Solvent levelling effect

Heteropolyacids are well known to be strong Bronsted acids and their acidity has been quantitatively characterized and compared with the acidity of mineral acids such sulfuric acid [10]. Differently of H2SO4 which has only one totally ionizable proton, heteropolyacids when in aquous solution are completely dissociated at the first three steps because of the solvent leveling effect [11]. Several works described the H3PW12O40 heteropolyacid as an efficient super-acid, which has been used either as soluble catalyst in polar solvents, or in the heterogeneous phase, supported silica or on active coal as solid matrix. 

Solvent levelling effect upon Lewis acidity 236... 

Because of the mentioned leveling effect of the solvent (or excess acid itself acting as such) the acidity cannot exceed that of its conjugate acid. In the case of water the limiting acidity is that of HsO. Proton-ated water, H30 (hydronium ion), was first postulated in 1907, and its preeminent role in acid-catalyzed reactions in aqueous media was first realized in the acid-base theory of Bronsted and Lowry. Direct experimental evidence for the hydronium ion in solution and in the... 

The equilibrium in this reversible reaction will be greatly influenced by the nature of the acid and that of the solvent. Weak acids are normally used in the presence of strongly protophilic solvents as their acidic strengths are then enhanced and then become comparable to those of strong acids — this is referred to as the levelling effect . 

Determinations in non-aqueous solvents are of importance for substances which may give poor end points in normal aqueous titrations and for substances which are not soluble in water. They are also of particular value for determining the proportions of individual components in mixtures of either acids or of bases. These differential titrations are carried out in solvents which do not exert a levelling effect. 

It should be mentioned that the predicted curve at highest benzene level in Figure 13 agrees with classical kinetics (no diffusion-control). It is not clear therefore why measured data at even higher benzene concentrations do not agree with classical kinetics. There may be some subtle chemical interactions at these high solvent levels. Duerksen(lT) fomd similar effects with styrene polymerization in benzene and had to correct kp for solvent. 

In this case, the alkaline pH of the aqueous solvent exhibits a "leveling effect on dissolved amines in that all the amines are unprotonated and become equivalent in reactivity. Challis also noted that under their experimental conditions 2 x 10 ... 

Comparison of reactions 4.9, 4.10, 4.12, 4.13 and 4.15 leads to another important conclusion, viz., in an amphiprotic solvent its own solvonium cation represents the strongest acid possible, and its own anion the strongest base. Even when a very strong foreign acid or base is dissolved, excessive proton donation to and proton abstraction from the solvent molecule yield the respective acid or base this phenomenon is generally known as the levelling effect, which in an amphiprotic solvent takes place on both the acid and the basic... 

The pyridinium ion (acid 2) as the analyte can be titrated with quaternary ammonium hydroxide (base 3) as it concerns the determination of H+ of the Brensted acid pyridinium, a potentiometric measurement of the pH titration curve and its inflection point is most obvious. In the aprotic, but protophilic, solvent pyridine no stronger acid can exist (see reactions 4.37 and 4.38) than the pyridinium ion itself hence there is a levelling effect but in theory only on the acid side. 

In Fig. 4.1a one sees the occurrence of an increasing levelling effect on the acid side when changing from protogenic solvents via neutral to proto-philic solvents (dotted lines below). 

The leveling effect is the effect by which all acids stronger than the acid that is characteristic of the solvent react with the solvent to produce that acid. For example, in water, the strongest acid and base that can exist in water is H+ and OH , respectively. In our example, the oxide ion (02 ) is a stronger base than OH . When K20 dissolves in water, the O2- cannot exist in the solution. O2- immediately removes a proton from H20, forming OfT ... 

Acetonitrile, acetone and dimethylformamide—these non-aqueous solvents exert a greater differential in the protophillic properties of many substances than in the corresponding aqueous solutions, due to the levelling effect of water in the latter solutions. Hence, the most acidic substance in aqueous solutions of a number of acids is the formation of the hydronium ion as shown below ... 

The balance between stationary-phase hydrophobicity and solvent exclusionary forces is the major determinant of selectivity on nonphenyl columns. On strongly hydrophobic columns, hydrophobic interactions between the proteins and the column are most likely to be the dominant determinant of selectivity. The low levels of binding salts required to achieve retention exert relatively less influence. On weakly hydrophobic columns, the high levels of binding salts cause solvent exclusionary effects to exert a greater influence. 

Buschmuller et have demonstrated that microwave resonance can be used effectively as means to monitor the moisture levels in a fluidized-bed dryer during the granulation process. The penetration depth of microwave resonance may be limited to a few microns, and hence this technique may not have any real advantages over NIR which has also been used for monitoring moisture in dryers, and has the advantage of providing chemical information such as solvent levels in addition to water, and other important properties such as polymorphic form, and particle size. 

With reference to a solvent, this term is usually restricted to Brpnsted acids. If the solvent is water, the pH value of the solution is a good measure of the proton-donating ability of the solvent, provided that the concentration of the solute is not too high. For concentrated solutions or for mixtures of solvents, the acidity of the solvent is best indicated by use of an acidity function. See Degree of Dissociation Henderson-Hasselbalch Equation Acid-Base Equilibrium Constants Bronsted Theory Lewis Acid Acidity Function Leveling Effect... 

A cation produced by the protonation of a solvent molecule. See also Leveling Effect... 

Interestingly, on changing from a purely aqueous medium to 70% DMSO-30% H2O, the leveling effect was no longer observed (Figure 5) . Other important manifestations of solvent effects are considered further below. 

The ionization of the acid depends not only on the basicity of the solvent, but also on its dielectric constant and its ion-solvating ability. The dependence of the acidity and basicity constants of a compound on the basicity and acidity, respectively, of the solvents, leads to a distinction between leveling and differentiating solvents. When the solvent is a stronger base than water, its leveling effect will apply also to weaker acids. Similarly, strong bases will also have equal basicities in sufficiently acidic solvents. All bases stronger than the HO ion are adjusted to the basicity of this ion in water. 

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