Mixed solvent

The second difficulty is that of product distribution. This is well illustrated by the reports of kinetic studies of solvolysis of complexes [Cr(NCS)4LJ , where both solvent components seem to enter the primary co-ordination shell of the chromium(m) during the reaction. The claims that the two components selectively displace the ligands NCS or L do not appear to be supported by unequivocal evidence, which would indeed not be easy to obtain. 

The third difficulty lies in the imprecise picture and limited physical data available for many solvent mixtures. This is particularly relevant to mixed aqueous solvents, where both inter- and intra-component interactions have such an important effect on the overall structure and properties of the solvent mixtures. 

Composition.—The dependence of aquation rate on the concentration of water, using dilute solutions of water in an inert solvent, has not usually proved a satisfactory way of deciding whether the aquation of a particular complex is associative or dissociative in mechanism. However, this approach has recently been applied to the aquation of the [Cr(ox)3] anion in benzene-water solvents containing surfactants. Here a first-order dependence of aquation 

Conversely, the rate of catalysed isomerization of platinum(n) complexes in inert solvents is decreased by the addition of polar solvents. The reduction in rate parallels the reciprocal of the concentration of the added polar solvent. Here the effect on the kinetics can be ascribed to competition between the catalyst and the polar solvent at the platinum(u). From these kinetic results one can estimate the solvation number of the platinum(n) complex with respect to the polar solvent. Surprisingly, in most cases this solvation number turns out to be one - only with methanol as added solvent does one get the expected solvation number of two.  

Some time ago it was reported that rates of dimethyl sulphoxide exchange with nickel(ii) in dimethyl sulphoxide-dichloromethane and dimethyl sulphoxide-nitromethane solvent mixtures were almost independent of solvent composition. Recently, rates of dimethylformamide exchange at nickel(n) in dimethylformamide-nitromethane solvent mixtures have also been found to be remarkably insensitive to solvent-composition variation. Whereas in the dimethyl sulphoxide-dichloromethane solvent mixtures nickel(n) exists as [Ni(DMSO)e] +, in dimethylformamide-nitromethane mixtures the composition of the nickel(n) co-ordination shell varies with solvent composition, with preferential solvation by the nitromethane rather than by the dimethylformamide. These observations are difficult to explain in terms of an /d 

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To estimate Henry s constant for solute i in a mixed solvent, we use the approximation... 

For a mixed solvent, it follows from Equation (18) that... 

C) Mixed solvent. A well-shaken mixture of 20 ml. of redistilled w-butanol, 9-5 ml. of water, and 5-5 ml. of 95% ethanol. 

Now place 35 ml. of the mixed solvent (C) in the clean cylinder E, and suspend the strip, as described above, to the horizontal arm of G (Fig. 25(A)) adjust the position of the strip so that, when the bung is firmly in position, the bottom of the paper-strip is about 5 mm. above the solvent. Place the cylinder for 5-8 hours in a draught-free place, such as a cupboard, where the temperature is reasonably constant. 

Note. The period of 5-8 hours recommended above for attaining an equilibrium between the vapour molecules of the mixed solvent and those absorbed by the paper strip is essential if accurate R values are required for identification of mixed amino-acids. To illustrate the separation, as in the above experiment, this period may be reduced to about 2 hours. 

D) Mixed solvent. Add 10 ml. of concentrated aqueous ammonia (t/, o-88o) to 40 ml. of redistilled n-butanol and thoroughly mix to obtain a complete solution. 

Place 40 ml. of the mixed solvent (D) in the chromatogram cylinder, and suspend the dried strip in the closed cylinder cf. p. 223) for at least 30 minutes to allow the paper to come into complete equilibrium with the mixed solvent. Then gently lower the strip until the bottom edge dips about 5 mm. into the solvent. Allow the chromatogram to develop overnight protected from draughts. 

If an alternative mixed solvent is required, shake thoroughly a mixture of 20 ml. of distilled water and 40 ml. of the -butanol, then... 

When the correct solvent for recrystallisation is not known a procedure similar to that given on pp. 15-16 should be followed, but on the semi-micro scale not more than 10 mg. of the solid should be placed in the tapered-end test-tube (Fig. 29(B)) and about o i ml. of the solvent should be added from the calibrated dropping-pipette (Fig. 30(B)). If the compound dissolves readily in the cold, the solvent is unsuitable, but the solution should not be discarded. [In this case recourse should be had to the use of mixed solvents (p. 18). For example if the substance is very soluble in ethanol, water should be added from a calibrated pipette with shaking to determine whether crystallisation will now take place, indicated by a cloudiness or by the separation of solid.]... 

If the substance is found to be far too soluble in one solvent and much too insoluble in another solvent to allow of satisfactory recrystallisation, mixed solvents or solvent pairs may frequently be used with excellent results. The two solvents must, of course, be completely miscible. Recrystallisation from mixed solvents is carried out near the boiling point of the solvent. The compound is dissolved in the solvent in which it is very soluble, and the hot solvent, in which the substance is only sparingly soluble, is added cautiously until a slight turbidity is produced. The turbidity is then just cleared by the addition of a small quantity of the first solvent and the mixture is allowed to cool to room temperature crystals will separate. Pairs of liquids which may be used include alcohol and water alcohol and benzene benzene and petroleum ether acetone and petroleum ether glacial acetic acid and water. 

The experimental details for recrystallisation from mixed solvents (or solvent pairs) will be evident from the account already given the best proportions of the two solvents are determined by preliminary small-scale experiments. 

The properties of a copolymer can be viewed as hybrids of the properties of the separate homopolymers. Because of this, a good deal of refinement can be introduced into these properties by the use of copolymers. The situation is analogous to the use of pure liquids or binary solutions as solvents. The number of binary combinations, n(n - l)/2 as noted above, greatly exceeds the number of pure liquids, and any one of these combinations can be prepared over a range of compositions. Just as mixed solvents offer a wider range of properties than... 

Selectivity. Solvent selectivity is intimately linked to the purity of the recovered extract, and obtaining a purer extract can reduce the number and cost of subsequent separation and purification operations. In aqueous extractions pH gives only limited control over selectivity greater control can be exercised using organic solvents. Use of mixed solvents, for example short-chain alcohols admixed with water to give a wide range of compositions, can be beneficial in this respect (6). 

The activity of the hydrogen ion is affected by the properties of the solvent in which it is measured. Scales of pH only apply to the medium, ie, the solvent or mixed solvents, eg, water—alcohol, for which the scales are developed. The comparison of the pH values of a buffer in aqueous solution to one in a nonaqueous solvent has neither direct quantitative nor thermodynamic significance. Consequently, operational pH scales must be developed for the individual solvent systems. In certain cases, correlation to the aqueous pH scale can be made, but in others, pH values are used only as relative indicators of the hydrogen-ion activity. 

J. G. Sloan, The Thermodynamic Tehaviour of Electrolytic and Mixed Solvents, Symp., Advanced Chemistry Series, 155, 1975, pp. 128—142. 

The first aromatic sulfone polymer produced commercially was introduced as Bakelite polysulfone but now is sold by Union Carbide under the trade name Udel. It is made by reaction of the disodium salt of bisphenol A (BPA) with 4,4 -dichIorodiphenyl sulfone in a mixed solvent of chlorobenzene and dimethyl sulfoxide (eq. 12). 

Values for aqueous solutions unless otherwise noted, either by direct determination or by extrapolation from mixed solvents. 

For prediction of the liquid diffusivity of a solute in a pair of mixed solvents, the method of Tang and Himmelblau, Eq. (2-161), is recommended. 

Mixed Solvents. Where a substance is too soluble in one solvent and too insoluble in another, for either to be used for recrystallisation, it is often possible (provided they are miscible) to use them as a mixed solvent. (In general, however, it is preferable to use a single solvent if this is practicable.) Table 8 contains many of the common pairs of miscible solvents. 

The technique of recrystallisation from mixed solvents is as follows ... 

Mixed solvents are intermediate in strength, and so provide a finely graded series. In choosing a solvent for use as an eluent it is necessary to consider the solubility of the substance in it, and the ease with which it can subsequently be removed. 

Purified by repeated crystn from MeOH. It can also be purified by sublimation under vacuum. Purity can be checked by TLC using a mixed solvent (pet ether, diethyl ether, EtOH 10 10 1) on a silica gel plate. 

In contrast molecular interaction kinetic studies can explain and predict changes that are brought about by modifying the composition of either or both phases and, thus, could be used to optimize separations from basic retention data. Interaction kinetics can also take into account molecular association, either between components or with themselves, and contained in one or both the phases. Nevertheless, to use volume fraction data to predict retention, values for the distribution coefficients of each solute between the pure phases themselves are required. At this time, the interaction kinetic theory is as useless as thermodynamics for predicting specific distribution coefficients and absolute values for retention. Nevertheless, it does provide a rational basis on which to explain the effect of mixed solvents on solute retention. 

This is an oversimplified treatment of the concentration effect that can occur on a thin layer plate when using mixed solvents. Nevertheless, despite the complex nature of the surface that is considered, the treatment is sufficiently representative to disclose that a concentration effect does, indeed, take place. The concentration effect arises from the frontal analysis of the mobile phase which not only provides unique and complex modes of solute interaction and, thus, enhanced selectivity, but also causes the solutes to be concentrated as they pass along the TLC plate. This concentration process will oppose the dilution that results from band dispersion and thus, provides greater sensitivity to the spots close to the solvent front. This concealed concentration process, often not recognized, is another property of TLC development that helps make it so practical and generally useful and often provides unexpected sensitivities. 

Mikkelsen, S., Jorgensen, M., Browne, E., and Gyldensten, G. ( I 988). Mixed solvent exposure and organic brain damage A study of painters. Acta Neurol. Scand. 78, 1-143. 

The reaction was performed m a mixed solvent of THF-diethyl ether (1 4) under reflux... 

PSS columns for medium polar or mixed solvents PSS HEMA and PSS SUPREMA Basic were designed to allow SEC separations in polar media such... 

There is increasing interest in copolymer systems, which, due to their chemical heterogeneity, may require very complex eluent systems in order to dissolve the sample and ensure that the separation ensues hy a pure size exclusion mechanism. In these examples, the PLgel is also compatible with eluent systems containing mixed solvents of different polarity (including water as a cosolvent up to 10% hy volume) and in organic solvents modified with acids or bases (e.g., acetic or formic acid, triethanolamine) as it is stable in the pH range of 1-14. 

Unlike earlier sulfonated styrene/divinylbenzene copolymers, these sulfonated gels can he run in virtually any solvent from water and buffers to pure organics as well as most any mixed solvent systems desired. In aqueous systems they absorb water and in organic solvents they stay swollen by imbibing organic solvents. 

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