out. For comparisons and also to find out the structures of the products, model complexes Cu(DMTD)2, Ag(DMTD), and Hg(DMTD)2 have been prepared by refluxing the ethanol solutions of DMTD and the metal ions. A. Chemical Reaction of DMTD with Metallic Copper. The IR spectrum of DMTD shows a moderate strong band at 2480 cm-1 (Figure 1c), characteristic of the o˜ (S-H) band in DMTD.46 This band disappears completely in the IR spectrum of Cu-A (Figure 1a), suggesting the rupture of the S-H bond in DMTD and the formation of the S-Cu bond at the same time. The far-infrared band at 348 cm-1 confirms the presence of the S-Cu bond (not shown here). As shown in Table 1, the bands at 1505 and 1452 cm-1 in Figure 1c are assigned to the thioamide bands I of DMTD, which contain a major contribution from δ (N-H). However, only one
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SCHEME 1. Model of the Interaction Mechanism of DMTD with the Metal Surfaces: (a) One-Dimensional Polymer Chain and (b) Two-Dimensional Weblike Polymer Chain.
band appears at 1468 cm-1 for Cu-A (Figure 1a), indicating that only one N-H group exists in the reaction product. Therefore, DMTD must exist in the thiol-thione form in the reaction product. The strong band at 1265 cm-1 in Figure 1c is assigned as the thioamide band II of DMTD, which is mainly contributed by the o˜(CdN). For the IR spectrum of the reaction product (CuA) as shown in Figure 1a, this band is splitted into two bands at 1257 and 1405 cm-1 respectively. This can be explained based on the final product as an infinite polymer chain (see the reaction product a in Scheme 1). It can be seen that one DMTD molecular combines with two copper ions through both of its excocyclic sulfur atoms. One sulfur atom binds with the copper ion in the form of the monosulfide salt of DMTD (-NdCS-Cu), while the other sulfur atom forms a coordination bond from CdS to the copper ion (H-N-CdSfCu). This coordination reduces greatly the conjugate effect in H-N-CdSfCu, which makes the CN bond order of H-N-CdSfCu lower than that of -NdC-S-Cu. As a result, the band due to o˜(C-N) should be expected to appear at a lower-wavenumber side at 1257 cm-1 and the other due to o˜(CdN) at a lower-wavenumber position at 1405 cm-1. The thioamide bands III at 1050 cm-1 in Figure 1c remain nearly constant (at 1042 cm-1) in Cu-A (Figure 1a). Since o˜ (N-N) contributes mainly to the thioamide band III, we should not expect too much of shift in the position of this band if the bonding scheme is assumed as the above-discussed. The thioamide bands IV that contain a major contribution from o˜ (CdS), appear as sharp bands at 940 and 920 cm-1 for DMTD (Figure 1c). In Figure 1a, the two bands disappear completely. A new band appears at 722 cm-1 can be assigned to o˜(C-S). The result suggests that the bond order of CdS is reduced nearly to that of C-S in Cu-A. It further indicates that DMTD bonds to copper ion through both of its excocyclic sulfur atoms. To find out the structure of the product, we have measured the IR spectrum of Cu(DMTD) 2 (shown in Figure 1b). When compared with Figure 1a, we find that the two IR spectra are only a little different from each other. It has been known that DMTD coordinates to cupric ions in the thiol-thione form via both of its thiol and thiocarboxyl groups, forming a twodimensional weblike polymer. Since the copper powder does react with DMTD and its IR spectrum is slightly different from that of Cu(DMTD) 2, the only explanation is that the oxidation state of the copper ions in the two cases are different. The UVvis and XPS results provide further evidence for this conclusion. Figure 2 shows the UV-vis absorption spectra of DMTD and Cu-B. DMTD has two characteristic UV-vis absorption bands at 260 and 335 nm,47 as shown in Figure 3c. For Cu-B, two bands at 294 and 357 nm are observed (Figure 3a). The absorption differences are also found between Cu-B and the
Figure 2. UV-vis spectra of the reaction solution of DMTD with (a) metallic copper, (b) cupric ions, and (c) the alcoholic solution of DMTD.
Figure 3. XPS spectrum of the Cu (2p) region of the Cu ion and DMTD layer on the copper immersed in the 10-3 M alcoholic solution of DMTD for 1 day.
ethanol solution of Cu(DMTD)2 (Figure 3b). The latter has two absorption bands at 288 and 357 nm. It is thus obvious that the new band at 294 nm comes from the reaction product Cu-A. The oxidation state of the copper ions in Cu-A should be different from that in Cu(DMTD)2. Figure 3 shows the XPS spectrum in the Cu (2p) region of the Cu ion-DMTD layer on the copper disk surface. The two peaks at 932 eV (Cu 2p3/2) and 952 eV (Cu 2p1/2) give clear evidence that Cu is presented in the +1 oxidation state.48 Besides, since
2,5-Dimercapto-1,3,4-thiadiazole (DMTD)
Figure 4. IR spectrum of the reaction products of DMTD with (a) metallic silver, (b) silver ions, and (c) DMTD in solid state.
the mean free path of photoemitted electron in the energy range of interest is only a few to a few tenths of angstroms, the XPS analysis indicates that a very thin layer or even just a monolayer of the chemisorbed DMTD exists on the copper surface. The SERS results also provide useful information on the molecular structure of the monolayer adsorbed on the copper surface, which will be discussed in section 2. It can be concluded that after the reaction of DMTD with metallic copper, DMTD coordinates with Cu+ ion through both of its excocyclic sulfur atoms. One is the chemical bond, and the other is a coordination one. They can further interact each other and subsequently forms an infinite layer or even a monolayer of Cu+-DMTD on the copper surface. B. Chemical Reaction of DMTD with Metallic SilVer. When the silver powder was mixed with DMTD in alcoholic solution by vigorous agitating for 20 days, the gray product (Ag-A) was obtained. Figure 4a shows the IR spectrum of Ag-A. Compared with that of DMTD (Figure 4c), the corresponding bands are found to move in a way similar to those for Cu-A (Figure 1). The disappearance of the o˜ (S-H) band at 2480 cm-1 in Figure 4a indicates the rupture of the S-H bond in DMTD and, at the same time, the formation of the S-Ag bond in Ag-A. The bands at 1505 and 1452 cm-1 in the IR spectrum of DMTD are reduced into one band at 1470 cm-1 in Figure 4a, which suggests that DMTD exists in the thiol-thione form in Ag-A. The other bands such as 1402, 1272, 1037, and 721 cm-1 in DMTD also change with the same trends as those discussed in section 1A. The IR spectrum of Ag(DMTD) is shown in Figure 4b. It is the same as that in Figure 4a, not only in the positions but also in the relative intensities of all the bands. Obviously, Ag-A has the same stoichoimetry and the same structure as those of Ag(DMTD), which means that the metallic silver was oxidized into Ag+ by DMTD during the reaction, forming the Ag(DMTD) complex. The result is further confirmed by the UVvis spectra. The UV-vis absorption spectrum of the reaction solution (Ag-B) shows two new peaks at 242 and 288 nm (Figure 5a), suggesting the formation of a new product. The spectrum is essentially the same as that of the reaction solution between Ag+ and DMTD (Figure 5b). It also indicates that the reaction processes for both the Ag0 and the Ag + ion with DMTD are the same and their products have the same stoichiometery and the same structure. The peak at 335 nm is the characteristic band of DMTD-.40
J. Phys. Chem. B, Vol. 105, No. 33, 2001 7987
Figure 5. UV-vis spectra of the reaction solution of DMTD with (a) metallic silver, (b) silver ions, and (c) the alcoholic solution of DMTD.
Figure 6. IR spectrum of the reaction products of DMTD with (a) liquid mercury, (b) mercury ions, and (c) DMTD in solid state.
In conclusion, during the reaction of DMTD with metallic silver, the DMTD coordinates to the silver ion via both of its two sulfur atoms, forming a S-Ag chemical bond and a S f Ag coordination bond. The complexes interact further with each other and form a one-dimensional polymer chain on the silver surface. C. Chemical Reaction of DMTD with Liquid Mercury. Figure 6a shows the IR spectrum of Hg-A. Similarly, the o˜ (S-H) band at 2480 cm-1 disappears in Figure 6a, and the two thioamide bands I in DMTD are reduced into one band at 1469 cm-1. Compared to the thioamide band at 1265 cm-1 in DMTD, there appear two bands at 1411 and 1270 cm-1 in Hg-A. The thioamide band III at 1052 cm-1 in DMTD shifts negatively to 1040 cm-1 in Hg-A The thioamide bands IV at 940 and 920 cm-1 in DMTD are reduced into one band at 720 cm-1 in HgA, assigned to o˜ (C-S). As Figure 6a is exactly the same as the IR spectrum of the Hg(DMTD)2 (Figure 6b), the product Hg-A should be Hg(DMTD)2. In conclusion, during the reaction of DMTD with the liquid mercury, the mercury was oxidized into Hg2+ with the formation of Hg(DMTD)2. Therefore, the mercury surface is covered by an infinite two-dimensional weblike monolayer of Hg(DMTD)2,
7988 J. Phys. Chem. B, Vol. 105, No. 33, 2001
Figure 7. SERS spectra of DMTD on copper surface (a) immediately after withdrawing from the solution and (b) after being immersed in solution for 90 h.
where DMTD coordinates with the Hg2+ ion through both of its excocyclic sulfur atoms. To investigate the role of oxygen in the reaction, we carried out a parallel reaction where oxygen was removed from the solution by bubbling with purified nitrogen and at the same time, the predried solvent was used. We found that the reaction can still progress and there was no water detected in the solution after the reaction. Besides, the IR spectrum of the parallel reaction products (not shown here) is exactly the same as that of Figure 6a or Figure 6b.This indicates that the oxygen does not participate in the reaction and that oxygen is not the prerequisite of this kind of reaction. Apparently, DMTD itself behaves as the oxidant. The mercury was oxidized into Hg2+ by DMTD. 2. Discussion of the Reaction Mechanism. On the basis of the above discussion in section 1C, we know that DMTD can react with liquid mercury without the participation of oxygen and further, there is no water produced during the reaction process. Thus, this kind of reaction occurs only between DMTD and the zero oxidation state metals, and DMTD itself behaves as the oxidant. It is known that DMTD is a strong proton provider and can easily dissociate H+. As the adsorption of DMTD on metal surfaces progresses, the acidity near the metal surfaces is increased, and the formal potentials of Mn+/M0 is changed. It is the H+ from DMTD that oxidizes M0 to Mn+. Moreover, it is known that DMTD can steadily coordinate with many transition-metal ions including Cu+, Ag+, and Hg2+ to form complexes. These complexes can further interact with each other to form polymers that are insoluble in organic solvents, which may also pull the reaction to the product side and the process is irreversibly. Or, the formation of a very stable complex induces and accelerates the chemical reactions, with the monosulfide salt of DMTD being the ultimate product. However, the SERS results that obtained immediately after the adsorption provides different information. Figure 7a shows the SERS spectrum of DMTD adsorbed on the copper surface immediately after withdrawing of the sample from DMTD solution. The strong band at 1376 cm-1 indicates that the reaction product of DMTD with metallic copper surface is a disulfide salt of DMTD,49 although the IR spectrum of Cu-I
Huang et al. indicates a monosulfide salt of DMTD. Thus, a transition state must exist between the above two products during the surface reaction. Figure 7b shows the SERS spectrum of DMTD adsorbed on the copper surface after the sample was treated for 90 h. A shoulder at 1410 cm-1 is observed as well as the strong band at 1376 cm-1, which suggests the formation of the monosulfide salt of DMTD. So it is clear that the disulfide salt of DMTD is formed as transition intermediates at the very beginning of the surface reaction. Since there is too much DMTD in the reaction solution, when the reaction time is prolonged, the disulfide salt of DMTD can react with the excessive DMTD, forming the monosulfide salt of DMTD. Further, the monosulfide salt of DMTD can interact with each other and is polymerized into an infinite monolayer adsorbed on the metal surfaces. The proposed mechanism for the chemical reactions of DMTD with the zero oxidation state metals can be summarized in Scheme 1. Conclusions DMTD can react easily with metallic copper, silver, and mercury under mild conditions. The reaction consists of two steps. DMTD is first chemisorbed on the metal surfaces by the rupture of both of its two excocyclic S-H bonds, forming the disulfide salt of DMTD as the intermediate product. Then the unstable disulfide salt reacts further with the excessive DMTD in solution, with the monosulfide salt of DMTD being the final product, which forms a polymer layer or even a monolayer covering on the metal surfaces. A similar reaction has also been discovered between DMTD and the zero oxidation state iron. Acknowledgment. Financial support of this work by the Doctoral Program Foundation of the National Ministry of Education of China (Grant No. 98028451) is gratefully acknowledged. Ling Hang deeply thanks Prof. Zheng Zhang (Department of Chemistry, Nanjing University) for his kind help in the syntheses of DMTD and its derivatives. References and Notes (1) Sundberg, R. J.; Martin, R. B. Chem. ReV. 1974, 74, 471. (2) Katritzky, A. R.; Rees, C. W. ComprehensiVe Heterocycl. Chemistry, 1st ed.; Pergamon: Elmsford, NJ, 1984; Vol. 6, Part 4B. (3) Pappalardo, S. Trends Heterocyclic Chem. 1991, 21, 43. (4) Kester, J. J.; Furtak, T. E.; Becolo, A. J. J. Electrochem. Soc. 1982, 129, 1716. (5) Thierry, D.; Leygraf, C. J Electrochem. Soc. 1986, 133, 2236. (6) Hashemi, T.; Hogarth, C. A. Electrochim. Acta 1988, 33, 1123. (7) Zaidi, S. A. A.; Varshney, D. K.; Siddiqi, K. S.; Siddiqi, Z. A.; Islam, V. Acta Chim. Acad. Sci. Hung. 1975, 95, 383. (8) Fabrettic. A. C.; Franchini, G. C.; Peyronel, G. Spectrochim. Acta 1980, 36A, 517. (9) Zaidi, S. A. A.; Varshney, D. K. J. Inorg. Nucl. Chem. 1975, 37, 1804. (10) Gajendragad, M. R.; Agarwala, Z. Anorg. Allg. Chem. 1975, 415, 84. (11) Gajendragad, M. R.; Agarwala, U. Indian J. Chem. 1975, 13, 697. (12) Zaidi, S. A. A.; Farooqi, A. S.; Varshney, D. K. J. Inorg. Nucl. Chem. 1977, 39, 581. (13) Gajendragad, M. R.; Agarwala, U. J. Inorg. Nucl. Chem. 1975, 37, 2429. (14) Nagasawa, M. U.S. Patent 268,347 (cl. 106-15), Aug 23, 1966. (15) Gajendragad, M. R.; Agarwala, U. Aust. J. Chem. 1975, 28, 763. (16) Osman, M. M.; Makhyoun, M. A.; Tadros, A. B. Bull. Soc. Chim. Fr. 1980, Part I, 451. (17) Pnevamativakis, G. A.; Stathis, E. C. Chem. Ind. 1963, 30, 1240. (18) Bekleshova, G. E.; Vitkina, M. A. Khim. Tekhnol. Respub. MezhVed. Nauchno-Tekh. Sb. 1963, 5, 57. (19) Majumdar, A. K.; Chakrabartly, M. M. Anal. Chim. Acta 1959, 20, 386; 1958, 9, 372. (20) Anul Kumvar, Z. Anal. Chem. 1957, 156, 265.
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