Spectrophotometric study of acidity constants of Alizarine Red S in various water-organic solvent

The acidity constants of Alizarine Red S were determined spectrophotometrically at 25 degrees C and at constant ionic strength 0.1 M (KNO3) in pure water as well as in aqueous media containing variable mole percentages (5-70%) of organic solvents. The organic solvents used were methanol, ethanol, N,N-dimethyl formamide (DMF), dimethyl sulfoxide (DMSO), acetonitrile and dioxan. The acidity constants of all related equilibria are estimated using the whole spectral fitting of the collected data to an established factor analysis model. DATAN program was applied for determining of acidity constants and pure spectra of different form of Alizarine Red S. The obtained results indicated that acidity constants decrease as the content of an organic solvent in the medium increases. There are linear relationship between acidity constants and the mole fraction of various organic solvents in the solvent mixtures. Effect of various solvents on acidity constants and pure spectrum of each component are also discussed.     

Study of heterogeneous equilibria in saturated solutions of some sparingly soluble 8-hydroxyquinolines

Heterogeneous equilibria in saturated aqueous solutions of 8-hydroxyquinoline (A), 2-methyl-8-hydroxyquinoline (B) and 5-chloro-8-hydroxyquinoline (C) were investigated at constant ionic strength (1M sodium chloride), at 25.0 +/- 0.1 degrees , by the application of the following methods: the formation function method, the method of bound protons, the method of free proton sites and the solubility method. The following equilibrium constants were determined: K(s1)=[H(2)A(+)]/[H(3)O(+)], K(s2)=[HA] and K(s3)=[H(3)O(+)] [A(-)]. Their mean pK values are: -2.88, 2.42 and 12.09 for A, -3.27, 2.71 and 12.73 for B, and -0.34, 3.88 and 12.75 for C. In addition, the acidity constants of investigated 8-hydroxyquinolines were determined by pH-metric titrations. The values obtained for these constants were consistent with those calculated on the basis of corresponding equilibrium constants determined in heterogeneous systems.     

The mandelamide keto-enol system in aqueous solution. Generation of the enol by hydration of phenylcarbamoylcarbene

Flash photolysis of diazophenylacetamide in aqueous solution produced phenylcarbamoylcarbene, whose hydration generated a transient species that was identified as the enol isomer of mandelamide. This assignment is based on product identification and the shape of the rate profile for decay of the enol transient, through ketonization to its carbonyl isomer, as well as by the form of acid-base catalysis of and solvent isotope effects on the decay reaction. Rates of enolization of mandelamide were also determined, by monitoring hydrogen exchange at its benzylic position, and these, in combination with the ketonization rate measurements, gave the keto-enol equilibrium constant pK(E) = 15.88, the acidity constant of the enol ionizing as an oxygen acid, pQ(E)(a)= 8.40, and the acidity constant of the amide ionizing as a carbon acid pQ(K)(a)= 24.29. (These acidity constants are concentration quotients applicable at ionic strength = 0.10 M.) These results show the enol content and carbon acid strength of mandelamide, like those of mandelic acid and methyl mandelate, to be orders of magnitude less than those of simple aldehydes and ketones; this difference can be attributed to resonance stabilization of the keto isomers of mandelic acid and its ester and amide derivatives, through electron delocalization into their carbonyl groups from the oxygen and nitrogen substituents adjacent to these groups. The enol of mandelamide, on the other hand, again like the enols of mandelic acid and methyl mandelate, is a substantially stronger acid than the enols of simple aldehydes and ketones. This difference can be attributed to the electronegative nature of the oxygen and nitrogen substituents geminal to the enol hydroxyl group in the enols of mandelic acid and its derivatives; in support of this, the acidity constants of these enols correlate well with field substituent constants of these geminal groups.     

Accurate determination of low pK values by (1)H NMR titration

The NMR titration methodology to determine acid dissociation constants in aqueous solutions is extended for pK(a) values between 0 and 2, where potentiometric titrations are no longer applicable. (1)H NMR spectra are acquired for single samples of constant acid concentration (e.g. 0.02M), controlled ionic strength (I=1M with HNO(3)/NaNO(3)) and varying pH. To avoid biased pH readings due to the acid error of the glass electrode, true, concentration-based pH values are deduced by combination of the charge balance equation with information from (1)H NMR chemical shifts of the investigated acid. The method has been tested on histidine (pK(1)=1.83+/-0.02) and yielded the dissociation constant of dichloroacetic acid (pK=1.06+/-0.01) for the first time with good accuracy and precision. Dichloroacetic acid is recommended as an NMR spectroscopical "indicator molecule" for in situ monitoring the pH in strong acidic solutions of other equilibrium systems.     

Spectrophotometric determination of the dissociation constants of n.n'-bis(2-hydroxyethyl)dithio-oxamide

The dissociation constants of N, N'-bis(2-hydroxyethyl)dithio-oxamide were determined by a spectrophotometric method. The dissociation constants were calculated by means of a weighted least squares technique. N, N'-Bis(2-hydroxyethyl)dithio-oxamide was found to be a dibasic acid with a thermodynamic dissociation constant pK1-11/04. At ionic strength μ - 0.5, pK1 - 10.71 and pK2 13.92.     

Spectrophotometric determination of the dissociation constants of methyl yellow in mixed protic solvents

The concentration dissociation constants (pK(a)) of methyl yellow, MY (H(+)In) in mixed aqueous solvents of methanol, ethanol, iso-propanol, tert-butanol have been accurately determined from spectrophotometric measurements at 25 degrees C and a constant ionic strength of 0.1 mol l(-1). It has been shown that in these solvents, the pK(a) values decrease with increasing composition of the organic co-solvent. A linear relationship between pK(a) and the mole fraction (x(2)) of the co-solvent was observed in a limited range of the compositions for each of the solvent systems. The results have been discussed in the light of transfer thermodynamic properties of the species existing in the dissociation equilibrium, solvent basicity and solute-solvent interactions. Furthermore, it was also observed that with the change of the solvents, the absorption spectra of MY shifted apparently and the color transition changed accordingly. The solvent effect on the spectra has been attributed to the isomerization equilibria of MY. A simple application of MY was also shown to the sodium acetate-hydrochloric acid titrations in the mixed solvents.     

A comparison of the acidity and the complexing ability of (o-hydroxyphenyl) bis- and (o-hydroxyphenyl) mono-methylenephosphonic acids towards lanthanide(III) ions

The characterization of the acid-base properties of a family of three phenoldimethylenephosphonic acids differently substituted by auxo- or chromophoric groups was studied. These ligands may be considered as pentaacids. Their pK(a) were determined in aqueous solution at 25 degrees C and constant ionic strength 0.1 mol dm(-3) by potentiometry and UV-visible spectrophotometry. Eventually, the complexing power of these molecules with lanthanide cations, Ce(3+), Pr(3+), Nd(3+), Sm(3+) and Eu(3+), was studied keeping in view the possible application to the treatment of radioactive liquid waste. The number, the nature of the species in solution, their stability constants and their individual electronic spectra as restored by computation have been determined. In all cases, complexes of 1:1 stoichiometry accompanied by hydroxy and protonated species are postulated. The comparison of tri- and pentaacids confirms that the acids having the methyl group at the para position with respect to the phenolic OH, have the highest complexing power.     

Determination of the overlapping pK(a) values of chrysin using UV-vis spectroscopy and ab initio methods

The overlapping pK(a) values of 5,7-dihydroxyflavone (chrysin) in EtOH-water solutions were determined by means of a UV-vis spectroscopic method that uses absorbance diagrams, at constant ionic strength (0.050 M) and temperature (25.0+/-0.1 degrees C). It was observed that the pK(a) values increase when the polarity-polarizability and solvation abilities of the reaction medium decrease. In order to calculate the pK(a1) and pK(a2) of chrysin in pure water, various relationships between the determined pK(a) and properties of solvents (relative permittivity, alpha-parameter of Taft and parameter Acity), are proposed. Moreover, with the aim of explaining the first pK(a1) value obtained, the molecular conformations and solute-solvent interactions of the 7(O(-))chrysinate monoanion were also investigated, using ab initio methods. Several ionization reactions and equilibria in water, which possesses a high hydrogen-bond-donor ability, are proposed. These reactions and equilibria constituted the necessary theoretical basis to calculate the first acidity constant of chrysin. The HF/6-31G(d) and HF/6-31+G(d) methods were used for calculations. Tomasi's method was used to analyze the formation of intermolecular hydrogen bonds between the 7(O(-))chrysinate monoanion and water molecules. It was proposed that in alkaline aqueous solutions the monoanion of chrysin is solvated with one water molecule. The agreement between the experimental and theoretical pK(a1) values provides good support for the acid-base reactions proposed in this paper.     

Spectrophotometric determination of acidity constants of 4-(2-pyridylazo)resorcinol in binary methanol-water mixtures

The acidity constants of 4-(2-pyridylazo)resorcinol (PAR, Scheme 1) in binary mixtures of methanol-water at 25 °C and an ionic strength of 0.1 M have been determined spectrophotometrically. DATa ANalysis (DATAN) program applied for determination of acidity constants. As percent of methanol increases in solvent mixtures the pK_a constants also increased. There is linear relationship between acidity constants and the mole fraction of methanol in the solvent mixtures. Effect of solvent composition on acidity constants and pure spectrum of each component are also discussed.     

Spectrophotometric determination of acidity constants of alizarine red S in water, water-Brij-35 and water-SDS micellar media solutions

The acidity constants of Alizarine Red S in water, water-Brij-35 and water-SDS micellar media solutions at 25 degrees C and an ionic strength of 0.1 M have been determined spectrophotometrically. To evaluate the pH-absorbance data, a resolution method based on the combination of soft- and hard-modeling is applied. The acidity constants of all related equilibria are estimated using the whole spectral fitting of the collected data to an established factor analysis model. DATAN program applied for determination of acidity constants. Results show that the pKa values of Alizarine Red S are influenced as the percentages of a neutral and an anionic surfactant such as Brij-35 and SDS, respectively, added to the solution of this reagent. Effect of surfactant on acidity constants and pure spectrum of each component are also discussed.     

Studies on fluorescein-II The solubility and acid dissociation constants of fluorescein in water solution

The solubility of yellow fluorescein and of red fluorescein as a function of pH has been measured in water at ionic strength 0.10. The pH of minimum solubility is the same for both, 3.28. The intrinsic solubility, defined as the solubility of the undissociated species, H(2)Fl, and assumed to be constant and independent of pH, was calculated from the observed solubilities on the low-pH side of the minimum: S(i, yellow) = 3.80 x 10(-4)M: S(i, red) = 1.45 x 10(-4)M. The first dissociation constants were evaluated from the intrinsic solubilities and the observed solubilities on the low-pH side: both fluoresceins yielded the same value, pK(H3Fl) = 2.13. In using the observed solubilities on the high-pH side of the minimum to evaluate the intrinsic solubility and the second dissociation constant it was necessary to modify the existing theoretical approach by taking into account the presence of the fully dissociated anion. Appropriate mathematical treatments were devised to handle the more complex equations. Both fluoresceins yielded the same value for the second dissociation constant, pK(H2Fl) = 4.44. Both fluoresceins give the same yellow colour in saturated solution and the results just reported for the pH of minimum solubility and for the dissociation constants also indicate that for each of the three prototropic forms of fluorescein present in solution, H(3)Fl(+), H(2)Fl, and HFl(-), only one structure exists.     

Determination of pK(a) values of diastereomers of phosphinic pseudopeptides by CZE

A CE method was used for the determination of acidity constants (pK(a)) of a series of ten phosphinic pseudopeptides, which varied in number and type of ionogenic groups. Effective electrophoretic mobilities were measured in the 1.8-12.0 pH range in the BGEs of constant ionic strength of 25 mM. Effective electrophoretic mobilities, corrected to standard temperature of 25 degrees C, were subjected to non-linear regression analysis and the obtained apparent pK(a) values were recalculated to thermodynamic pK(a)'s by extrapolation to zero ionic strength according to the extended Debye-Hückel model. The pK(a) values of the phosphinic acid group fell typically in the 1.5-2.25 interval, C-terminal carboxylic groups in the 2.94-3.50 interval, carboxylic groups of the lateral chain of glutamate and aspartate in the 4.68-4.97 interval, imidazolyl moiety of histidine in the 6.55-8.32 interval, N-terminal amino groups in the 7.65-8.28 interval and epsilon-amino group of the lateral chain of lysine in the 10.46-10.61 interval. Further, separation of diastereomers of the phosphinic pseudopeptides was investigated in achiral BGEs. Evaluation of the resolution of the diastereomers as a function of pH of the BGE revealed that most suitable pH region for separation of the diastereomers is around the pK(a) values of the central phosphinic acid group of the pseudopeptides. Successful separation of some diastereomers was, however, achieved in the neutral and alkaline BGEs as well.     

Kinetic and equilibria studies of the aquation of the trinuclear platinum phase II anticancer agent [(trans-PtCl(NH(3))(2))(2)(mu-trans-Pt(NH(3))(2)(NH(2)(CH(2))(6)NH(2))(2))](4+) (BBR3464)

The hydrolysis profile of the bifunctional trinuclear phase II clinical agent [(trans-PtCl(NH(3))(2))(2)(mu-trans-Pt(NH(3))(2)(NH(2)(CH(2))(6)NH(2))(2))](4+) (BBR3464, 1) has been examined using [(1)H,(15)N] heteronuclear single quantum coherence (HSQC) 2D NMR spectroscopy. Reported are estimates of the rate and equilibrium constants for the first and second aquation steps, together with the acid dissociation constant (pK(a1) approximately equal to pK(a2) approximately equal to pK(a3)). The equilibrium constants for the aquation determined by NMR at 298 and 310 K (I = 0.1 M, pH 5.3) are similar, pK(1) = pK(2) = 3.35 +/- 0.04 and 3.42 +/- 0.04, respectively. At lower ionic strength (I = 0.015 M, pH 5.3) the values at 288, 293, and 298 K are pK(1) = pK(2) = 3.63 +/- 0.05. This indicates that the equilibrium is not strongly ionic strength or temperature dependent. The aquation and anation rate constants for the two-step aquation model at 298 K in 0.1 M NaClO(4) (pH 5.3) are k(1) = (7.1 +/- 0.2) x 10(-5) s(-1), k(-1) = 0.158 +/- 0.013 M(-1) s(-1), k(2) = (7.1 +/- 1.5) x 10(-5) s(-1), and k(-2) = 0.16 +/- 0.05 M(-1) s(-1). The rate constants in both directions increase 2-fold with an increase in temperature of 5 K, and rate constants increase with a decrease in solution ionic strength. A pK(a) value of 5.62 plus minus 0.04 was determined for the diaqua species [(trans-Pt(NH(3))(2)(OH(2)))(2)(mu-trans-Pt(NH(3))(2)(NH(2)(CH(2))(6)-NH(2))(2))](6+) (3). The speciation profile of 1 under physiological conditions is explored and suggests that the dichloro form predominates. The aquation of 1 in 15 mM phosphate was also examined. No slowing of the initial aquation was observed, but reversible reaction between aquated species and phosphate does occur.     

Equilibrium studies of the diorganotin(IV) complexes with some amino acids and related compounds

The interaction of dimethyltin(IV) and diethyltin(IV) cations with water and some amino acids and related compounds was investigated at 25 degrees C and ionic strength 0.1 M NaNO(3) using a potentiometric technique. The results showed the formation of 11 and 12 (organotin:ligand) complexes and the corresponding stability constants were determined. The participation of different ligand functional groups in binding to organotin is discussed. The effect of the pK(a) value of the respective ligand on the stability constant of its complex species was elucidated. The concentration distribution of the complexes in solution was evaluated.     

Multiwavelength spectrophotometric determination of acidity constants of 1-（2-thiazolylazo）-2-naphthol in methanol-water mixtures

The acid-base properties of 1-(2-thiazolylazo)-2-naphthol (TAN) in mixtures of methanol-water at 25℃and an ionic strength of 0.1 mol/L are studied by a multi-wavelength spectrophotometric method.The acidity constants of all related equilibria are estimated using the whole spectral fitting of the collected data to an established factor analysis model DATAN program was used for determination of acidity constants.The corresponding pK_a values in methanol-water mixtures were determined.There is a linear relationship between acidity constants and the mole fraction of methanol in the solvent mixtures.     

Ionization and tautomerization of 2-nitrocyclohexanone in aqueous solution

The keto-enol tautomerism of 2-nitrocyclohexanone (2-NCH) was studied in aqueous solution under different experimental conditions. Ketonization rate constants were measured spectrophotometrically at 25 degrees C at an ionic strength of 0.4 mol dm-3 (NaCl) in diluted hydrochloric acid, in diluted sodium hydroxide, and in several buffers by using NaHSO3 as the scavenger of the keto form. A value of pK(a)(EH) = 4.78 for the enol form was obtained from the rate-pH profile of the reaction. A value of pK(a)(KH) = 5.97 for the keto form was directly obtained from the UV-vis spectra of 2-NCH recorded at different pHs. The equilibrium constant for the keto-enol tautomerism, pK(T) = -log([enol]/[ketone]) = 1.19, was obtained by combining the two pKa values (pK(T) = pK(a)(KH) - pK(a)(EH)). A comparison of these results with the corresponding values (Keefe, J. R.; Kresge, A. J. In The Chemistry of Enols; Rappoport, Z., Ed.; Wiley & Sons: New York, 1990; pp 399-480) for cyclohexanone shows the dramatic effects of an alpha-nitro substituent on the keto-enol acidities and the tautomerization constant of alicyclic ketones. Rates and equilibria were discussed in the light of the Br?nsted equation, the principle of nonperfect synchronization, and the Marcus theory. It turns out that, on passing from nitroalkanes to nitroketones, the resonance contribution to pKa and deprotonation rate decreases, being overwhelmed by steric and inductive effects.     

Phototautomerism in the lowest excited singlet state of 1-hydroxy-2-carboxyanthraquinone

The dependence of the absorption and fluorescence spectra of 1-hydroxy-2-carboxy-anthraquinone on pH and Hammett acidity have been studied. This compound exhibits phototautomerism in its uncharged and its singly-charged anionic species in aqueous media. Its ground state (pK(a)) and lowest excited singlet-state (pK(a)( *)) dissociation constants have been determined by absorptiometric and fluorimetric titrations and the assignment of the pK(a) and pK( *)(a) values to the equilibria concerned has been carefully considered.     

Acid-Base Properties of Some Penicillins

The acidity constants of benzylpenicillin, carbenicillin, and ampicillin were determined by pH-metric titration at 20°C and an ionic strength of 0.1 M (KCl). Diagrams of distribution of ionic and molecular forms of these compounds vs. pH were calculated.     

Stability constants of the ternary complexes of CuDTPA, NiDCTA, CrEDTA, CoHEEDTA, NiHEEDTA and CuHEEEDT Aheedta with OH-

The acid dissociation constants of the metal chelates H(3)CuDTPA, H(2) NiDCTA, HCrEDTA, HCoHEEDTA, HNiHEEDTA and HCuHEEDTA were determined by potentiometric titration. The constants determined at an ionic strength of 0.1 were pK(a,1) = 2.1; pK(a,2) = 2.8 and pK(a,3) = 4.75 for H(3) CuDTPA (296 K), pK(a,1) = 2.16 for HCrEDTA (298 K); pK(a,1) = 1.6 and pK(a,2) = 2.0 for H(2) NiDCTA (298 K); pK(a,1) = 2.24 for HCoHEEDTA, pK(a,1) = 2.47 for HCuHEEDTA and pK(a,1) = 1.73 for HNi-HEEDTA. At high pH the formation of ternary hydroxo-complexes was observed for the chelates CrEDTA(-) (pK(a,1) = 7.35; pK(a,1) = 12.35), CoHEEDTA(-) (pK(a,1) = 11.74), NiHEEDTA(-) (pK(a,2) = 12,44) and CuHEEDTA(-) (pK(a,2) = 10.45).     

Studies on fluorescein-III The acid strengths of fluorescein as shown by potentiometric titration

Potentiometric back-titration of yellow solid fluorescein (H(2)Fl) and of red solid fluorescein in alkali with acid yielded titration curves that were practically identical in shape and position. The end-points at pH 8.5, 5.40 and 3.3 corresponded, respectively, to titration of the excess of standard alkali, and the successive protonations Fl(2-) + H(+) = HFl(-) and HFl(-) + H(+) = H(2)Fl. The pH at the mid-point of the first protonation yielded a value of 6.36 for pK(HFl) (ionic strength 0.10). Because of precipitation of yellow fluorescein during the second protonation step, a value for pK(H(2)Fl) could not be obtained. The total concentration of fluorescein at the first appearance of the precipitate fell on the curve for the solubility of yellow fluorescein as a function of pH. The titrations and the pK values found for the three acid groups of protonated fluorescein (H(3)Fl(+)) have been interpreted on the basis that in water fluorescein exists in only one structural form the yellow zwitterion. Similar back-titrations of alkalinized solutions of yellow or red fluorescein in 50% aqueous ethanol showed that in this medium fluorescein is present in only one form, presumably the quinonoid structure, with much weaker apparent acid functions, pK'(1) = 6.38 and PK'(2) = 7.16 (ionic strength 0.10).