Evaluation of the acidic properties of hydroxycoumarins and choice of solvents for performing potentiometric analysis

The relative acidity constants (pK_a) for 17 hydroxycoumarins in water, methanol, acetone (Ac), dimethylformide (DMFA), and dimethyl sulfoxide (DMSO) have been determined by Henderson's method. The existence of a linear relationship between pK_a in water and pK_a in acetone, dimethylformamide, and dimethyl sulfoxide has been established. From the pK_a values the sequence of neutralization of the hydroxy groups has been determined: their acidic properties decrease in the sequence 4-OH > 7-OH > 6-OH > 8-OH. A quantitative evaluation of the conditions of titration in five solvents on the basis of the titration constants (pK_t) and of the values of the potential jumps and the shape of the potentiometric titration curves has permitted acetone to be proposed as the optimum solvent for the performance of potentiometric analysis.All-Union Scientific-Research Institute of Drug Chemistry and Technology, Khar'kov. Translated from Khimiya Prirodnykh Soedinenii, No. 6, pp. 770–773, November–December, 1985.     

Evaluation of the acidic properties of hydroxycoumarins and choice of solvents for performing potentiometric analysis

The relative acidity constants (pK_a) for 17 hydroxycoumarins in water, methanol, acetone (Ac), dimethylformide (DMFA), and dimethyl sulfoxide (DMSO) have been determined by Henderson's method. The existence of a linear relationship between pK_a in water and pK_a in acetone, dimethylformamide, and dimethyl sulfoxide has been established. From the pK_a values the sequence of neutralization of the hydroxy groups has been determined: their acidic properties decrease in the sequence 4-OH > 7-OH > 6-OH > 8-OH. A quantitative evaluation of the conditions of titration in five solvents on the basis of the titration constants (pK_t) and of the values of the potential jumps and the shape of the potentiometric titration curves has permitted acetone to be proposed as the optimum solvent for the performance of potentiometric analysis.     

Anthill Earth as a Gold Occurrence Indicator,and Gold Determination by Solid Sampling Flame Atomic Absorption Spectrometry

With the aim of avoiding cumbersome sample treatment, we present a device for the introduction of solid soil samples into AAS-flames for gold determination, as well as the proposition of earth from anthills as a gold occurrence indicator. A previous ground sample of anthill earth (0.50 mg) was weighed directly into a small recipient of polyethylene which was then connected to a sampling boronsilicate glass chamber. The sample was carried by an airflow (5 L min⁻¹) to a quartz cell positioned between the burner top and the optical beam. The generated atomic vapor produced a transient signal which was totally integrated in three seconds. The performance was compared with conventional flame atomic absorption spectrometry after proper sample digestion. No significant differences were observed between both procedures (mean deviation ±1.90%), and a LOQ of 0.03 μg Au was achieved using the proposed method. The anthill earth was found to be very suitable for indication of gold occurrence in soils and related materials.     

Basicities and tautomerism of 4,4,6-trimethyl-2-aryl-amino-4H-1, 3-thiazines

The relative basicities of 4,4,6-trimethyl-2-arylamino-4H-1,3-thiazines, which are capable of amine-imine tautomerism, and of model compounds with fixed amino and imino structures were determined by potentiometric titration in methanol. Good correlation was observed between the pK_a values and the Hammett σ constants in the investigated reaction series. The anomalous pK_a values of the model amines and imines are explained by the different character of the electronic interactions in them, which is due to the change in the three-dimensional orientation of the aromatic ring and the amidine fragment.     

Indole derivatives. 131. The basicity of 5- and 6-aminoindoles

The pK_a values for the 5- and 6-aminoindoles were determined from potentiometric titration curves and from ¹³C NMR data on the total change of the chemical shifts of the carbon atom signals on protonation of the amino group. The pK_a values obtained (5.99 and 5.53) were higher than those of aniline (3.92) or -naphthylamine (3.39).For Communication 130, see [1].Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 6, 766–769, June, 1988.     

The Acidity and Solvation of Lignin-Related Phenols in Water—1,4-Dioxane Mixtures

The pK _a values were determined for 10 guaiacyl phenols by UV spectrophotometric and potentiometric titration in the water—dioxane binary system over the composition range 0–80% organic solvent. The differentiating action of the solvent and para-substituent effects on the acidity of guaiacol derivatives were analyzed. The dependences of acidity constants on the mixed solvent composition were interpreted using the preferential solvation model.     

Determination of Acid Dissociation Constants of Neamine by Potentiometric and Electrospray Mass Spectral Techniques

The acid dissociation constants of neamine have been measured using potentiometric titrations. The pK_a1, pK_a2, and pK_a3 values of neamine are 6.35 ± 0.2, 7.73 ± 0.15, and 8.62 ± 0.08, respectively. Neamine is readily characterized using positive-ion electrospray ionization–mass spectrometry (ESI–MS). Various protonated species and their solvated ions are mainly observed. Nevertheless, the abundances of the observed speciation over the pH range 5.0–9.8 do not reflect the variation in the bulk solution. Hence, reaction quotients determined from the ESI–MS investigations vary significantly from the pK_a's determined from potentiometric titrations, thereby illustrating that care must be taken in determining thermodynamic properties using the former technique.     

Determination of dissociation constants of species oxidizable in aqueous solution by air oxygen on an example of quercetin

The comparison of literature data on the dissociation constant of one of the most abundant natural flavonoids, e.g., quercetin, demonstrates their irreproducibility. The reason for this likely corresponds to the easiness of its oxidation by air oxygen during the titration process. To eliminate such problems, a modified version of potentiometric titration was proposed with bubbling a weak flow of an inert gas (nitrogen) through the solution to be titrated in the presence of minimal amounts of a nonionic surfactant. By virtue of the technique proposed, the values of pK _a for quercetin were measured to be 6.62 ± 0.04 and 9.7 ± 0.3. The first one corresponds to the hydroxyl group in the γ-pyrone fragment of the molecule, while another agrees with the typical values of pK _a for phenols.     

Comparison of a single-use pH test strip and a glass electrode for potentiometric titration

The potentiometric titration of a carbonate mixture or an acetate solution is a common experiment in analytical laboratories. Typically, a glass electrode combined with a calomel or Ag/AgCl reference electrode is used to locate the equivalence points in neutralization titrations. The dissociation constants of weak acids and bases can be calculated from the pH at the half-neutralization point. Recently, a new commercial product for measuring pH has been developed. This novel acid–base detection strip is a single-use sensor that requires neither storage in a preservation liquid nor calibration prior to use. This study examined its suitability for the continuous monitoring of pH changes in potentiometric titrations of carbonate mixtures, acetate solutions, or ammonia solutions. There were no significant differences in the concentrations of solutions tested using a glass electrode and a pH test strip. The pK_a, pK_b, and pH values determined using the two systems differed by less than 5%. The results confirmed that the pH strips are suitable for continuously monitoring pH changes during neutralization titrations. However, the strips can only be used once.     

Comparison of properties of an oxime-bound partially quaternized poly-4-vinylpyridine and a monomer analogous oxime

Six copolymers of 4-vinylpyridine and 4-vinyl-N-phenacyloximepyridinium bromide (PPyOx-12, 23, 40, 50, 60 and 72 per cent) were prepared from poly-4-vinylpyridine partially quaternized by phenacylbromide (PPyKt) and hydroxylamine. The potentiometric data of the polymer acid groups (oxime, keto-enol tautomerism) in PPyOx and PPyKt were compared with those of 4-ethyl-N-phenacyloximepyridinium bromide (PyOx) and 4-ethyl-N-phenacylpyridinium bromide (PyKt) respectively. In addition, the second-order constants of the nucleophilic reaction for all the copolymers (PPyOx) and the analogue towards p-nitrophenylacetate (NPA) were determined at various pH values in water at 25°. In contrast to the known potentiometric behaviour of polyacrylic and polymethacrylic acids, the dissociation constants (pK_a) of acid groups in PPyKt and PPyOx were found to be independent of the degree of ionization. Moreover, the values of pK_a of these groups for all PPyKt and PPyOx samples were appreciably less than the pK_a of the corresponding analogues. As a consequence, the oxime of PPyOx-12 in the range of pH between 7·5 and 9·0 reacts about two order of magnitude faster than the monomer with the ester. These results indicate that, in contrast to the known correlation between reactivity (nucleophilicity) and basicity for monomer compounds, the activity of the polymer oxime does not diminish with a decrease of the basicity. On the contrary, the reactivity of polymer anion towards the ester is markedly higher than that of the monomer anion of PyOx. An explanation may be found in the effects of microenvironment on the property of the functional groups attached to the side pyridine residues of PVP.     

Basicities and structures of 4,4,6-trimethyl-2-arylamino-5,6-dihydro-4H-1,3-thiazines and 4,4,6-trimethyl-2-arylamino-5,6-dihydro-4H-1,3-oxazines

The relative basicities of 4,4,6-trimethyl-2-arylamino-5,6-dihydro-4H-1,3-thiazines and 4,4,6-trimethyl-2-arylamino-5, 6-dihydro-4H-1,3-oxazines, which are capable of amine-imine tautomerism, and of model compounds with fixed amine and imine structures were determined by potentiometric titration in methanol. Good correlation of the pK _a values with the Hammett constants in the investigated reaction series was found. The inapplicability of the use of the pK _a values of model compounds for the determination of the tautomeric equilibrium constants in the case of some N-heterocyclic amines of nonaromatic character with an aryl substituent attached to the exocyclic nitrogen atom of the amidine fragment of the molecule is demonstrated.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 12, pp. 1621–1624, December, 1976.     

Multifunctional substances — Determination of pKa-values in overlapping ranges using an iteration procedure

Summary In multifunctional substances, pK_a-values from different functional groups may overlap. Therefore, systematically wrong results are obtained, if pK_a-values are used from titration charts without appropriate corrections. The iteration procedure presented in this article starts with the pK_a-values at the half-way neutralisation points. From these preliminary values, new positions in the charts are calculated, from which better approximations of the pK_a-values are obtained. These steps are repeated several times, until the results converge. Cefodizime (HR 221) with three acid/base functional groups was used as model substance. The disodium salt was titrated using three molar equivalents of 0.1 mol/l hydrochloric acid. Using the iteration procedure, three pK_a-values were obtained from the titration chart: 1. The amino-function: pK₁=2.85; 2. the first carboxyl-function: pK₂=3.37; 3. the second carboxyl-function: pK₃=4.18.     

Esr and polarography of nitroazoles. 4. Polarographic study of nitrobenzimidazoles

The electrochemical reduction of 2-substituted 5(6)-benzimidazoles has been studied with the aid of classical polarography and cyclic voltammetry in acetonitrile. The influence of the substituents in position 2 on the magnitudes of the half-wave potentials of the first stage is exerted by induction and resonance mechanisms with approximately the same contributions, and that on the magnitudes of the half-wave potentials of the second stage predominantly by the resonance mechanism. The possibilities of taking the radical-stabilizing factor into account in the correlations are discussed. In order to study the 2-substituted 5(6)-nitrobenzimidazole series, their pK _a values in acetonitrile have been determined by potentiometric titration.For communication 3, see [1].Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 9, pp. 1246–1251, September, 1984.     

Quantitative estimation of the conditions of titrating hydroxyanthraquinones in nonaqueous solvents

In order to determine the optimum conditions of potentiometric titration, an investigation has been made of the relative acidities of 13-hydroxyanthraquinones in water, methanol, acetone (ac), dimethylformamide (DMFA), and dimethyl sulfoxide (DMSO) on the basis of a calculation of the indices of the relative acidity constants (pK_a) by Henderson's method. The existence of a relationship between pK_a in water and pK_a in acetone, dimethylformamide, and dimethyl sulfoxide has been established which is characterized by the linear equations $$pK_a^{DMSO} = 1.54pK_a^{H_2 O} + 11.88$$ , $$pK_a^{DMFA} = 1.38pK_a^{H_2 O} + 8.50$$ , $$pK_a^{ac} = 1.11pK_a^{H_2 O} + 10.26$$ . The sequence of neutralization of the hydroxyls in the titration of polyhydroxy-anthraquinones has been determined from the pK_a values in DMSO and the results of a calculation of electronic structures by the Pariser-Parr-Pople method. A quantitative evaluation of the conditions of titration in the five solvents on the basis of indices of the titration constants (pK_t) has shown that the optimum conditions for the quantitative determination by potentiometric titration are achieved in dimethyl sulfoxide.     

Nucleophilic substitution in 1-alkyl-4,5-dichloro-3-nitropyridazin-6-ones

Treatment of 1-alkyl-4,5-dichloro-3-nitropyridazin-6-one with C-nucleophiles and with ambident nucleophiles (2-azahetarylacetonitriles) leads to a selective substitution of a chorine atom by the quaternary carbon atom of the carbanion formed from a substituted acetonitrile. The pK_a of the CH-acid 2-(1-alkyl-5-chloro-3-nitro-6-oxo-1,6-dihydro-4-pyridazinyl)malononitrile was determined by potentiometric titration. Reaction of 2-(1-alkyl-5-chloro-3-nitro-6-oxo-1,6-dihydro-4-pyridazinyl)-2-hetarylacetonitriles with primary amines gives 6,7-dihydro-1H-pyrrolo[2,3-d]pyridazin-7-ones. __________ Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 4, pp. 556–564, April, 2006.     

Acid-base strengths in 1,2-dichloroethane

The pK_a value of hydriodic acid in 1,2-dichloroethane was determined from conductivity measurements. A glass electrode was calibrated for dichloroethane in the potentiometric titration of hydriodic acid with tetramethylguanidine. From potentiometric titrations, the pK_a values in dichloroethane of hydrobromic acid, hydrochloric acid, picric acid and some sulfonphthaleins as well as some protonated nitrogen bases were determined. In the curves of the titrations of the carboxylic acids and the hydrogen halides with TMG, evidence was found for the formation of the complex B(HX)₂.     

Research on naphthyridines. 10. Synthesis and ionization constants of 10-alkylaminobenzo[b]-1,8-naphthyridines

10-Alkylaminobenzo[b]-1,8-naphthyridines were synthesized by cyclization of 2-arylaminonicotinic acid alkylamides by means of phosphorus oxychloride. The pK_al values of the 10-alkylaminobenzo[b]-1,8-naphthyridines, which range from 8.74 to 8.50, were determined by potentiometric titration in anhydrous ethanol.See [1] for communication 9.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 6, pp. 799–801, June, 1979.     

Acid-base properties of substituted tetraphenylmethylenediphosphine dioxides

Conclusions Some tetraphenylmethylenediphosphine dioxides, substituted in the methylene bridge, were subjected to potentiometric titration in nitromethane with perchloric acid. The obtained values of the pK _a basicity constants (CH₃NO₂) obey a linear relationship with the ^* constants of the substituents.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 1, pp. 199–201, January, 1972.     

Calculation on ion exchange capacity for an ion exchanger using the potentiometric titration

We calculated the characteristics of a phosphoric cation exchanger and studied an accurately computable method for ion exchange capacity for a type of potentiometric titration curve. The ion exchanger was prepared by phosphorylation of a styrene‐divinylbenzene copolymer. The ion exchange capacity was 5.7 meq/g. The experimental pK values versus χ in a phosphoric cation exchanger can explain a linear equation. The ΔpK values were obtained from the slope of a linear equation. The ΔpK values were the differences of pK values between the apparent equilibrium constant at complete and zeroth neutralization of the ion exchanger. The experimental pK values at χ = 0.5 (χ:degree of neutralization of ion exchanger) showed good agreement with the theoretical data. When it was titrated with NaOH and Ba(OH)₂ solutions, a good agreement between experimental and theoretical pK values for various χ was found in all potentiometric titration curves. The potentiometric titration curve near the inflection point in the case of divalent ions was changed more sharply than that for monovalent ions. The plot of ∂pH/∂g versus g (number of moles of alkali to 1 g of ion exchanger) was fitted to the Lorenzian distribution, from which ion exchange capacity was accurately evaluated. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 3181–3188, 2000