Methods Involving Nonaqueous Solvents

An acid-base scale for solids based on titrations with 1-butylamine or trichloroacetic acid in benzene in the presence of indicators was described in [642]. In [643], 0.2 g of material was mixed with 3 g of heptane and 0.01 cm of pH indicator solution. A series of indicators was used, and the apparent pH detected by these indicators was identified with the PZC. These methods are reported as historical curiosities, and they are not recommended by the present author. 

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Double layer at both pc-Au and singlecrystal Au electrodes brought in contact with such nonaqueous solvents as dimethylsulfoxide (DMSO), dimethylfor-mamide (DMF), acetonitrile, propylene carbonate (PC), and selected alcohols has also been studied. The experimental methods used involved CV and impedance measurements, except for butanol isomers, for which surface-enhanced... 

The third approach to solving this problem (Farber, 1999) involves the preparation of an enzyme-intermediate complex at high substrate concentration for X-ray data collection. Under such a condition active sites in the crystal lattice will be filled with intermediates. Using a combination of flow cell experiments and equilibrium experiments, it is possible to obtain the structure of important intermediates in an enzyme reaction (Bolduc et al., 1995). It was also discovered that some enzyme crystals can be transformed from their aqueous crystallization buffer to nonaqueous solvents without cross-linking the crystals before the transfer (Yennawar et. al., 1995). It is then possible to regulate the water concentration in the active site. The structure of the first tetrahedral intermediate, tetrapeptide -Pro-Gly-Ala-Tyr- in the y-chymotrypsin active site obtained by this method is shown in Fig. 1.1. 

Conversion of mononuclear oxoanions into polyoxoanions requires the consumption of acid. Simple examples involving Bronsted acids see Acids Acidity) in aqueous solutions are given in equations (1-3), although alternative routes to polyoxoanions have been developed, for example, through the use of acidic oxides in aqueous and nonaqueous solvents, or by hydrothermal methods. 

A study of the acid-base properties of solutes in nonaqueous solvents must include consideration of hydrogen ion activities and in particular a comparison of their activities in different solvents. Attempting to transpose interpretations and methods of approach from aqueous to nonaqueous systems may lead to diflSculty. The usual standard state (Section 2-2) for a nonvolatile solute is arbitrarily defined in terms of a reference condition with activity equal to concentration at infinite dilution. Comparisons of activities are unsatisfactory when applied to different solvents, because different standard states are then necessarily involved. For such comparisons it would be gratifying if the standard state could be defined solely with reference to the properties of the pure solute, as it is for volatile nonelectrolytes (Section 2-7). Unfortunately, for ionic solutes a different standard state is defined for every solvent and every temperature. 

Accurate solubility data is a crucial part of the design, development, and operation of a crystallization process. When confronted with the need for accurate solubility data, it is often common to find that the data is not available for the solute at the conditions of interest. This is especially true for mixed and nonaqueous solvents, and for systems with more than one solute. In addition, most industrial crystallization processes involve solutions with impurities present. If it is desired to know the solubility of the solute in the actual working solution with all impurities present, it is very unlikely that data will be available in the literature. Methods for the calculation of solubility have been discussed previously. These can be quite useful, but often are not possible because of lack of adequate thermodynamic data. This means that the only method available to determine the needed information is solubility measurement. 

Ghlorohydrination with Nonaqueous Hypochlorous Acid. Because the presence of chloride ions has been shown to promote the formation of the dichloro by-product, it is desirable to perform the chlorohydrination in the absence of chloride ion. For this reason, methods have been reported to produce hypochlorous acid solutions free of chloride ions. A patented method (48) involves the extraction of hypochlorous acid with solvents such as methyl ethyl ketone [78-93-3J, acetonitrile, and ethyl acetate [141-78-6J. In one example hypochlorous acid was extracted from an aqueous brine with methyl ethyl ketone in a 98.9% yield based on the chlorine used. However, when propylene reacted with a 1 Af solution of hypochlorous acid in either methyl ethyl ketone or ethyl acetate, chlorohydrin yields of only 60—70% were obtained (10). 

The cleavage of a glycosidic bond by acetolysis is an alternative method to hydrolysis. Although both methods are acid-catalyzed, and presumably in the case of glycopyranosides involve the formation of a cyclic oxocarbenium transition state, they also have their differences. Thus, for example, the most important difference is that the hydrolysis is always performed either in aqueous solutions or in a solvent containing water, whereas the acetolysis is performed in nonaqueous... 

It has been established that any rational process design method of gas absorbers (gas-liquid reactors) may conveniently be divided into three main stages (see. Figure 1). The first stage which is essential for any rational process design approach seems to be still one of the main problems. That is, the values of the mass transfer coefficients, interfacial areas, hold-ups etc, are difficult to estimate under the conditions of the reaction system. There is a real dearth of such data in all absorbers where liquid other than water or aqueous solutions (for instance, nonaqueous, nonnewtonian, viscous solvents etc.) are involved and various reported data and correlations are not all reliable when employed under different experimental conditions and they should therefore be used cautiously. In this respect, although there is some dispute, the packed column seems to be one of the gas absorbers whose characteristics (i.e. k a, a etc.) can be predicted with a fair degree of confidence. 

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