Solvent less-volatile

The ability of diethyl ether to extract uranyl nitrate from aqueous solution has been known for a hundred years and was the method chosen by the Manhattan Project to purify the uranium used in the first nuclear chain reactors. This solvent has numerous disadvantages. It is very volatile, very flammable, and toxic, and it requires addition of sodium, aluminum, or calcium nitrate to the aqueous phase to enhance extractions. When solvent extraction was first applied to recovery of uranium and plutonium from irradiated fuel, other oxygenated solvents less volatile than diethyl ether that were first used were methyl isobutyl ketone, dibutyl... 

Sample is injected at a temperature above the solvent boiling point. The sample is completely evaporated during injection. No flooded zone is formed. Volatile analytes co-evaporate with the solvent. Less volatile components remain spread over the retention gap and are reconcentrated by the phase-ratio-focusing effect. 

A small amount of a higher boiling co-solvent (e.g. octadecane) is added to the main solvent to create a layer of condensed liquid ahead of the main evaporation site. The main solvent evaporates concurrently, and part of the co-solvent evaporates together with the main solvent. Boiling point and amount of co-solvent must be adjusted such that some co-solvent is left behind as a liquid and spreads into the retention gap. Volatile analytes are reconcentrated due to solvent trapping in the co-solvent. Less volatile components remain spread over the retention gap and are reconcentrated by the phase-ratio-focusing effect. 

Pressure, psia At 100 F for leas-volatile constituents of these molecular weights At 160 F for less-volatile constituents of these molecular wmghts At 220 F fur less-volatile constituents of these mole< uIar wnghts For a 200-moIecular-weight solvent less volatile constituent at these temperatures ... 

Le Chatelier s principle applies to these equilibria, as it does to all equilibria. One way to exert a stress on a solubility equilibrium is to change the amount of solvent. Adding solvent reduces the concentration of dissolved substance more solid then tends to dissolve to restore the concentration of the dissolved substance to its equilibrium value. If an excess of solvent is added so that all of the solid dissolves, then obviously the solubility equilibrium ceases to exist and the solution is unsaturated. In a vaporization-condensation equilibrium, this corresponds to the complete evaporation of the condensed phase. Removing solvent from an already saturated solution forces additional solid to precipitate in order to maintain a constant concentration. A volatile solvent is often removed by simply letting a solution stand uncovered until the solvent evaporates. When conditions are right, the solid forms as crystals on the bottom and sides of the container (Fig. 16.1). 

Incidentally it may be pointed out that by the aid of the Le Chatelier and Braun principle one can predict that the vapour pressure (of the solvent, of course) over a solution containing a non-volatile solute is less than the vapour pressure over the pure solvent itself at the same temperature Thus, consider the solvent alone in equilibrium with its vapour Dissolve some non-volatile substance in the liquid According to the principle the system tends to remain in its former state, 1 e it tends to lower the concentration of the solute as far as possible since it will thereby be approximating to the state of pure liquid It effects this by causing some of the vapour to be con- densed, thereby making the vapour more... 

This mode is used for the exhaustion of flowers, leaves, roots, seeds, and other substances of delicate textui e, which are easily penetrable and readily yield their soluble matters and especially for the purpose of extracting volatile ingredients. The heat applied to the solvent increases its energy but as the material is only in contact for a limited time, the interval between the commencement and completion of the operation is not sufficient to affect the material or solution, even though one or more of its components are alter le by heat. 

The theoretical treatment which has been developed in Sections 10.2-10.4 relates to mass transfer within a single phase in which no discontinuities exist. In many impDitant applications of mass transfer, however, material is transferred across a phase boundary. Thus, in distillation a vapour and liquid are brought into contact in the fractionating column and the more volatile material is transferred from the liquid to the vapour while the les.s volatile constituent is transferred in the opposite direction this is an example of e(]uimolecular counterdiffusion. In gas absorption, the soluble gas diffuses to the surface, dissolves in the liquid, and then passes into the bulk of the liquid, and the carrier gas is not transferred. In both of these examples, one phase is a liquid and the other a gas. In liquid -liquid extraction however, a solute is transferred from one liquid solvent to another across a phase boundary, and in the dissolution of a crystal the solute is transferred from a solid to a liquid. 

Diffuse reflectance or DRIFTS (diffuse reflectance infrared Fourier-lransform spectroscopy) allows the sain)le to be analysed neat, ot diluted in a non-absorbing matrix (e.g. KCl or KBr at 1-5% w/w analyte). DRIFTS also may be used to obtain the spectrum of a solute in a volatile solvent by evaporating the solution onto KBr. When the IR radiation interacts with the powdered sample it will be absorbed, reflected and diffracted. The radiation which has been diffusely reflected contains vibrational information on the molecule. This technique allows non-destructive testing of neat materials and is suited to quantitative analysis, although care must be taken to ensure that a consistent particle size is used. 

A number of pore-water or solute extraction methods are reported in the literature. Vacuum filtration (Bufflap Allen 1995) involves the application of a vacuum to the sample causing pore-water to be drawn-out from the pore spaces. Dialysis or diffusion methods involve equilibration of pore-water in the sample with deionized water (Brandi Hanselmann 1991) or a solid phase material which takes up the chemical constituents of the pore-water (Zhang et al. 1998). Both methods are generally restricted to relatively high moisture content, unconsolidated sediments and have limited potential to the characterization of barrier materials. Distillation of water vapour and other volatile solvents may be used to determine stable isotope contents (Moreau-Le Golvan et al. 1997), but pore-water solutes are left behind in the rock matrix so cannot be characterized. 

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