Ex-situ complex formation

Fig. 7.4. Determination of copper as Cu(dedtc)2 by the reduction process at a glassy carbon electrode and ex situ complex formation (I) current response for reduction of oxygen (2) current response for reduction of Cu(dedtc)2- Reverse phase chromatography used with a Cig column. DC current measured at a potential of —0.6 V vs Ag/AgCl. Flow rate, 2 ml min . Injection volume, 20 n (200 ng copper). Temperature, 20 1°C. Reproduced by courtesy Anal. Chem. 53 (1981) 1209.

Fig. 7.7. Variation of chromatographic peak current with flow rate using ex situ complex formation (1) gold or platinum electrodes (2) glassy carbon electrodes. A total of 200 ng of copper was ityected as Cu(dedtc)2. Other conditions as in Fig. 7.5. Reproduced by courtesy Anol. Chem. 53 (1981) 1209.

INSTRUMENTATION FOR 7-DAY CONTINUOUS MONITORING OF METALS USING EX-SITU COMPLEX FORMATION... 

Ex situ complex formation, while more difficult to automate, eliminates or minimizes many of the above difficulties. Ex situ complex formation relies upon external preparation of the metal complex before injection into the chromatographic system. The ligand may therefore be stored in an organic solvent such as acetonitrile and subsequently mixed with the aqueous sample. The dithiocarbamate ligand is considerably more stable in organic solvents than in aqueous media, so that stock solutions can be stored for lengthy periods, particularly when light is excluded. 

Fig. 7.16. Flow diagram (a) and schematic representation (b) of operational procedures required to produce automatic sample iiyection and ex situ complex formation. Reproduced by courtesy Anal. Ckem. 60 (1988) 1357.

Industrial liquors and effluents from electroplating or electrorefining industries often have considerably elevated cobalt and cadmium levels, which can be monitored routinely by the method described in this work for one week periods, without operator intervention or maintenance. The concept of using an exchange reaction with Zn(hedtc)2, instead of in situ complex formation with an excess of dithiocarbamate ligand or ex situ complex formation in the reaction chamber, offers a considerable improvement in reproducibility, stability, cost of reagents and safety. 

Many of the characterization techniques described in this chapter require ambient or vacuum conditions, which may or may not be translatable to operational conditions. In situ or in opemndo characterization avoids such issues and can provide insight and information under more realistic conditions. Such approaches are becoming more common in X-ray adsorption spectroscopy (XAS) methods ofXANES and EXAFS, in NMR and in transmission electron microscopy where environmental instruments and cells are becoming common. In situ MAS NMR has been used to characterize reaction intermediates, organic deposits, surface complexes and the nature of transition state and reaction pathways. The formation of alkoxy species on zeolites upon adsorption of olefins or alcohols have been observed by C in situ and ex situ NMR [253]. Sensitivity enhancement techniques play an important role in the progress of this area. In operando infrared and RAMAN is becoming more widely used. In situ RAMAN spectroscopy has been used to online monitor synthesis of zeolites in pressurized reactors [254]. Such techniques will become commonplace. 

Characterization of in situ and ex situ synthesized catalysts on a silica support confirms the presence of tin oxides and tin hydroxyl species [80]. In the same report, the authors determined that a 1 2 Pd Sn stoichiometry is optimal and implicated PdSn2 particles as being important for effective catalysis. It was proposed that in situ catalyst formation is a two-step process that involves the formation of a Pd Sn 2(OAc)g complex followed by the decomposition of this complex to give oxygenated PdSn2 clusters. Other Pd-based catalysts have also been developed. The addition of Bi [71,81], persulfate/Sn (Phillips Petroleum Co.) [82], Sn/Sb (BP) [83], and ultrafine Au [84] have been shown to be beneficial. 

A widespread property of intermetallic compounds is the ability to form hydrides (7). The formation of a hydride involves changes in the structural and electronic properties. And, in general, hydrides are very active hydrogenation catalysts as was shown above. As a result, the observed catalytic properties are not specific for the intermetallic compound, which was placed in the reactor, but belong to the hydride formed in situ. The whole situation becomes even more complex due to the instability of some hydrides under normal conditions. This results in the decomposition of the hydride as soon as the reactive atmosphere is removed, which makes the ex situ detection difficult or even impossible. Thus, to be able to assign the observed catalytic properties to a specific intermetallic compound, the possibility of hydride formation under reaction conditions has to... 

On the basis of findings from the preliminary investigation of the above kind in a conventional electrochemical cell, a suitable mobile phase for the reverse phase liquid chromatographic separation of Cu(dtc)2 complexes with electrochemical detection would be 70% acetonitrile-30% water (0.02 M acetate buffer) with NaNOa as supporting electrolyte. Electrodes investigated in the published paper [3] were the same as in the stationary cell and both oxidation and reduction processes for Cu(dtc)2 were compared. A Metrohm ElA 1096 detector cell (wall jet electrode) was used in this particular cell and a Cig reverse phase chromatographic column was employed. Retention volumes of 14.4 and 10.4 ml were obtained for Cu(dedtc)2 and Cu(pydtc)2, respectively. This smaller retention volume of Cu(pydtc)2 may be attributed to the more polar nature of the complexes. Other experimental details are available in reference 3. In addition, the possibility of in situ formation of the Cu(dtc)2 complex as an alternative to ex situ formation of the complex externally to the column also was examined. 

Fig. 7.15. Schematic diagram of the second generation METSCAN system used for trace metal determinations with ex situ formation of metal dithiocarbamate complexes. Reproduced by courtesy Anal. Chem. 60 (1988) 1357.

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