Picric acid acidity constant

In this experiment the method of continuous variations is used to determine the stoichiometry and equilibrium constant for the organic complex of 3-aminopyridine with picric acid in CHCI3, and the inorganic complex of Fe +with salicylic acid. 

The applicability of the equation to predict pK values was confirmed by the good fits obtained for experimental and predicted values. A graphical example is given in Figure 1 for the dissociation constants of picric acid in binary mixtures62. 

FIGURE 1. Dissociation constants of picric acid in binary mixtures62. Reprinted with permission from Reference 62. Copyright (1994) American Chemical Society... 

The LFP studies of the reaction of the A-methyl-A-4-biphenylylnitrenium ion with a series of arenes showed that no detectable intermediate formed in these reactions. The rate constants of these reactions correlated neither with the oxidation potentials of the traps (as would be expected were the initial step electron transfer) nor with the basicity of these traps (a proxy for their susceptibility toward direct formation of the sigma complex). Instead, a good correlation of these rate constants was found with the ability of the traps to form n complexes with picric acid (Fig. 13.68). On this basis, it was concluded the initial step in these reactions was the rapid formation of a ti complex (140) between the nitrenium ion (138) and the arene (139). This was followed by a-complex formation and tautomerization to give adducts, or a relatively slow homolytic dissociation to give (ultimately) the parent amine. 

This last induction is confirmed by observations on nitro-mannite, a crystalline solid which appears by reason of this circumstance better suited than liquid methyl nitrate for transmitting detonation. It has in fact given practically constant velocities of 7700 meters per second in lead tubes of 1.9 mm. internal diameter at a density of loading of 1.9. Likewise picric acid, also crystalline, 6500 meters per second.. . . ... 

The benzimidazoles form crystalline salts also when warmed with an equal weight of picric acid in aqueous ethanol.16 The melting points of the aldo-benzimidazole pierates (see Table I) thus furnish still another set of constants for characterization and identification purposes. 

Fig. 5. Typical dependence of the formation constants of arene complexes with different acceptors on the oxidation potential of the arene. Data from refs. [23, 28]. I2 = iodine, TCNE — tetracyamethylene PA — picric acid.

Heat of combustion and heat of formation of picric acid and of other nitrophenols. The heat of combustion of picric acid at constant volume (corrected for the nitric acid formed) is 621.2 kcal/mole and the heat of formation is 63.3 kcal/mole or 276.4 kcal/kg (Gamer and Abemethy [51]). 

Koelsch [67], from observation of 1200 workers engaged in handling picric acid, described the symptoms of poisoning as follows a yellow colouration of unprotected parts of the skin and hair, irritation of the mucous membranes and of the upper parts of the respiratory tract and the digestive tract. In addition, a constant bitter taste and a lack of appetite appear. Cases of acute or chronic poisonings were not observed. 

Conductometric and spectrophotometric behavior of several electrolytes in binary mixtures of sulfolane with water, methanol, ethanol, and tert-butanol was studied. In water-sulfolane, ionic Walden products are discussed in terms of solvent structural effects and ion-solvent interactions. In these mixtures alkali chlorides and hydrochloric acid show ionic association despite the high value of dielectric constants. Association of LiCl, very high in sulfolane, decreases when methanol is added although the dielectric constant decreases. Picric acid in ethanol-sulfolane and tert-butanol-sulfolane behaves similarly. These findings were interpreted by assuming that ionic association is mainly affected by solute-solvent interactions rather than by electrostatics. Hydrochloric and picric acids in sulfolane form complex species HCl and Pi(HPi). ... 

Also, in this case, conductivity data were analyzed by Fuoss-On-sager-Skinner equations and limiting equivalent conductances A<> and association constants KA are collected in Table II together with physical properties of solvent mixtures. Furthermore, Table III shows ethanol and tert-butanol concentration in the mixture [ROH], the relevant dielectric constant c, and the pK of picric acid. 

Thus picric acid reacts with n alcohol molecules forming a complex (Equation 13) which rearranges to form an ion pair (Equation 14) which then dissociates (Equation 15). The processes involved in Equations 13 and 14 are conditioned by specific solute-solvent interactions, while the process depicted by Equation 15 is controlled by electrostatics. K3 is equal to 1/KA, where KA is the theoretical association constant given by Equation 2. It can be easily shown that the experimental association constant is given by the equation ... 

A number of other diazocompounds have been studied. Warren (1961) has shown that the reaction of 9-diazofluorene in ethanol and in ethanol-water mixtmes is similar to that of diphenyldiazomethane. The rate of reaction is slower in deuteriated solvent and the operation of general acid catalysis was inferred from measurements with picric acid. As would be expected from the conjugation between the diazo and dibenzo-cyclopentadienyl groups in 13, the rate of reaction is slower than that of DDM (by a factor of about twelve) and there seems little doubt that both substrates react with a rate-determining proton transfer. Bate studies with substituted diazofluorenes yield a value of — p = l 6-l-9 depending on the substituent constants used (Warren, 1963 Warren and Yandle, 1965). This value is similar to that for the reaction of substituted diphenyldiazomethanes in toluene and is a further indication that the stability of diazoalkanes is quite sensitive to substituent effects. 

The simpler equation (4) embodies the Ostwald conception with the difference that Km is not the true dissociation constant but the apparerd constant of the indicator since it represents the product of the true dissociation constant and the equilibrium constant for the normal and aci-forms. The latter equilibrium favors the normal compound in the case of p-nitrophenol so that this substance appears to be a very weak acid. With o-nitro-phenol, however, the existence of the aci-form is favored so that this compound behaves as a stronger acid. The ratio of aci to normal is so large in the case of picric acid that relatively much of the aci- or ionogen form, as compared with the pseudo-compound, is present in aqueous solution. Consequently this substance is a rather strong acid. As the apparent dissociation constant increases, the intensity of the yellow color of aqueous solutions must likewise grow because more of the aci-form will be found in solution. This statement can be confirmed easily. Picric acid in water solutions is yellow, but colorless in organic solvents due to the predominance of the pseudo-form. 

Table VI. The Computation of the Thermodynamic Dissociation Constant of Picric Acid at 25° in Methyl Alcohol...

The conductance method is satisfactory only if the solvent can be rigorously purified. Through failure to appreciate this, the first values of pKa of picric acid in acetonitrile proved to be much too small, 5.6 and 8.9 as compared with 11.0 from electromotive force measurements on buffered solutions. D Aprano and Fuoss found that acetonitrile having a satisfactory specific conductance of about 10 cm still contained a trace of ammonia. This was converted to ammonium pic-rate when acid was added to the solvent giving a spurious contribution to the conductance of picric acid solutions. This discovery moved them to make the flat assertion that dissociation constants of weak acids cannot be determined in aprotic solvents conductimetrically . This may be an overly pessimistic view, conductance values of pKa for acids in di-methylsulphoxide and dimethylformamide agree well with those from spectrophotometric and electromotive force measurements. Approximate values of pKa and pKf can be obtained from conductometric titrations of a weak acid with a weak base. ... 

Other contributions to medium effects are generally small. Approximately one unit in ApATarises from the decrease in dielectric constant from 78 for water to 47 for dimethylsulphoxide. Dispersion interactions are more important in dimethylsulphoxide because it is more polarisable than water. We expect to see evidence of this effect only for acids, such as picric acid or certain hydrocarbons, that form coloured anions. Picric acid, unlike many other molecular acids, is indeed stronger in dimethylsulphoxide than in water. 

I.M. Kolthoff and M.K. Chantooni, Jr., Calibration of the glass electrode in acetonitrile. Shape ofpotentiometric titration curves. Dissociation constant of picric acid, J. Am. Chem. Soc. 87 (1965), pp. 4428-4436. 

A comparison of the rates of decomposition shows that the decomposition of BPO (Section 20.1.2.1) is influenced much more strongly by solvent than is the decomposition of AIBN (Table 20-2). With BPO, several irregularities are also observed. For example, is no longer constant when the range of BPO concentrations is large. The decomposition can also be accelerated by free radicals, such as triphenylmethyl, and can be retarded by inhibitors, such as quinone or picric acid. 

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