The Leveling Effect

The strongest acid that can exist in water is HjO+ and the strongest base is OH If an acid stronger than H30+ is dissolved in water, it protonates H20 to make H,0+. If a base stronger than OH is dissolved in water, it deprotonates H20 to make OH. Because of this leveling effect, HC104 and HC1 behave as if they had the same acid strength both are leveled to H30+  

The pure commercial material is dried at 105°C and used to standardize base. A phenolphthalein end point is satisfactory. 

HC1 and water distill as an azeotrope (a mixture) whose composition ( 6 M ) depends on pressure. The composition is tabulated as a function of the pressure during distillation. See Problem 11-55 for more information. 

This is a strong acid, so any indicator with an end point between 5 and 9 is adequate. 

I mol of commercial grade sulfosalicylic acid is combined with 0.75 mol of reagent-grade KHC03, recrystallized several times from water, and dried at 110°C to produce the double salt with 3 K+ ions and one titratable H+.15 Phenolphthalein is used as the indicator for titration with NaOH. 

A similar situation holds for base strength. For a dilute solution, no base stronger than HO can exist to any appreciable extent in water. Adding a base whose conjugate acid has a pKa above 15.7 will cause the products in Eq. 5.6 to dominate. For example, if one makes up a solution of the base Na NH2 (the pXa of NH3 is 35) in water, the NH2 will completely de-protonate water, and therefore the real base in such a solution is simply HO . These phenomena have been discussed in terms of water, but are general to any solvent, leading to what is known as the leveling effect. 

The leveling effect creates a limitation on the strengths of acids and bases that can be determined in particular solvents  

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The significance of the possible diprotonation of water under extremely acidic conditions directly affects the question of acid strength achievable in superacidic systems. The leveling effect mentioned above limits the acidity of any system to that of its conjugate acid. Thus, in... 

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 . 

The conductivities of melts, in contrast to those of aqueous solutions, increase with decreasing crystal radius of the anions and cations, since the leveling effect of the solvation sheaths is absent and ion jumps are easier when the radius is small. In melts constituting mixtures of two salts, positive or negative deviations from additivity are often observed for the values of conductivity (and also for many other properties). These deviations arise for two reasons a change in hole size and the formation of new types of mixed ionic aggregates. 

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... 

In Fig. 4.1b one sees the occurrence of the levelling effect on the basic side only in protogenic solvents (dotted lines above). 

Their relative acidities (pKas) thus cannot be measured in water at all. Further, when acids are sufficiently strong (low enough pKa), they will all be essentially fully ionised in water, and will thus all appear to be of the same strength, e.g. HC1, HN03, HC104, etc. This is known as the levelling effect of water. 

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 ... 

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 phenomenon described above for water also applies to other amphoteric solvents. It is termed the leveling effect, and may be summarized by the following statements ... 

In water, our effective prange is only -1.74 to 15.74, that is, it is determined by the solvent. This is known as the levelling effect of the solvent. This is an important point. It means that, if we want to remove the proton from something with a high piCa, say 25-30, it would be impossible to do this in water since the strongest base we can use is hydroxide. If we do need a stronger base than OH , we must use a different solvent system. 

Fig. 3-3. Schematic description of the levelling effect of water on (a) acids and (b) bases in aqueous solution [2]. Relative orders of acidity and basicity are not invariable to changes of solvent and of the conjugated acid or base, respectively.

The development of these ion-molecule equilibrium measurements has completely changed the status of acid/base reactions (and of other reactions cf. Sechon 5.2) in the gas phase. It is now possible to compare the complex and poorly understood situahon in solution with the simple state in the gas phase. It is also possible to determine the acidity of all acids in the gas phase, from the weakest such as methane to the strongest. In solution, however, due to the levelling effect of the solvent or solubility problems, only a certain range of acids can be measured in a given solvent. 

Aprotic Solvents.—The colorimetric method of studying dissociation constants has found a special application in aprotic solvents such as benzene these solvents exhibit neither acidic nor basic properties, and so they do not have the levelling effects observed with acids in proto-... 

Nonaqueous solvents have the advantage of enhanced solubility of organic reactants and products and avoidance of the leveling effects of aqueous solvent. 

This is called the leveling effect, in which acids or bases are brought down to the limiting conjugate acid or base of the solvent. Because of this, nitric, sulfuric, perchloric. 

FIGURE 6-17 The Leveling Effect and Solvent Properties. (Adapted from R. P. Bell, The Proton in Chemistry, 2nd edition, 1973, p. 50. Second edition, copyright 1973 by R. P. Bell. Used by permission of Cornell University Press.)... 

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