Measurement of Acidity

Table 8.17 Standard Reference Values pH for the Measurement of Acidity in...

Determination of Lignin Content. Lignin content in plants (wood) is determined by direct or indirect methods (21). The direct method includes measurement of acid-insoluble (ie, Klason) lignin after digesting wood with 72% sulfuric acid to solubilize carbohydrates (22). The Klason lignin contents of representative lignifted materials are shown in Table 2. 

A particular concentration measure of acidity of aqueous solutions is pH which usually is regarded as the common logarithm of the reciprocal of the hydrogen-ion concentration (see Hydrogen-ION activity). More precisely, the potential difference of the hydrogen electrode in normal acid and in normal alkah solution (—0.828 V at 25°C) is divided into 14 equal parts or pH units each pH unit is 0.0591 V. Operationally, pH is defined by pH = pH(soln) + E/K, where E is the emf of the cell ... 

In the discussion of the relative acidity of carboxylic acids in Chapter 1, the thermodynamic acidity, expressed as the acid dissociation constant, was taken as the measure of acidity. It is straightforward to determine dissociation constants of such adds in aqueous solution by measurement of the titration curve with a pH-sensitive electrode (pH meter). Determination of the acidity of carbon acids is more difficult. Because most are very weak acids, very strong bases are required to cause deprotonation. Water and alcohols are far more acidic than most hydrocarbons and are unsuitable solvents for generation of hydrocarbon anions. Any strong base will deprotonate the solvent rather than the hydrocarbon. For synthetic purposes, aprotic solvents such as ether, tetrahydrofuran (THF), and dimethoxyethane (DME) are used, but for equilibrium measurements solvents that promote dissociation of ion pairs and ion clusters are preferred. Weakly acidic solvents such as DMSO and cyclohexylamine are used in the preparation of strongly basic carbanions. The high polarity and cation-solvating ability of DMSO facilitate dissociation... 

That the rates of many reactions are markedly dependent upon the acidity or alkalinity of the reaction medium has been known for many decades. In this section, the kinetic analysis of reactions in dilute aqueous solution in which pH is the accessible measure of acidity is presented in sufficient detail to allow the experimentalist to interpret data for most of the systems likely to be encountered and to extend the treatment to cases not covered here. This section is based on an earlier discussion.The problem has also been analyzed by Van der Houwen et al. "... 

The parameters and Ca are associated with the Lewis acid, and Eg and Cb with the base. a and b are interpreted as measures of electrostatic interaction, and Ca and Cb as measures of covalent interaction. Drago has criticized the DN approach as being based upon a single model process, and this objection applies also to the — A/y fBFs) model. Drago s criticism is correct, yet we should be careful not to reject a simple concept provided its limits are appreciated. Indeed, many very useful chemical quantities are subject to this criticism for example, p o values are measures of acid strength with reference to the base water. 

From measurements of acid constants, > " and rates of esteri-... 

The determination of molecular structure by microwave spectroscopy (Section VII,A), the measurement of acid-base equilibria by ICR (Section VII,F), and theoretical calculations, especially accurate for the gas phase, are the main novelties of this section. 

Acidity constant, Ka (Section 2.8) A measure of acid strength. For anv acid HA, the acidity constant is given bv... 

Tables of KA furnish a quantitative measure of acid strengths with which we can compare different acids and predict their properties. Several values of KA are given in Table 11-IV.

Measurement of Acidity or Alkalinity of Explosives. (Measurement of pH value). These tests consist essentially of extrg the sample of expl with w (either cold in the case of liquids such as NG, or hot for solids such as TNT, Tetryl, PA, etc), and detg the acidity of the resulting extr either by titration, colorimetric methods or by means of a pH meter. These tests are described under the individual compds... 

Alkalinity measurement is also required for the determination of active matter by difference and equivalent weight calculations. It can be determined as two of the following compounds sodium bicarbonate, sodium carbonate, or sodium hydroxide. The sample is titrated to a phenolphthalein endpoint to determine the sodium hydroxide/sodium carbonate content. An added measure of acid converts any bicarbonate to carbon dioxide, which is subsequently removed from the solution. Back-titration of the excess acid gives a measure of the amount of bicarbonate and/or carbonate present. 

Bacarella, A. L. Grunwald, E. Marshall, H. R Purlee, E. L., The potentiometric measurement of acid dissociation constants and pH in the system methanol-water. pKa values for carboxylic acids and anilinium ions, j. Org. Chem. 20, 747-762 (1955). 

There are countless situations where the relative acidity or alkalinity of a substance or system is of critical importance. Agricultural conditions, water quality, food preservation, lung and kidney function—all are circumstances in which quantitative measures of acidity or basicity enable proper maintenance and regulation of vital processes. The term pH is widely used as an expression of acid/base content, but what is the origin of such nomenclature ... 

Table 1. Selected absorption indicators for measurements of acidic or basic pHs, and respective pKa values at room temperatures1.

Similarly, all pH indicators mentioned can be used for measurement of acidic and basic gases. Figure 14 shows the example of deprotonated bromophenol blue when dissolved in silicone and upon exposure to ammonia when the color of bromophenol blue changed from yellow to blue40 41. 

Similarly, all pH indicators mentioned in sections 4.1-4.3 can be used for measurement of acidic and basic gases. 

Alkenes frequently have two kinds of C—H bonds, vinyl and allyl, that are generally more acidic than the C—H bonds of saturated alkanes. Quantitative measures of acidity are related to the chemistry of the corresponding carbanions and carbanion salts or organometallic compounds. Several methods have been used for the study of anions in the gas phase1. For many acids it is possible to measure equilibrium constants for equilibria of the type in equation 1. From such equilibrium constants with compounds RH of independently known gas-phase acidity, it has been possible to determine the acidities of a wide range of compounds2. 

A further measure of acidity is provided by rates of deuterium exchange between a labeled base such as DO and a proton acid. The mechanism involves exchange within weak ion-molecule encounter complexes as shown in equation 4. 

Johnson, C. A., and L. Sigg (1985), "Acidity of Rain and Fog Conceptual Definitions and Practical Measurements of Acidity", Chimia 39, 59-61. 

When the acidity or the basicity of the solute molecule is high enough to stretch the OH or OD bond to the point of rupture, then the molecule dissociates into ions in solution. Therefore the dissociation constants also serve as a measure of acidity or basicity of solute molecules, especially those which are subject to significant ionization. Since the coulombic forces causing repulsion of ions at membrane-solution interfaces extend to distances farther than those involved in the polar hydrogen bonding repulsions of nonionized solutes at such interfaces, one would expect that a dissociated molecule to be repelled and, in... 

Another problem is the very high concentrations of reactants present in the low-conversion region. The correct derivation of any rate expression such as Eqs. 2-20 and 2-22 requires the use of activities instead of concentrations. The use of concentrations instead of activities assumes a direct proportionality between concentration and activity. This assumption is usually valid at the dilute and moderate concentrations where kinetic studies on small molecules are typically performed. However, the assumption often fails at high concentrations and those are the reaction conditions for the typical step polymerization that proceeds with neat reactants. A related problem is that neither concentration nor activity may be the appropriate measure of the ability of the reaction system to donate a proton to the carboxyl group. The acidity function ho is often the more appropriate measure of acidity for nonaqueous systems or systems containing high acid concentrations [Ritchie, 1990]. Unfortunately, the appropriate ho values are not available for polymerization systems. 

Measurement of Acid-Base Interactions in Group IIIA Containing... 

MEASUREMENT OF ACID-BASE INTERACTIONS IN GROUP IMA CONTAINING SAMPLES... 

If pKa is used as a measure of acidic or basic strength/or an acid, the smaller the pKa value the stronger the acid for a base the larger the pKa value the. stronger the base. 

The main area of consideration will therefore be that of tautomerism between forms of type (165) and (166). Let us consider first the sort of substituents X which can participate in this sort of situation, and secondly what influence the position of the substituent will have. Substituents of the requisite character will be those for which the hydrogen in XH has a measure of acidic character, particularly therefore OH, SH, NH2 and Me (or amino or alkyl groups in general in which N or C bears at least one hydrogen atom). Where such substituents are in conjugation with the heterocyclic nitrogen, e.g. the 2- and 4-position in pyridine, or the 2-, 4-, 6- and 8-positions of quinoline, both tautomeric forms may exist... 

Most ILMs are less acidic than the commonly used acidic matrices alone. This leads to the possibility to synthesize matrices with only weakly acidic or even neutral or basic pH values [48]. These matrices may be beneficial for the analysis of acid-labile compounds [40]. For example, these matrices were successfully used for the measurement of acid-labile compounds like sulphated oligosaccharides, which are a class of compounds with high biological relevance [49]. Using classical preparations, the detection of these challenging analytes was only possible after derivatization or in the form of noncovalent complexes formed with basic peptides. Upon use of the ILM,... 

To extend this measure of acidity to higher acid concentrations, use is made of the following facts (1) over the range of sulfuric acid concentrations from 9 to 24%, / is measurable for the weaker base o-nitroaniline as well as for p-nitroaniline (2) over this range, the difference between the log / values for the two bases does not change measurably (3) the difference has nearly the same value in a variety of strong acid-water mixtures. 

The pH scale in water is widely used as a measure of acid-base properties in aqueous solutions. It is defined by pH=-log a(H+). In Section 3.1, we dealt with the poH value, defined by poH=-log a(H+), for solutions in amphiprotic and aprotic solvents of high permittivity. Recently, however, the symbol pH has also been used for the value of -log o(H+) in such non-aqueous solutions. Therefore, hereafter, the symbol pH is used instead of paH-... 

The pXa (- -log,oKJ is a measure of acid strength, with the lower the value the stronger the acid. It is the pH at which a weak acid is 50% ionized ... 

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