Common acidity scale

Fig. 1.1.2. The schematic common-acidity scale of protic and aprotic solvents constructed from the data of Refs. [16, 52] (where A designates acidic regions and, designates basic ones).

N.A. Izmailov, Common Acidity Scale, Doklady Akad. Nauk. SSSR 127 (1959) 104-107. 

However, for several reasons it is difficult to compare the acidity of solids in definitive terms as demanded above. They are related to the experimental methods for characterizing surface acidity or to the theoretical definition of a common acidity scale. 

The values of the prirrrary medium effect of proton were used by Izmailov to con-stract the common acidity scale for protolytic solvents and for the solvents more basic than water having negative values of logy +, whereas for more acidic solvents these valuesarepositive. It is obviorrs that for the cafe of molten salts values of logy 2- will... 

Further, Izmailov proposed to constract common acidity scale for solutions in different solvents using pA index, which is equal to ... 

Where do hydrocarbons lie on the acidity scale As the data in Table 8.1 show, both methane (pKa 60) and ethylene (plC, = 44) are very weak acids and thus do not react with any of the common bases. Acetylene, however, has piCa = 25 and can be deprotonated by the conjugate base of any acid whose pKa is greater than 25. Amide ion (NH2-), for example, the conjugate base of ammonia (pKa - 35), is often used to aeprotonate terminal aikynes. 

For NO2 the recommended values of am and ap (benzoic acid scale) in the IUPAC document79 are 0.73 and 0.78, respectively, compared with the traditional values of 0.71 and 0.78, respectively (Section III.B). When these values are used for correlations of processes taking place in other than highly aqueous media, the possibility of specific solvent effects should be borne in mind. The fact that the 0 values of NO2 are very much at the upper end of the scale for commonly used substituents means that they exert a strong influence in regression analysis and there is danger of their biassing a correlation unduly. 

Antoine Baum , 1728-1804. French pharmacist and chemist. Author of a Chymie expenmentale et raisonnee in which he discussed chemical apparatus, chemical affinity, fire, air, earth, water, sulfur, gypsum, alum, clay, niter, gunpowder, borax, arsenic, glass, porcelain, and the common acids, alkalies, metals, and ores used m 1773. His hydrometer scale is still used. He was one of the first chemists to investigate platinum... 

UV-Vis spectroscopy is a widely used technique to monitor the acid-base equilibrium in common organic solvents. This is also true for ILs. However, considering ionic nature of this solvent, the interpretation of the results is not such simple. The work is going on to establish the Bronsted acidity scale in ILs [30] which can later be used to predict various interactions and structure-property relationships. 

In the above, the pH of the solution of a(H+) = lmolkg 1 was defined equal to zero in each solvent.10 However, the solvation of H+ differs from one solvent to another and, even in a solution of pH = 0, the reactivity of H+ differs drastically by solvent. In order to compare the acid-base properties in different solvents, it is convenient to define a pH scale that is common to various solvents [20]. Figure 3.6 shows the pH windows in various solvents in such a common pH scale, using the pH in water as reference. In the figure, the left margin of the pH window corresponds to the value of -log yt(H+,W->S) (see Table 2.7) and the width of the window corresponds to the value of pKSH. If the solvent is of weaker basicity than water, the pH window expands to the left (more acidic) side than water. On the other hand, if the solvent is of weaker acidity than water, the pH window expands to the right (more basic) side than water. 

Several dye coloration scales are used to characterize starch varieties. Perhaps the oldest is the differentiation between starches based on the uptake of Saphranin and Gentiana Violet (see Table XXXVIII).787 Co-vello788 presented another coloration scale which is based on the use of six common acidic and basic dyes (see Table XXXIX). Like the Saphranin and Gentiana Violet dyes, these dyes adsorb directly on starch. Table XL presents a list of synthetic dyes tested in starch dyeing.789-790 Zwikker791 observed the reactions of mechanically damaged starch granules and amylo-... 

This range shows that plant lipids as a whole can produce a wide variety of fatty acids, sometimes at very high levels. It follows that the enzymes necessary to produce these less common acids are available somewhere within the plant kingdom. It is then possible to identify these, clone them, and introduce them into species that are already cultivated on a large scale such as rapeseed, soybean, maize (corn), sunflower, and linseed among others (143, 202-204). 

This work, however, did not completely eliminate the controversy over the toxicity of volatile acids. Subsequently, Buswell and Morgan 20) reported that propionic rather than acetic acid was toxic to the methane bacteria. McCarty et al. (9) investigated the effect of various volatile acids on methane bacteria in order to clear up this controversy. They added 6000 mg/liter of acetic, propionic, and butyric acid (the three most common acids produced during anaerobic breakdown of complex substances) individually to laboratory scale sewage sludge digesters. Prior... 

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. 

Acidity scales are used commonly to assess the chemical and biological state of seawater. The international operational scale of pH fulfills the primary, requirement of repro ducibility and leads to useful equilbrium data. Nevertheless, these pH numbers do not have a well defined meaning in respect to all marine processes. Seawater of 35%o salinity behaves as a constant ionic medium, effectively stabilizing both the activity coefficients and the liquid junction potential. It may be possible, therefore, to determine hydrogen ion concentrations in seawater experimentally. One method is based on cells without a liquid junction and is used to establish standard values of hydrogen ion concentration (expressed as moles of H /kg of seawater) for reference buffer solutions. 

It is obvious that the expression enclosed in the brackets by the author of the present book is nothing but the primary medium effect of O2- expressed via the difference in the values of the equilibrium constants of equation (1.3.6) for the media compared the molten equimolar KCl-NaCl mixture, which was chosen as a reference melt, and for which pKHa/H20 was found to be 14 at 700 °C, and the melt studied. As to the physical sense of the common acidity function Cl, this is equal to the pO of the solution in the molten equimolar KCl-NaCl mixture, whose acidic properties (oxide ion activity) are similar to those of the solution studied. Moreover, from equation (1.3.7) it follows that solutions in different melts possess the same acidic properties (f ) if they are in equilibrium with the atmosphere containing HC1 and H20 and Phc/Ph2o — constant. This explanation confirms that the f function is similar to the Hammett function. Therefore, Cl values measured for standard solutions of strong bases in molten salts allow the prediction of the equilibrium constants on the background of other ionic solvents from the known shift of the acidity scales or the f value for the standard solution of a strong Lux base in the solvent in question. According to the assumption made in Refs. [169, 170] this value may be obtained if we know the equilibrium constant of the acid-base reaction (1.3.6) in the solvent studied. 

The concentration of H" " or OH in aqueous solution can vary over extremely wide ranges, from 1 M or greater to 10 M or less. To construct a plot of concentration against some variable would be very difficult if the concentration changed from, say, 10 M to 10" M. This range is common in a titration. It is more convenient to compress the acidity scale by placing it on a logarithm basis. The pH of a solution was defined by Sprenson as... 

The concepts of hydrogen ion concentration and pH discussed above are meaningful only for dilute aqueous solutions of acids. In water-like solvents such as methanol, similar concepts may be developed, but in most organic solvents, in concentrated aqueous solutions and in the anhydrous state the concept is meaningless and some other scale of acidity is required. There are a number of different acidity scales but the one most commonly used is the Hammett acidity function, which allows comparison of the same acid in different media as well as intercomparisons of acids. The function H0 pertains to the equilibrium between a base, B, its conjugate acid, BH+, and the proton, H+ n+H+ RH+... 

The pA" of several common amines used in synthesis have been determined in THF at 25°C. All will be used as bases in reactions used later in this book. 4-Al,Al-Dimethylaminopyridine (DMAP, 16), proton sponge (17), quinuclidine, (18), DBU (l,8-diazabicyclo-[5.4.0]-undec-7-ene, 19), and diazabicyclo-[2.2.2]octane (DABCO, 20) are shown, along with triethylamine, tributylamine, and tribenzylamine for comparison. The p/T values for these compounds are shown in THF, DMSO, and acetonitrile in Table 2.4.27 These values were determined from proton-transfer ion pairs of the type BH" " with acidic indicator hydrocarbons in THF. The values obtained in THF differ substantially from the ionic pKa values for NH+ in DMSO or acetonitrile. Streitwieser and co-workers27 states that "at the present time amines cannot be placed quantitatively on any of the ion pair acidity scales currently in use for neutral acids in THF". The realization that it may be necessary to use different bases in different solvents, despite the fact that all are considered to be good bases in organic reactions, makes it clear why a quantitative comparison of bases would be useful. 

The most common acid solvent is inhibited muriatic acid. It is inhibited, as all acids solvents must be, to reduce chemical attack on metal surfaces to an acceptable level. Muriatic acid is cheap and effective. It forms reaction products that are generally water-soluble and so are removed in the used solvent. Sulfuric acid is seldom used because of its insoluble reaction products. Nitric acid cannot be prevented from attacking carbon steels or copper alloys. It has certain special applications where the substrate metal is stainless steel or aluminum. Of special interest for small-scale maintenance cleaning by plant personnel is sulfamic acid, mainly because it is a dry solid product that is safely to handle. It has acid properties only after it is dissolved in water. Solid inhibitors and wetting additives can be premixed into the packaged product. 

Recently Himmel et al. have proposed a unified pH scale for all phases [534]. This general Brpnsted acidity scale is based on the absolute chemical potential of the proton in any medium. They set the standard chemical potential of the proton in the gas phase Pabs°(H gas) to 0 kJ/mol as a reference value. They report the Gibbs energies of proton solvation Asoi °G(H+) (based on a reference value -1105 8 kJ/mol for water) in most common solvents. Absolute pH values are assigned according to the relation... 

It follows that the acidity of an acid can be measured only with reference to a base. If a common base is chosen, a relative acidity scale for several acids can be established. Water is a very common solvent. A scale of values... 

---