Multiwavelength Spectrophotometric Determination of Dissociation Constants of Mercaptopyrimidines

Abstract

The dissociation constants of some 2- and 6-mercaptopyrimidine derivatives with amino or hydroxy substituents are reported. The constants have been determined at 25° C in 0.1 M KCl aqueous solution. The multiwavelength spectrophotometric data obtained have been 3mlyzed by means of the SQUAD program (Stability Quotients from Absorbance Data).     

Acid Dissociation Constants and Rare Earth Stability Constants for DTPA

Diethylenetriamine-N,N,N′,N″,N″-pentaacetic acid (DTPA) is an octadentate aminopolycarboxylate complexing agent whose f-element complexes find important practical applications in nuclear medicine and in advanced nuclear fuel reprocessing. This investigation focuses primarily on the latter application, specifically on characterization of lanthanide–DTPA complexes of relevance to the Trivalent Actinide–Lanthanide Separations by Phosphorus reagent Extraction and Aqueous Komplexants (TALSPEAK) process. To function acceptably, the TALSPEAK process requires the presence of moderate concentrations (0.5–2.0 mol·L^?1) of a (Na⁺/H⁺) lactate (or citrate) buffer. Competition between DTPA, lactate, and the extractant bis(2-ethylhexyl)phosphoric acid (HDEHP) for the lanthanides and trivalent actinides governs the course of the extraction process. To facilitate modeling and to support process improvements, the acid dissociation constants and stability constants for rare earth complexes with DTPA have been determined in 2.0 mol·L^?1 ionic strength (NaClO₄) media. The acid dissociation constants for DTPA and the stability constant for [Eu(DTPA)]^2? also were determined in sodium trifluoromethanesulfonate at 2.0 mol·L^?1 ionic strength to evaluate the potential impact of changing the nature of the electrolyte. The thermodynamic data are compared with earlier reports of similar data at lower ionic strength and used to complete calculations exploring the relative stability of lanthanide–DTPA and lactate complexes under TALSPEAK extraction conditions. Lanthanide–DTPA stability trends are discussed in comparison with literature data on a variety of other metal ions.     

聚天冬氨酸酸解离常数的测定

采用pH电位滴定法，利用先进的BEST程序，针对聚羧酸在水溶液中解离及平衡常数测定的复杂性，构建了聚天冬氨酸（PASP）在水溶液中解离的H4L和H2L两种模型，分别测得了相应的各级酸解离常数，并得到了PASP在不同pH条件下的物种分布曲线。在35℃，I=0．1时，对于H4L模型，各级酸解离常数的pK,值为：2．27、3．66、4．22、5．40；对于H2L模型，相应的pK,值为2．92、4．84。研究结果表明，两种模型计算结果均能与实验结果很好吻合，但H4L模型误差更小，因此是一种更为合理的模型。     

Spectrophotometric Determination of Thioctic Acid in its Dosage Forms through Complex Formation with Pd(II)

A simple and sensitive spectrophotometric method has been developed for the determination of thioctic acid in pharmaceutical preparations. The proposed method is based upon the formation of a complex with palladium(II) in acetate buffer of pH 4.8 with an absorption maximum at 318 nm. The absorbance obeyed Beer's law over the range of 2–20 μg mL^?1 with a minimum detection limit of 0.15 μg mL^?1 and molar absorptivity (ζ) of 7 × 10³ L mol^?1.cm^?1. The different experimental parameters affecting the development and stability of the color were carefully studied and optimized. The proposed method was successfully applied to the analysis of commercial tablets and ampoules. The results obtained were in good agreement with those obtained using a reference spectrophotometric method. A proposal of the reaction pathway is presented.     

Multiwavelength Spectrophotometric Determination of Acid Dissociation Constants. Deconvolution of Binary Mixtures of Ionizable Compounds

ABSTRACT

A spectral deconvolution method based on target factor analysis has been developed to determine pK_a values of binary mixtures of monoprotic and/or diprotic ionizable compounds. The technique makes use of the approach of Frans and Harris, which has been implemented previously for mixtures of monoprotic compounds (Anal. Chem. 1985, 57, 1718-1721), to extract the unknown pK_a values. The method has been illustrated by using the multiwavelength spectrophotometrictitation data of binary mixtures of: I. benzoic acid and phenol, 2. benzoic acid and nicotinic acid, 3. p-aminosalicylic acid and phthalic acid. It was demonstrated that the pK_a values as determined in this study are in good agreement with the literature.     

稀释溶液吸光度比值法测定不稳定络合物的稳定常数

本文利用溶液的稀释度与其吸光度的关系,导出一个适用于不稳定络合物稳定常数的测定方法。当络合物的摩尔吸光系数(ε)不能直接测定时,本法可直接求算它的稳定常数及摩尔吸光系数。本文还讨论了方法误差与络合物离解度以及溶液稀释度的关系。     

线性扫描伏安法测定铅-L-谷氨酸络合物稳定常数

本文介绍了用线性扫描伏安法以玻碳电极为工作电极测定铅-L-谷氨酸络合物的稳定常数,研究了支持电解质及酸度的影响,在最佳实验条件下,获得了络合物的配位数和累积稳定常数。方法简便,快速准确,有实际意义。     

分光光度法测定I3-的稳定常数

用分光光度法测定了反应I2+I-=I3-的平衡常数.在350nm处,I3-的摩尔吸光系数为2.3×104L*mol-1*cm-1,而I2和I-在这一波长没有吸收,据此提出了用分光光度法测定I3-稳定常数的方法.研究了适当的实验条件,测定结果为lgK=2.96.     

硫酸软骨素A的解离常数及其金属配合物稳定常数的测定

硫酸软骨素A(ChSA)是葡萄糖醛酸和半乳糖胺通过β-1,3糖苷键所形成的二糖为基本单位的大分子酸性粘多糖。它广泛存在于人及脊椎动物血管及其它脏器结缔组织之中,有关它的生理功能及其在医药等领域中的用途,文献亦有报导。我们曾以镇江肉联厂生产的硫酸软骨素粗品为原料,精制得到纯度较高的硫酸软骨素A制品,并经阳离子交换树脂转化为氢型,测定了它与Cu²⁺、Ca²⁺离子的结合度。     

Spectrophotometric Study of 1,2,7-Trihydroxyanthraquinone-Mg(II) Complex. Determination of Mg(II)

Abstract

The spectrophotometric study of violet complex Anthrapurpurin-Mg(II) in a basic medium and a hydroalcoholic solution was made (δ_max - 530 nm., ? = 3.5 × 10³ 1 mol^?1 cm^?1, stoichiometry 1:1, apparent constant of stability log K = 9. 26). A new method for the spectrophotometric determination of Mg(II) is proposed for concentrations between one and six ppm. Relative errors between replicate samples were 0.90 %.     

Chromatographic Determination of Dissociation Constants of Pramipexole and its Impurities

The main goal in this investigation was monocratic HPLC determination of dissociation constant values (pK _a) of pramipexole and its impurities, BI-II 546 CL, BI-II 751 xx and 2-aminobenzothiazole. The chromatographic method is advantageous as a small amount of substance is needed and the degree of substance purity is less important. Analysis was carried out using stationary phases stable in a wide pH range. Triethylamonium phosphoric buffer was selected as appropriate pH controlling solution because it can cover a wide pH range. Detection was done on two different wavelengths, 262 nm for pramipexole, BI-II 751 xx and 2-aminobenzothiazole and at 326 nm for BI-II 546 CL. The constants were calculated from the typical sigmoidal curves of analyte obtained as retention factor versus the pH of the mobile phase.     

Stability Constants of Cu(II) Hydroxypropylenediaminetetraacetates

Equilibrium of complexation between 2-hydroxypropylene-1,3-diamine-N,N,N,N-tetraethanoic acid (H₄L) and u²⁺ ions was studied by potentiometric and spectrophotometric methods. Stability constants of CuL^2– and CuHL^– complexes were determined at 298.15 K and I = 0.5 (KNO₃).     

Determination of the Dissociation Constants of Urocanic Acid Isomers in Aqueous Solutions

(E)- and (Z)-Urocanic acids are endogenous chemicals in the normal mammalian skin. The first and the second thermodynamic dissociation constants (pK _a1 and pK _a2) of urocanic acid isomers were determined using UV spectrophotometry in aqueous solutions. The values with standard deviation (pK _a1 = 3.43 ± 0.12 and pK _a2 = 5.80 ± 0.04) and (pK _a1 = 2.7 ± 0.3 and pK _a2 = 6.65 ± 0.04) were obtained to (E)- and (Z)-urocanic acids, respectively. The second dissociations were studied also by potentiometric titration in aqueous sodium chloride solutions up to the isotonic salt concentration (0.154 mol dm⁻³), and the second thermodynamic dissociation constants as well as activity parameters for both isomers were determined at temperature 25°C and for (E)-urocanic acid also at 37°C. The obtained pK _a2 values were close to those found by UV spectrophotometry. The equations for the calculation of the second stoichiometric dissociation constants of urocanic acid isomers on molality and molarity scale in aqueous sodium chloride solutions were derived. The obtained pK _a1 and pK _a2 values for (Z)-urocanic acid appear to be essentially lower than some previously reported values in literature.     

Stability Constants of Manganese(II) Alkylenediaminetetraacetates

Stability constants of the manganese(II) complexes with 2-hydroxypropylene-1,3-diamine-N,N,N,N-tetraethanoic and ethylenediamine-N,N,N,N-tetraethanoic acids were determined by the potentiometric method at 298.15 K and ionic strengths of 0.1, 0.5 and 1.0 (KNO₃). The data obtained were extrapolated to the zero ionic strength of the solution using the equation with one individual parameter.     

Determination of the Dissociation Constants of Urocanic Acid Isomers in Aqueous Solutions

Summary. (E)- and (Z)-Urocanic acids are endogenous chemicals in the normal mammalian skin. The first and the second thermodynamic dissociation constants (pK _a1 and pK _a2) of urocanic acid isomers were determined using UV spectrophotometry in aqueous solutions. The values with standard deviation (pK _a1 = 3.43 ± 0.12 and pK _a2 = 5.80 ± 0.04) and (pK _a1 = 2.7 ± 0.3 and pK _a2 = 6.65 ± 0.04) were obtained to (E)- and (Z)-urocanic acids, respectively. The second dissociations were studied also by potentiometric titration in aqueous sodium chloride solutions up to the isotonic salt concentration (0.154 mol dm⁻³), and the second thermodynamic dissociation constants as well as activity parameters for both isomers were determined at temperature 25°C and for (E)-urocanic acid also at 37°C. The obtained pK _a2 values were close to those found by UV spectrophotometry. The equations for the calculation of the second stoichiometric dissociation constants of urocanic acid isomers on molality and molarity scale in aqueous sodium chloride solutions were derived. The obtained pK _a1 and pK _a2 values for (Z)-urocanic acid appear to be essentially lower than some previously reported values in literature. An erratum to this article is available at .     

荧光法测定核黄蛋白核黄素的解离常数

本文推荐一个用荧光法测定配合物解离常数的实验,实验原理简单,知识内容跨学科,兼有生物化学和化学知识,有利于生化专业和化学专业的学生知识面的拓宽.     

Stability Constants for Dimercaptosuccinic Acid with Bismuth(III), Zinc(II), and Lead(II)

Abstract

Stability constants for the complexation of zinc(II), lead(II), and bismuth(III) by the vicinal dithiolate chelating agent meso-dimercaptosuccinic acid (DMSA) have been determined by a combination of potentiometric titration and spectrophotometric competition at 25°C and 0.1 M ionic strength. The spectrophotometric studies use the shifts in the ultraviolet bands of the thiol groups to quantitate metal binding to DMSA in the presence of competitive aminocarboxylic acids. Bismuth(III) forms a bis(DMSA) chelate with an exceptionally high stability constant of 10^43,87. This complex undergoes a series of protonations over the pH range 10 to 2, but there appears to be no measurable dissociation of ligand over this pH range. The zinc-DMSA system is dominated by a Zn₂(DMSA)₂ dimer, which has a protonation constant of 10⁶ and dissociates completely at lower pH. No more than 20% of total zinc exists as a monomeric complex at any pH. Lead forms a 1:1 complex with a stability constant of 10^17,4. Insoluble protonated lead complexes precipitate at pH < 6.5. Speciation calculations have been used to evaluate the potential competition from serum zinc to the binding of Pb²⁺ and Bi³⁺ by DMSA. The results indicate that DMSA should be relatively effective for the in vivo chelation of both Bi³⁺ and Pb^{2 +}.