Solubility of metal chelates

There are principally two ways of modifying the solubility of metal chelates such as dithiocarbamates or beta-diketonates ... 

To increase the understanding of factors determining the solubility of metal chelates in ScF we will report our concept and recent results. We discuss the influence of ligand structure modifications and the influence of the scCOa density on the solubility of the chosen chelates. 

A variety of techniques have been used to measure the solubility of metal chelates in ScFs, including gravimetric, (45,46) spectroscopic, (36,37,47,46,48) and chromatographic methods. (36,37,45-51). No method is ideal, and the relative merits of these techniques are discussed in the literature. As previously shown, laser induced fluorescence (UF) is a fast, sensitive, and selective method for in-situ determination of uranyl chelates in ScF CO2. In this section we discuss the methods and merits of using LIF to determine solubilities of uranyl chelates in ScF CO2. 

The solubility models presented in this section provide general guidelines for predicting solubilities of metal chelates in supercritical CO2. Based on some experimental measurements, solubilities of the metal chelates of interest may be estimated using these models. However, accurate calculation of metal chelate solubilities in supercritical CO2 is still difficult to accomplish. 

M Ashraf-Khorassani, MT Combs, LT Taylor, Solubility of metal chelates and their extraction from an aqueous environment via supercritical CO2. Talanta 44 755-763, 1997. 

Cloud point extraction of metal ions. The use of cloud point extraction as a separation technique was first introduced by Watanabe for the extraction of metal ions forming sparingly water soluble complexes [109], Since then, the technique has been applied successfully to the extraction of metal chelates for spectrophotometric, atomic absorption, or flow injection analysis of trace metals in a variety of samples [105-107,110]. Other metal complexes such as AUCI4 or thiocyanato-metal complexes can be extracted directly using nonionic surfactants such as polyoxyethylene... 

The solubilities of metal 8-hydroxyquinolates vary widely from cation to cation and are pH dependent because 8-hydroxyquinoline is always deprotonated during chelation reaction. Therefore, we can achieve a considerable degree of selectivity in the use of 8-hydroxyquinoline by controlling pH. 

While ranges of total concentration serve to set bounds for experimentally determining effects on marine populations, the actual species of metal ion available to the biological population is of importance. Sillen, in a classic paper, has computed the stable species of many metals in sea water21). He concluded, for example, that Hg+2, Cd+2, and Pb+2 exist primarily as chloride complexes. pH determines the availability of the hydroxide ion and thereby the solubility of metal hydroxides. Sillen assumed a pH of 8.1 0.2 as representative. Significant variations could occur, however, in estuarine waters. When concentrations of trace elements were compared with calculations of their solubility products and stability constants, the observed values were considerably less than the calculated values. The implication is that the heavy metals are not in equilibrium with solid phases of their salts, but that other processes, such as chelation and adsorption, control their concentration. 

The trimethylsilylated silicic acids formed in this instance are soluble in conventional organic solvents, and their volatility is sufficiently high for them to be analysed by gas chromatography. Carzo and Hoebbel [411] carried out a comprehensive study of the chromatographic retention of various trimethylsilylated silicic acids on different stationary phases Apiezon L and silicone OV-1 and OV-17. The analysis of metals in the form of volatile complexes continues to attract attention, and have been described for analysing sodium [412], potassium [412], radium [413], caesium [413], barium [414], calcium [414], strontium [415], beryllium [416, 417], magnesium [418], zinc [419, 420], nickel [419], mercury [421], copper [422, 423], silver [424, 425], cadmium [421], indium [426, 427], g ium [428], scandium [217], cobalt [421], thallium [426], hafnium [429, 430], lead [431, 432], titanium [430], vanadium [433], chromium [434-436], manganese [426], iron [437], yttrium [438], platinum [439,440], palladium [439, 441, 442], zirconium [430], molybdenum [443], ruthenium [444], rhodium [445], rare earths [446—449], thorium [221, 450, 451] and uranium [221, 452]. The literature on GC analysis of metal chelates was reviewed by Sokolov [458]. 

Solubility and Modifications of Metal Chelates in Supercritical Carbon Dioxide... 

Modem catalytic exhaust converters used in cars contain the precious metals palladium and rhodium in the gram level. (25) Common techniques of recycling heavy metals use the transformation of the metals into chloro complexes which are extracted by means of organic solvents. In order to evaluate the replacement of organic solvents by SCCO2 by a different extraction process the solubility of different chelates of palladium(II), rhodium(III), lead(II) and copper(II) was investigated. 

We suppose that the planar quadratic coordination of the palladium(II) chelates allow CO2 molecules or other small molecules like traces of the synthesis solvent to be coordinated to the Pd(II) centre and therefore change the polarity /solubility of the chelate. Voluminous and flexibel residual ligand substitutes protect the central palladium ion against interaction of small molecules and we can observe a drastic increase of solubility. Otherwise, if the coordination sphere of the chelate centre ion is octahedral or tetrahedral, other molecules (e.g. the solvent) cannot interact with the metal centre easily. 

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