General acidity scale

Another approach to this problem was proposed by Tremillon et al. [169-171], Their idea consists in the construction of the general acidity scale for chloride melts and determination of the positions of standard solutions of strong Lux bases on this scale. As media for acid-base reactions, chloride melts should be classified among solvents of the second kind, and, therefore, the positions of such solvents on the acidity scale should be described by only one point—the acidity function for the standard solution of a strong base. 

To construct the general acidity scale in molten media Combes et al. [169, 170] proposed to estimate the acidity functions for ionic melts by... 

These chloride melts are referred to as ionic media with low-acidic properties. As seen from the previous Part, they can be used as a zero (reference melts) for the construction of the general acidity scale of high-temperature ionic melts. The behaviour of most oxygen electrodes was studied in the molten KCl-NaCl, which is generally accepted as the classic molten medium. 

These are constant numbers obtained from experiments or independent calculations. They are properties of the atom or the molecule and have been useful in predicting the chemical behavior of the system. In addition to its use as a scale of electronegativity, the chemical potential is also a measure of the intrinsic strength of generalized acids and bases [12]. 

Several alternative attempts were made to quantify Lewis-type interactions235,236. Following the HSAB principle, the applicability of a one-parameter Lewis acidity scale will inevitably be restricted to a narrow range of structurally related Lewis bases addition of parameters results in more general relationships237-239. The quantitative prediction of the gas-phase stabilities of Lewis acid-Lewis base complexes is still difficult. Hence the interpretation, not to mention the prediction, of solvent effects on Lewis acid-Lewis base interactions is often speculative. 

Decay of arene hydrocarbon radical-anions formed during preparative scale electrochemical reduction in the presence of general acids involves protonation as... 

Displacement reactions observed in the gas phase are generally exothermic or thermoneutral as in the case of simple isotope exchange. This requirement is consistent with the limited dynamic range of the experimental techniques which precludes the observation of reactions with sizable activation energies. The relevant thermochemical data for negative ions have become available in recent years through the determination of electron affinities (Janousek and Brauman, 1979), and indirectly from gas-phase acidity scales (Bartmess and Mclver, 1979). Relative gas-phase acidities available at present (Bartmess et al., 1979 Cumming and Kebarle, 1978) are an important consideration in... 

An acidity scale has been proposed in which the difference in the acidity parameters. (aB — aA), of a metal oxide and a nonmetal oxide is the square root of the enthalpy of reaction of the acid and base.4 Thus for reaction 9.5, the enthalpy of reaction is —8fikJ mol-1 and so the a values of CaO and SiO, differ by about 9 units. Selected values are fisted in Table 9.1. Although based on the Lux-Flood concept, the values are obviously of more general interest. The most basic oxide, as expected, is cesium oxide, amphoteric oxides have values near zero (water was chosen to calibrate the scale at a value of 0.0). and the most acidic oxide is CLO,. the anhydride of perchloric acid. 

Aminotri(methylenephosphonic acid) (ATMP or AMP) is the lowest cost phosphonate. It is a good general-purpose scale inhibitor. It is also an effective chelant and can provide good corrosion inhibition when in combination with zinc or phosphates. 

For carbocations, an electrophilicity (Lewis acidity) scale can be based on ions other than the hydroxide ion as is shown in general for X- in Equation (6), for which the equilibrium constant can be denoted A1R. Scales based on chloride ion, for example, have been used in the gas phase2,17,36 and are also appropriate for nonaqueous solvents. 

In principle, the A 0(H) function is of limited interest for kinetic applications because the indicators are chemically very different from the organic substrates generally used. On the other hand, as the measurements are based on pH determination, the length of the acidity scale is limited by the pA" value of the solvents. However, very interesting electrochemical acidity studies have been performed in HF by Tremillon and co-workers, such as the acidity measurement in anhydrous HF solvent and the determination of the relative strength of various Lewis acids in the same solvent. By studying the variation of the potential of alkane redox couples as a function of acidity, the authors provide a rational explanation of hydrocarbon behavior in the superacid media.48... 

DSM has developed a general, industrial-scale process for the production of either l- or D-amino acids through the hydrolysis of the amide (Scheme 2.22).75 78 The product amino acid and untouched amide are easily separated. The amide can be recycled. The resolution method has been extended to a,a-disubstituted of which L-methyl-dopa (14) is an example.79... 

The general SPP scale of solvent dipolarity/polarizability and the specific SB and SA scales of solvent HBA basicity and HBD acidity, respectively, are orthogonal to one another and they can be used in the correlation analysis of solvent effects in single- or, in combination with the others, in two- or three-parameter correlation equations, depending on the solvent-influenced process under consideration see also Section 7.7. Examples of the correlation analysis of a variety of other solvent-dependent processes by means of SPP, SB, and SA values, including those used for the introduction of other solvent polarity parameters, can be found in references [335-337, 340-342]. In particular, comparisons with Kamlet and Taft s n scale [340] and Winstein and Grunwald s Y scale [341] have been made. 

The influence of solvents on the ionization equilibrium is related to their electrostatic and their solvation properties. The value of the ionization constant of an analyte is closely determined, in practice, by the pH scale in the particular solvent. It is clear that it is most desirable to have a universal scale which is able to describe acidity (and basicity) in a way that is generally valid for all solvents. It is, in principle, not the definition of an acidity scale in theory which complicates the problem it is the difficulty of approximating the measured values in practice to the specifications of the definition. The pH scale, as is common in water, is applicable only to some organic solvents (i.e., mainly those for which the solvated proton activity is compatible with the Brpnsted theory of acidity). The applicability of an analog to the pH scale in water decreases with decreasing relative permittivity of the solvents and with their increasing aprotic character. 

C whereas the minimum of the melting-point plot for the KC1-NaCl binary system is achieved at the component ratio of 1 1 and corresponds to a temperature near 658 °C. Naturally, such a considerable difference in temperature should cause appreciable changes in the acidic/basic properties of both substances arranged in one conjugate pair. Therefore, we shall consider further in Part 3 the general oxoacidity scales corresponding to the definite temperatures 600, 700 and 800 °C. 

Fig. 1.1.3. The general oxoacidity scale of ionic melts. The positions of the conjugated acid-base pair corresponds to the KCl-NaCl eutectic at 700 °C.

Considerations similar to those lying at the basis of the determination of the acidic properties of ordinary room-temperature molecular solvents [174] can be used to determine the relative acidic properties of high-temperature ionic melts-solvents. To estimate the positions of melts within the general oxoacidity scale, it is necessary to choose the indicator equilibrium and the reference melt. The basicity of the standard solution of a strong base in the reference solvent according to the definition should be equal to zero (0). 

As heterogeneous equilibria in solid oxide-ionic solvent systems are the most investigated in molten chlorides, the general acid-base scale for such melts may be constructed using the solubility method. 

Further development of the problem of general oxoacidity scales in molten salts may be connected with finding more suitable indicator equilibria, and obtaining new data on buffer solution acidities in various melts that will allow the calibration of different oxide ion sensors against 12 values. 

Another peculiarity of the alkali metal chlorides doped with alkali metal chlorides appears in the values of the slopes of the peroxide section they are appreciably lower than 2.3RT/F. The behaviour of the Pt(02)lYSZ membrane oxygen electrode in chloride melts based upon alkali- and alkaline-earth metal chlorides is similar to that in the KCl-LiCl. In this case, the changing position of the inflection point of the E-pO calibration plot agrees with the positions of these melts doped with acidic cations (Ba-Sr-Li-Ca) in the general oxoacidity scale. 

From the viewpoint of the general oxoacidity scale, it is of interest to continue investigations of phenomena such as the levelling of acidic and basic properties by ionic solvents and the effect of temperature on the reciprocal arrangement of ionic melts in the oxoacidity scale. 

---