Equilibrium constants in reactions of 2-aminoethanol and ammonia with isophthalaldehyde and terephthalaldehyde.

Equilibria established in reactions between isophthalaldehyde (IPA) and terephthalaldehyde (TPA) on one side and 2-aminoethanol and NH3 on the other side were followed in solutions, where these amines acted both as reagents and as buffers. The equilibrium between TPA and 2-aminoethanol was followed spectrophotometrically; all others, of reactions of TPA with NH3 and of IPA with both NH3 and H2N-CH2CH2-OH, were followed by using polarography. Separate limiting reduction currents of the dialdehyde and its imine enable simultaneous determination of the starting material and the product. The equilibria are shifted more in favor of the imine in reactions of TPA than those of IPA. Equilibrium constants for reactions of both TPA and IPA with 2-aminoethanol are 3 orders of magnitude higher than those observed for the reactions of dialdehydes with NH3, even when the pKa values of these two reagents differ only slightly.

Comparison of solution chemistries of orthophthalaldehyde and 2,3-naphthalenedicarboxaldehyde.

Polarography was used to obtain the concentrations of the dialdehydic (10%), monohydrated acyclic (5%), and cyclic hemiacetal form (85%) of orthophthalaldehyde (OPA). For 2,3-naphthalenedicarboxaldehyde (NDA) these values were estimated to be 15, 7, and 78%. Addition of water in unbuffered solutions followed first-order kinetics with rate constants 0.0018 s-1 for OPA and 0.0012 s-1 for NDA. Dehydration to form both the dialdehyde and the monohydrate is both acid- and base-catalyzed. Both dialdehydes yield on reaction with OH- ions geminal diol anion, which is electro-oxidized to a carboxylic acid. In the most frequently used pH range for the determination of amino acids, NDA can undergo reaction with OH- ions, but OPA does not. In aqueous solutions, NDA is less strongly hydrated than OPA.

Polarographic and voltammetric behavior of selenious acid and its use in analysis.

Solution chemistry of Se(IV), in particular the acid-base properties, salt and complex formation, chemical reduction and reaction of Se(IV) with organic and inorganic sulfur compounds are briefly summarized. The electrochemical reduction of Se(IV) on dropping and stationary mercury electrodes is dealt with in some detail. The effects of antecedent acid-base equilibria and of consecutive reactions of the reduction product, Se(2-), adsorption of their products, and effects of added metal ions are discussed. The principles and applications of stripping analyses for determination of ultratraces of Se(IV) are summarized. The behavior on unreactive (Au, Pt, carbon) and reactive (Hg, Ag, Cu) electrodes are compared.

Acid hydrolysis of 1,6-dihydro-4-amino-3-methyl-6-phenyl-1,2, 4-triazin-5(4H)-one (1,6-dihydrometamitron).

Metamitron (1) does not undergo hydrolysis at pH 1-8 and up to 5 M H(2)SO(4). The product of its two-electron reduction, 1, 6-dihydrometamitron (2), on the other hand, undergoes at pH <3 relatively fast hydrolysis. The dependence of the measured rate constant on acidity indicates that the completely protonated form (AH(2)(2+)) predominating in strongly acidic media undergoes hydrolysis slower than the species bearing one less proton (AH(+)). The latter most reactive species is present in highest concentration in solutions of pH between 0 and 2. This species is protonated on the 2,3-azomethine bond and yields as final products 2-hydrazino-2-phenylacetic acid (4) and acethydrazide (5). Kinetic, polarographic, and spectrophotometric measurements indicated for the first dissociation an average value pK(a) = -0.8, for the second pK(a) = 0.95. These observations together with the easy reduction of the 1,6-bond in metamitron (1) indicate that in nature the cleavage of metamitron may be preceded by its reduction to 1, 6-dihydrometamitron (2), which is then hydrolyzed. Thus, anaerobic, reductive conditions are likely preferable for the total microbial degradation of metamitron.

Interaction of iron(II) with bile salts.

Iron(II) ions react with small aggregates of cholate, glycocholate, chenodeoxycholate, and deoxycholate to form soluble and colloidal compounds. Taurocholate under conditions used does not react with the Fe2+ ion. Small aggregates of dihydroxy bile salts (predominating in the premicellar region, at concentrations of the bile salt above 1 mmol dm-3) have a larger affinity for Fe2+ compared to those formed from cholate anions. In their interactions with small aggregates of cholate anions, the Fe2+ ion shows an affinity comparable to that of Cu2+ and Cd2+ and somewhat larger than that of Zn2+. Small aggregates of cholate show a higher ability to mask Fe2+ than those of taurocholate and glycocholate. Interaction of glycocholic acid anions with Fe2+ ions is sufficient to prevent iron(II) precipitation.

Interaction between Cu2+ ions and cholic acid derivatives followed by polarography.

Interaction of bile salts with Cu2+ ions in unbuffered systems containing 0.15 M NaNO3 was followed by measuring polarographic limiting currents and half-wave potentials. Whereas taurocholate forms neither soluble complexes nor compounds of limited solubility, cholate, glycocholate, and dehydrocholate from both soluble complexes and slightly soluble salts of copper(II) with small aggregates of bile salts. The stability of soluble complexes is comparable for cholates, dehydrocholates, acetates, and acetylglycinates, but smaller for glycocholates. The solubility of the copper(II) salts with small aggregates decreases in the sequence: glycocholate > cholate >> dehydrocholate. It is proposed that these salts are formed by interaction of a copper(II) ion with two carboxylic groups located on the small aggregate in a sufficiently small distance. In the presence of excess cholate the precipitated copper(II) salts are dissolved. It is assumed that at high bile salt concentrations, where precipitates are not observed, larger aggregates are formed that have free carboxylate groups, which increase their solubility in aqueous solutions. For glycocholate, within the accessible concentration range and within the time-frame used (24 h for the establishment of the equilibrium), the formation of such larger aggregates was not observed, even when its "cmc" is comparable with that of cholate. The absence of formation of larger aggregates for dehydrocholate parallels its tendency not to form "micelles".

Chemical properties of bile acids. V. Interaction with concentrated sulfuric acid.

The reaction between concentrated sulfuric acid and steroid hydroxy compounds, previously studied for a series of sterols, was used to develop an analytical method for bile acids. The reaction generates species absorbing in the wavelength ranges 300-320 and 370-390 nm, corresponding to isomeric allylic and dienylic carbocations. Eleven bile acids were examined and influence of time, temperature and sulfuric acid concentration on the chromogenic reaction was studied. The linear dependence of absorbance on concentration of individual bile acids demonstrates that the method can be used to measure spectrophotometrically micromolar concentrations of bile acids.

Electroreduction of nifedipine.

At pH greater than 6, the nitro group of nifedipine is reduced in a four-electron step to a hydroxylamino group. At pH less than 6, the hydroxylamino derivative undergoes an acid-catalyzed dehydration, and the rate of this reaction governs the limiting current. Resulting quinonemethide is further reduced, and a six-electron reduction step results. Formation and reducibility of the quinonemethide is favored by conjugation with the 1,4-dihydropyridine ring. Further reduction of the hydroxylamino derivative is confirmed by the absence of the anodic oxidation peak on the reverse scan at pH less than 6.5. At potentials more negative than that of the nitro group, the protonated form of the hydroxylamino derivative is reduced between pH 4 and 8 in direct current (dc) polarography and between pH 6 and 11 in cyclic voltammetry, the difference resulting from the difference in the measurement time window. The yields and fate of the hydroxylamino derivative obtained in controlled potential electrolysis with a hanging mercury drop electrode (HMDE) differ from those in dc polarography, indicating the presence of different competing reactions. For analyses, pH 2 and 11 are most suitable.

Diffusion of naproxen in presence of beta-cyclodextrin across a silicone rubber membrane.

The diffusion of Naproxen through a silicone rubber membrane has been studied for four different release systems: solution and suspension both of Naproxen and Naproxen beta-cyclodextrin coprecipitates. Differences in transport between the two forms indicate the existence of an interaction between the acid and beta-cyclodextrin in solution. Independence of the transport on pH in the case of the coprecipitate indicates rapid liberation of Naproxen from the complex prior to entering the membrane. Increase in the cumulative amount diffused for the suspension of Naproxen with increasing pH was interpreted as due to an increase in its solubility with increasing pH.

Effect of pretreating samples with boric acid before liquid-chromatographic determination of urinary catecholamines.

Isolation of catecholamines on prepacked gravity-fed ion-exchange columns in conjunction with liquid-chromatographic separation is occasionally subject to disturbances of unknown origin, especially after injection of H3BO3 eluates from the ion-exchange columns onto the chromatography column. Peak spreading, sometimes even peak splitting, has been observed in many laboratories--effects easily confused with those sometimes caused by voided columns or by impurities. To elucidate the origins of these interferences, we performed a combined voltammetric and chromatographic investigation. We show that complexation of the catechol group by borate ions and dissociation of those pH-labile complexes in the course of the chromatographic separation is the cause of those peak distortions. Addition of a small volume of diluted HCI to the borate eluates eliminates such interference and leads to reproducible, easily interpretable chromatographic separations.

Development and publication of new polarographic and related methods of analysis.

A review is made of the factors that should be investigated in the development of new polarographic methods of analysis. Recommendations are made concerning the preparation for publication of the results of such investigations.

Effect of pH on concentration of species present in aqueous solutions of carbonyl compounds affected by hydration-dehydration and keto-enol equilibria.

Equations have been derived for hydration-dehydration and keto-enol equilibria of carbonyl compounds, showing the changes, with pH, of individual species present in aqueous solutions. An Egtran computer program has been constructed, so that the pH-variation of individual species for widely varying values of equilibrium constants can be evaluated. The treatment enables isolation of individual equilibrium constants from experimental data and identification of the system involved. The ratios of hydrated and non-hydrated, as well as of keto and enol forms, are pH-independent.