Indicators Hammett acidity

Indicators, Hammett acidity. With a set of 17 basic indicators any acidity lying in the range from a dilute aqueous mineral acid to that of pure sulfuric acid may be determined in any solvent. The range is from -t-2.8 (4-aminoazobenzene) to —9.3 (2,4,6-trinitroaniline). Aldrich supplies a set of 500 mg. of each analytically pure indicator. L. P. Hammett and A. J. Deyrup, Am. Soc., 54,2721 (1932) L. P. Hammett and M. A. Paul, im,56,827 (1934)... 

The spectral changes which occur in increasingly acid solutions of polyaza-heterocycles may indicate a second ionization. This event, however, can readily be distinguished from dehydration by measuring the spectra in anhydrous dichloroacetic acid, provided that the pKa value for the anhydrous species is above 1. Anhydrous dichloroacetic acid has a Hammett acidity function (Hq) of — 0.9 (as determined using o-nitroaniline as the solute), and the ultraviolet spectrum of a base with a p > 1 would be that of the anhydrous cation in this 2 A. Albert and W. L. F. Armarego, J. Chem. Soc. 4237 (1963). 

Using these indicators, Hammett was able to define a scale of acidity over the whole range from dilute aqueous solution to 100 per centl SC. 1... 

J(PQ) and J(PN)). Trends in J(P170) have been used to study the protonation of phosphoryl tribromide, phosphoryl trifluoride and dif1uorophosphoric acid. Nitric acid had a greater protonation power than Hammett acidity functions indicated.77 This coupling has been the subject of a semiempirical sum over states theoretical study.7 ... 

The acidity function is determined by successive use of a range of indicators. Hammett and Deyrup started with p-nitroaniline, the pKAl of which in a dilute aqueous solution is 1.11 (the solvolysis constant has been identified with the acidity constant). Since in a dilute aqueous solution, Yb = Ybh+ 1, the acidity function for the aqueous media goes over to the PH scale. By means of p-nitroaniline, the acidity constant of another somewhat more acidic indicator is obtained under conditions such that both forms of each indicator are present at measurable concentrations. Then H0 as well as the p A2 of the other indicator is determined by using Eq. (1.4.40). By means of this indicator, values of H0 not accessible with p-nitroaniline may be reached. The H0 scale is further extended by using a third indicator and its p A, is determined in the same way (see Fig. 1.11). The concentration ratios are determined photometrically in the visual or ultraviolet region. Figure 1.12 shows the dependence of H0 on the composition of the H2S04-H20 mixture and was obtained as indicated above. 

Hammett acidity function chem An expression for the acidity of a medium, defined as ho = Kbh+ BH / B, where Kbh+ is the dissociation constant of the acid form of the Indicator, and BH+ and B are the concentrations of the protonated base and the unprotonated base respectively. ham at a sid ad e. faijk shan ) hand sugar refractometer analy chem Portable device to read refractive Indices of sugar solutions. Also known as protelnometer. hand shCig ar. re.frak tam-3d-3r1... 

The Hammett acidity function only applies to acidic solvents having a high dielectric constant, further requiring that the fi/fm+ ratio be independent of the nature of the indicator. Thus, the Hammett acidity function is applicable for uncharged indicator bases that are anhine derivatives. As mentioned above, other acidity functions have been suggested. No single formulation has been developed that satisfies different solvent systems or types of bases. 

Titration method (consisting in the stndy of the interaction of indicator dyes with solids from solntions) is a techniqne for both qnalitative and qnantitative characterization of solid snrfaces. If a basic indicator B is nsed, the proton acidity of the snrface is expressed by the Hammett acidity fnnction. Similarly, the basicity can be defined when an acid is converted by its conjngated base. This allows defining acidity and basicity in the same scale. 

Hammett acidity function indicator error transition range... 

The presence of Lewis acidity [type (2)] or Bransted acidity [type (1)] is revealed by the IR spectrum of pyridine sorbed on AI1.5PW12O40 (138, 139). IR spectra of sorbed of NH3 show that Cui.5PW]204o has Lewis acidity as well as Bransted acidity (140). Ghosh and Moffat (141) measured the acidities of several salts of H3PW12O40 with Hammett indicators. The acid strength increases with an increase in the calculated charge on the peripheral oxygen atom of the polyanion Zr > Al > Zn > Mg > Ca > Na (141). 

H0 is known as the Hammett acidity function, and the series of substituted anilines used to establish the scale are called Hammett indicators. 

Paul and Long35 have tabulated pATBH+ values for indicators, which were used to establish Hammett acidity functions for aqueous acids between the years 1932 and 1957. The data were summarized as a set of best values of pAiBH+ f°r the bases. Since then, subsequent work seems to suggest that some of these values are incorrect. This is particularly the case for some of the weaker bases whose quoted pAfBH+ were based on a stepwise extrapolation of results of some indicators that have since been proven to be unsatisfactory based on the strict definition of H0- These data, as well as those for weaker bases that have been studied since, covering the whole acidity range from dilute acid to the superacid media are collected in Table 1.1. 

Lewis and Bigeleisen,71 who first determined the Hammett acidity function values for oleums, used a method to derive them from the vapor pressure measurements. Brand et al.,72 however, subsequently by the use of nitro compound indicators showed that the H0 values are not directly related to the vapor pressure. The most accurate Ho values for oleums so far have been published by Gillespie et al.7 (Table 2.4). 

The acid strength of a solid is defined as the ability of the surface to convert an adsorbed neutral base into its conjugate acid (2). If the reaction proceeds by means of proton transfer from the surface to the adsorbate, the acid strength is expressed by the Hammett acidity function Ho, Ho = pKa + log[B]/[BH + ], where [B] and [BH + ] are, respectively, the concentrations of the neutral base (basic indicator) and its conjugate acid, and pK.d is p BH +. If the reaction takes place by... 

Values of the Hammett acidity function for these systems were first given by Lewis and Bigeleisen (1943) who proposed that for fuming sulphuric acid the acidity function could be determined from the vapour pressure of S03 above the solution. However, as pointed out by Paul and Long (1957), there is no particular reason to expect the acidity to be related to the S08 pressure. The acidity function measurements of Brand (1950) and Brand et al. (1952) confirmed, by the use of aromatic nitrocompound indicators, that the acidity does indeed increase with increasing concentration of S03 but Brand s H0 values do not appear to be related to the vapour pressure of S03. 

The concepts of hydrogen ion concentration and pH discussed previously are meaningful only for dilute aqueous solutions of acids. A widely used means of gauging acidity in other media and at high concentrations is the Hammett acidity function H0, which is defined in terms of the behavior of one or more indicator bases B, for which there is the protonation equilibrium... 

Compounds which are vanishingly weakly basic in H20, such as nitro-anilines, nitro-aromatic or halo-nitro-aromatic compounds are protonated to a greater or less extent in superacids, allowing them to be used as indicators in Hammett Acidity Function measurements. Cl- which has essentially zero base strength in H20 is protonated by the superacids to HC1, which being monomolecular is expelled as a gas from the highly associated solvents. This provides a commonly-used important route to synthesis of anhydrous fluorides, fluorosulfates and triflates. 

Hammett Acidity Function measurements for AsF5-HF solutions [10], as shown in Fig. 2, indicate that AsF5 is a moderately strong Lewis acid of the HF system, being well over one H0 unit weaker than SbF5 in 0.5 mol% solution, but considerably stronger than TaF5. 

In dilute aqueous solution, the acidity is measured using pH values. For concentrated acid solutions and non-aqueous acid solutions, pH values are no longer available. Hence, the Hammett acidity function Ho is used as a measure of the acidity of such media [130]. The proton donor ability of an acid in such media is measured by studying the equilibria of a series of indicator bases B (mostly nitroanilines), the UV/Vis absorption spectra of which are markedly different from those of their conjugate acids, so that the indicator concentration ratio I = [B]/[BH+] can be measured spectrophotometri-cally. The acidity function Ho is then given by Hq = p.Kbh+ + Ig f with the subscript zero indicating that the Hq function applies only to neutral bases B [130, 170], For dilute solutions, Ho corresponds exactly to pH in concentrated solutions, the two functions differ appreciably. 

Catalysis by protonic acids accounts for the hydrolysis of fluorides, such as trityl fluoride [31], or elimination of hydrogen fluoride from various systems (Chapter 6), frequently being autocatalytic. The hydrolysis of benzyl fluoride is roughly proportional to the Hammett acidity function [32], which is consistent with the scheme indicated in Figure 5.14 [1, 33]. Indeed, the decomposition of benzyl fluoride, on storage, may be violent [34]. 

---