Solvent Extracts - Electrochemical Methods

The study of carbonate complexes of Pu is complicated by various experimental difficulties. The low solubility of many carbonates (7), leaving a very dilute Pu concentration in solution, results in difficulties to the experiments with electrochemical or spectrophotometric methods. However, the radiometric method with solvent extraction or solubility measurement is easily applicable for the purpose. Unlike the solution with anions, like Cl, N03 etc., the concentration of which can be varied at a constant pH, the preparation of solutions with varying carbonate concentration accompanies indispensably the change of pH of the solution. As a result, the formation of carbonate complexes involves accordingly the hydrolysis reactions of Pu ions in solutions under investigation. It is therefore prerequisite to know the stability constants of Pu(IV) hydroxides prior to the study of its carbonate complexation. 

Separation and detection methods The common methods used to separate the Cr(III)/(VI) species are solvent extraction, chromatography and coprecipitation. In case of Cr(VI) from welding fumes trapped on a filter, a suitable leaching of the Cr(VI) from the sample matrix is needed, without reducing the Cr(VI) species. The most used detection methods for chromium are graphite furnace AAS, chemiluminescence, electrochemical methods, ICP-MS, thermal ionization isotope dilution mass spectrometry and spectrophotometry (Vercoutere and Cornelis 1995)- The separation of the two species is the most delicate part of the procedure. 

A modification of the RDC design, based on the ring-disk arrangement of the RDE [36], incorporated an arc electrode [37,38] deposited on the surface of the membrane around the untreated area. This facilitated the electrochemical detection of species reacting at the interface at short times following the reaction. This method was used to study the solvent extraction of cupric ions, which were detected by reduction to copper metal at the arc electrode. The resulting current flow was related to the interfacial flux at the membrane. 

Determination of trace metals in seawater represents one of the most challenging tasks in chemical analysis because the parts per billion (ppb) or sub-ppb levels of analyte are very susceptible to matrix interference from alkali or alkaline-earth metals and their associated counterions. For instance, the alkali metals tend to affect the atomisation and the ionisation equilibrium process in atomic spectroscopy, and the associated counterions such as the chloride ions might be preferentially adsorbed onto the electrode surface to give some undesirable electrochemical side reactions in voltammetric analysis. Thus, most current methods for seawater analysis employ some kind of analyte preconcentration along with matrix rejection techniques. These preconcentration techniques include coprecipitation, solvent extraction, column adsorption, electrodeposition, and Donnan dialysis. 

An electrochemical reduction method was found to be more effective for extraction of these insoluble EMFs into solution [71]. After sublimation and washing with xylene, the insoluble residue was reduced electrochemically and a dark brown solution was obtained, indicating that almost all the insoluble materials entered the solution as anions, which are dominated by Gd C6o and Gd C74. This method is clearly more powerful than the solvent extraction it is suitable for large-scale industrial production. 

In the analysis of high-purity substances, general matrix removal is often very important so as to pre-concentrate the elements to be determined. To this aim all separation techniques such as ion exchange, liquid-liquid extraction of a metal complex with organic solvents, fractionated crystallization, precipitation and coprecipitation as well as electrochemical methods may be used (for a systematic treatment, see Ref. [300]). These principles can also be applied in on-line systems, as is now possible with solid phase extraction. Here matrix elements or the analytes can be adsorbed as complexes onto the column and eluted for direct determination by AAS. 

Available techniques for the removal of metal ions include chemical precipitation, ion exchange, evaporation, solvent extraction and a variety of membrane separation processes including reverse osmosis, ultrafiltration and electrodialysis [3]. Each of these methods has its own advantages but all lack the ability of certain electrochemical techniques to produce metal directly in a controlled fashion. 

Advantages and Limitations of Radiometric Titrations. Radiometric detection of the equivalence point is a general method that does not depend on the chemical reaction employed. This contrasts with other methods of detection, which depend on specific chemical or physical transitions at the equivalence point. Amperometric titrations are applicable only to electrochemically active systems conductometric titrations apply only to ionic solutions, and so on. In principle, any titration system in which a phase separation can be effected is amenable to radiometric detection, provided there exist suitable radioactive labels. The major limitation of the method is the requirement for phase separation. In precipitation titrations, the phase separation is automatic and the method is well suited to this class of titrations. For other classes of titrations, special phase-separation methods, such as solvent extraction, need to be applied. At the present time, the method suffers from a lack of phase-separation techniques suitable for continuous monitoring of the titration curves. 

The sensitivity of any analytical technique can be greatly increased by introducing a preliminary pre-concentration step, eg solvent extraction. In stripping voltammetry an electrochemical preconcentration technique is used. The analyte is concentrated, from very dilute solutions, by electrolysis to an insoluble product which collects at the electrode and can be subsequently determined with a very high sensitivity. The method is applicable only to a limited number of important analytes. Stripping voltammetry requires the use of solid or stationary electrodes, (2.7). 

Substoichiometric separation is performed by ordinary chemical separation methods such as solvent extraction, ion exchange, precipitation, and electrochemical methods. In recent years, however, the ion exchange and electrochemical methods have not been used very much in substoichiometric separation. The precipitation technique is often used due to its simplicity, while solvent extraction is most widely employed. This is because the procedure for solvent extraction is very simple and an appropriate extraction system can usually be selected from the great number of research papers dealing with solvent extraction of many different elements. Two extraction systems are commonly used chelate extraction of metal ions with chelating agents and ion-association extraction of metal ions with simple negative or positive ions. 

Operative conditions can be adjusted so that only metals with rate of dissociation of their complexes, within a desired range, are included in the electroactive fraction. Conditions that can be adjusted to achieve selectivity are deposition potential, electrode rotation rate, solution stirring, pulse frequency, potential scan rate, temperature, pH, etc. As electrochemical techniques require much less sample handling than other speciation methods, such as solvent extraction, dialysis or ultrafiltration, the potential sources of contamination are highly reduced. An in depth discussion of the pro and cons of electrochemical speciation is far beyond this article. Theoretical aspects and applications have been covered in great detail by Niirnberg, Florence et al., cf. ° and references therein. 

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