Liquid-phase precipitation impurities

Several microwave-assisted protocols for soluble polymer-supported syntheses have been described. Among the first examples of so-called liquid-phase synthesis were aqueous Suzuki couplings. Schotten and coworkers presented the use of polyethylene glycol (PEG)-bound aryl halides and sulfonates in these palladium-catalyzed cross-couplings [70]. The authors demonstrated that no additional phase-transfer catalyst (PTC) is needed when the PEG-bound electrophiles are coupled with appropriate aryl boronic acids. The polymer-bound substrates were coupled with 1.2 equivalents of the boronic acids in water under short-term microwave irradiation in sealed vessels in a domestic microwave oven (Scheme 7.62). Work-up involved precipitation of the polymer-bound biaryl from a suitable organic solvent with diethyl ether. Water and insoluble impurities need to be removed prior to precipitation in order to achieve high recoveries of the products. 

Polymerization can be catalytic or noncatalytic, and can be homogeneously or heterogeneously catalyzed. Polymers that form from the liquid phase may remain dissolved in the remaining monomer or solvent, or they may precipitate. Sometimes beads are formed and remain in suspension sometimes emulsions form. In some processes solid polymers precipitate from a fluidized gas phase. Polymerization processes are also characterized by extremes in temperature, viscosity, and reaction times. For instance, many industrial polymers are mixtures with molecular weights of 104 to 107. In polymerization of styrene the viscosity increased by a factor of 106 as conversion increased from 0 to 60 percent. The adiabatic reaction temperature for complete polymerization of ethylene is 1800 K (3240°R). Initiators of the chain reactions have concentration as low as 10-8 g-moFL, so they are highly sensitive to small concentrations of poisons and impurities. 

The solubility is defined with respect to a second precipitated phase. The solubility of an impurity is the maximum concentration, which can be incorporated in the liquid or solid phase without precipitating a second phase. For most impurities in solid silicon at high-temperatures, equilibrium is achieved with the liquid phase governed by the liquidus in the phase diagram. Solid solubility is temperature-dependent as represented by the solidus or solvent curves in the phase diagram. At lower temperatures, the reference phase is usually a compound or an impurity-rich alloy. When the impurity is volatile, the saturated crystal is in equilibrium with the vapor, and the impurity solubility also depends on its vapor pressure. 

Gold, silver, mercury, and platinum metals, as well as Se and Te, can be precipitated from acid solution in the elemental form by reduction with chemical reagents such as zinc, NH2OH, N2H4, SO2, or formic acid. In the trace analysis of high purity mercury the sample (about 100 g) is dissolved in HNO3 and the solution is warmed in the presence of formic acid. First of all, nitric acid, then mercury, is reduced. The mercury forms a separate liquid phase, and the impurities remain in the aqueous solution [102]. In the trace analysis of silver, the sample is dissolved in nitric acid, then formic acid and mercury are added. The silver liberated on reduction dissolves in the mercury to form an amalgam [102]. 

Mixtures of components that exhibit solid solution behaviour cannot be separated in a single step as can, for example, simple eutectic systems. Multistage or fractional precipitation schemes must therefore be employed (section 7.1). The distribution of an impurity between the solid (i.e. solid solution) and liquid phases may be represented by the Chlopin (1925) equation ... 

The main mechanism by which activated carbon removes impurities is one of physical adsorption, this being a reversible process. Consequently one can expect that desorption of the impurities will render the carbon surface available again for adsorption. Regeneration of spent activated carbon is not only important from the point of view of restoring the adsorption capacity of the carbon, but also because in many cases the recovery of the adsorbed species is important. If the adsorption is of chemical type (chemisorption), the formation of a bond between the carbon and the adsorbate makes the process non-reversible, and even if desorption is possible the desorbed species will be different to those originally adsorbed. Additionally, adsorption (especially in liquid phase) is often accompanied by precipitation of species which cannot be removed by simple desorption. 

A necessary preface to a description of the procedure is that the solvent and the precipitant must be purified to exhaustion by contact with successive specimens of the acid to be purified. The acid A is dissolved in the minimum amount of solvent S. The precipitant P is then added under isothermal conditions to the solution until roughly one half to three quarters of A has been precipitated. At this stage there is a three-phase system present (vapour and two liquids) with three (or more) components (A, S, and Imp where Imp denotes an impurity), and the impurities are partitioned between A and the mixture of S and P. This mixture is separated from A by decantation or syphoning, A is redissolved in S and reprecipitated by the addition of P. At all stages of this process the mixtures must be stirred efficiently but so gently that an emulsion is not formed. It happens quite often that an acid A with a melting point near or above ambient temperature will start to crystallise after the first or second extraction. 

During solidification of a Al-Si melt, the first phase that precipitated is solid Si. Most of the impurity elements have higher solubility in the liquid alloy than in solid Si and can, therefore, effectively be removed by the solvent refining method. The Si will of course be saturated with Al, which has to be removed at a later stage. 

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