The method of wet residues

The determination of equilibrium curves in three-component systems in some respects is simpler than in two-component systems. Consider the diagram in Fig. 15.29. Suppose that the system consists of a solution in equilibrium with solid and that the state point is at a. We do not know the location of a, but we do know that it lies on a tie line connecting the solid composition with the liquid composition. We proceed as follows some of the saturated liquid is removed and analyzed for A and B. This fixes the point s on the equilibrium line. After the removal of some of the saturated solution, the state point of the remainder of the system must lie at point r. So the remainder, that is, the solids together with the supernatant liquid, called the wet residue, is analyzed for two of the components. This analysis determines the point r. A tie line is drawn through s and r. The procedure is repeated on a system that contains a slightly different ratio of two of the components. The solution analysis yields the point 5, while the analysis of the wet residue yields a point r. The tie line is drawn through s and r. These two tie lines must intersect at the composition of the solid that is present. In this system, they would intersect at point D. This intersection point yields the composition of the solid phase D, which is in equilibrium with the liquid. 

The method of wet residues is superior to the procedure necessary in two-component systems, where the liquid and solid phase must be separated and analyzed individually. It is a practical impossibility to separate the solid phase from the liquid without some of the liquid adhering to the solid and thus contaminating it. For this reason, it is frequently easier to add a third substance to a two-component system, determine the equilibrium lines, as well as the composition of the solid phases by the method of wet residues, and infer the composition of the solid in the two-component system from the features of the triangular diagram. 

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Although extrapolations are commonly made graphically on phase diagrams, algebraic extrapolation is less subjective, more accurate, and lends itself to the application of statistical methods which minimize errors. Mathematical extrapolation procedures for the method of wet residues have been described by Ricci (1966) and Schott (1961). 

Schott, H. (1961) A mathematical extrapolation for the method of wet residues. Journal of Chemical Engineering Data, 6, 324—325. 

For multi-component systems the composition of the equilibrium sohd phase may be determined indirectly by the so-called wet residues method first proposed by Schreinemakers (1893) in which the need for solid-liquid separation by filtration, etc. is avoided. The experimental procedures, together with those of the alternative synthetic complex method, are fully described in section 4.6.5. 

The construction of a ternary solubility phase diagram for two solid phases in equilibrium with one solution was discussed in detail by Jacques et al. in the context of solubility phase diagrams of enantiomers in achiral solvents,2 where the method of algebraic extrapolation or wet residues is used. The biggest challenge with this classic method is the avaUabUity of pure components, enantiomers, or diastereomeric salts. The discontinuous isoperibolic thermal analysis (DITA) method developed by Marchand et al.22 overcame this barrier. In the DITA method, a mixture of an equal amount of diastereomeric salts is used. 

The distribution of impurities over a flat sihcon surface can be measured by autoradiography or by scanning the surface using any of the methods appropriate for trace impurity detection (see Trace and residue analysis). Depth measurements can be made by combining any of the above measurements with the repeated removal of thin layers of sihcon, either by wet etching, plasma etching, or sputtering. Care must be taken, however, to ensure that the material removal method does not contaminate the sihcon surface. 

Processing in Hquid sterilants results in wet products which require highly specialized packaging. Therefore, Hquid sterilization should only be considered if the sterilized article is to be used almost immediately. Liquid sterilants or their residues can be harmful to living tissues. Therefore it is always necessary to rinse articles with sterile water or saline solution foUowing treatment. Whereas Hquid sterilization is an extremely useful method for articles that caimot withstand the conditions of steam sterilization, the problems associated with its use limit its appHcation. 

Environmental monitoring of chloroacetanilides requires methods that have the capability to distinguish between complex arrays of related residues. The two example methods detailed here for water monitoring meet this requirement, but the method for metabolites requires sophisticated mass spectral equipment for the detection of directly injected water samples. In the near term, some laboratories may need to modify this method by incorporation of an extraction/concentration step, such as SPE, that would allow for concentration of the sample, so that a less sensitive and, correspondingly, less expensive, mass spectral detector can be used. However, laboratories may want to consider purchasing a sensitive instrument rather than spending time on additional wet chemistry procedures. In the future, sensitive instrumentation may be less expensive and available to all laboratories. Work is under way to expand the existing multi-residue methods to include determination of additional chloroacetanilides and their metabolites in both water and soil samples. 

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