Acetonitrile-water mixtures, dependence

The constants a, m, d depend on the type of the organic solvent in the mobile phase and on the solute. The value of the quadratic term dtp- in Eq. (1.17) determines the curvature of the log k versus tp plots. The parameter d increases with decreasing polarity of the organic solvent. Consequently, the log k versus cp plots are often linear in aqueous solutions of methanol (Fig. 1.9), slightly nonlinear in acetonitrile-water mixtures and... 

The products of hydrolysis of the 1,3-benzodithiolium cation 152 depend upon the experimental conditions. In acetonitrile-water (3 1) or acetone-water (4 1), after I hour, 153 is obtained. In an acetonitrile-water mixture (4 1), after several hours, or in triethylamine-water (1 15), 154 is obtained in excellent yields. - The mechanism probably involves nucleophilic attack of 152 by water and tautomerism of the 2-hydroxy compound 155 with the open-chain derivative 156. Reaction of 156 with another mole of 152 affords 153, which can lose carbon monoxide to give 157. This last can, in turn, react with 152, giving 154. Intermediate 155 has not been isolated but was identified by NMR in the reaction mixture (Scheme 29). ... 

This equation reproduced very well the general form of the dependence of for several nitrobenzene anion radicals dissolved in acetonitrile-water mixtures. However, agreement with the G.F.F. theory should not be taken as conclusive evidence for the existence in solution of 1 1 solvent-solute complexes, since detailed examination of alternative stoichiometries was not undertaken. Indeed, Gendell et al. made no attempt to fit eqn. 4.16.3 to any experimental ta, other than noting the similarity discussed above. 

Figure 2.14 Dependence of the (dynamic) viscosity t] on temperature for three representative HPLC solvents and one acetonitrile-water mixture. Curves show the progression as relative values of rj related to the viscosity at 25 °C.

The mixture of acetonitrile/water (1 1, v/v) was selected as most effective mobile phase. The optimum conditions for chromatography were the velocity of mobile phase utilization - 0,6 ml/min, the wave length in spectrophotometric detector - 254 nm. The linear dependence of the height of peack in chromathography from the TM concentration was observed in the range of 1-12.0 p.g/mL. 

Generally it was found that resolution R is practically the same for isoeluotropic mixtures methanol and acetonitrile with water. The dependencies were obtained between capacity factors for derivatives of 3-chloro-l,4-naphtoquinone at their retention with methanol and acetonitrile. Previous prediction of RP-HPLC behaviour of the compounds was made by ChromDream softwai e. Some complications ai e observed at weak acetonitrile eluent with 40 % w content when for some substances the existence of peak bifurcation. 

The sequence of the selectivities towards cations is also solvent dependent for dibenzo-18-crown-6 [11] the sequence is K+ > Na+ > Rb+ > Cs+ in water, methanol, dimethylformamide and dimethyl sulfoxide (Dechter and Zink, 1976 Srivanavit et al., 1977), whereas it is Na+ > K+ > Rb+ > Cs+ in acetonitrile (Hofmanova et al., 1978). A reversal of the K+/Na+ selectivity on going to apolar aprotic solvents was also observed for fluorenyl salts (Wong et al., 1970). Whereas for alkali cations the sequence of binding constants and enthalpies are the same in water (Izatt et al., 1976a), they differ considerably in methanol/water mixtures (Izatt et al., 1976b), dimethyl sulfoxide and acetone (Arnett and Moriarity, 1971). 

In spite of the obvious advantages of elevated temperature, there are examples of cases where better separation is achieved at a reduced temperature, even for simple solutes. Craft et al. [20] recently demonstrated an improved separation of P and y tocopherol at -20°C in THF/acetonitrile when compared to the ambient temperature separation of the compounds in Acetonitrile water. Bohm [21] reported the temperature dependence of the separation of a mixture of five xanthophylls and six carotenes on a C-30 column. The optimum temperature in this case was 23°C with a coelution of some peaks at temperatures below 20°C and others above 35°C. In a study using a 300 A pore C-18 column, Bohm [22] reported dramatic changes in the elution order over the temperature range -7°C to 35°C. On this column, the optimal separation was achieved at low temperatures... 

The operating conditions range from an excitation wavelength of 360-365 nm and an emission wavelength 425-435 nm. The intensity of aflatoxin fluorescence depends strongly on the injected solvent, with a higher response if the sample is injected in the mobile phase (usually a ternary mixture of methanol/acetonitrile/water) and a lower one if injected in the methanol or acetonitrile only. 

The crude reaction mixture is loaded onto an FSG column. First, the organic components are eluted with polar solvents (acetonitrile/water or methanol/water) while the perfluoro-tagged compounds are retained on the column. The fluorous compounds are then eluted with more fluorophilic solvents (acetonitrile, acetoni-trile/diethyl ether, diethyl ether, FC-72). How strongly a compound is retained depends on the size of the fluorous tag. Untagged molecules are not significantly retained [27]. 

The rate of photohydration of uracil with light of 265 n.m. in mixtures of acetonitrile and water is a linear function of the water content of the mixture the deuterium isotope effect, R//2o/R/)2o, is 1.18. These observations are shown to be consistent with the mechanism for uracil photohydrate formation proposed to explain the pH dependence of the rates. The rate of photohydrate formation for 1,3-dimethy-uracil is approximately proportional to the square of the water concentration in acetonitrile-H 20 mixtures, aceto-nitrile-D20, and in dioxane-water mixtures, RH20/Rn2o = 2.84. These data are also interpretable in terms of a common photohydration mechanism. 

The photo-aquation of [Pt(SCN)e]2 has been investigated in acetonitrile-water solvent mixtures.121 The mechanism appears to be different from that for [PtBre]2- as the reaction quantum yield is dependent on both wavelength and solvent. The intermediacy of Pt111 is suggested. 

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