Multiparametric curve fitting-XI: POLET computer program for estimation of formation constants and stoichiometric indices from normalized potentiometric data

The FORTRAN computer program POLET(84) analyses a set of normalized potentiometric titration curves to find a chemical model, i.e., the number of species present and their stoichiometry, and to determine the corresponding stability constants log beta(pqrs) and unknown stoichiometric indices p, q, r, and s of up to quaternary M(p)L(q)Y(r)H(s) complexes. The program belongs to the ABLET family, based on the LETAG subroutine, and can use an algorithmic and/or heuristic minimization strategy, or a beneficial combination of both. The data, a set of potentiometric titration curves plotted as volume and potential, are converted into normalized variables (formation function, pH) and then a computer-assisted search for a chemical model by POLET(84) is applied. The procedure for efficient application of POLET(84) in an equilibrium analysis is described and the program is validated by use of literature and simulated data. The reliability of the chemical model and its parameters is established by the degree-of-fit achieved, and the closeness of the stoichiometric indices to integral values.     

Multiparametric curve fitting--X: a structural classification of programs for analysing multicomponent spectra and their use in equilibrium-model determination

A functional structure-classification of programs for analysis of spectra elucidates their efficiency for determination of the stoichiometric indices, stability constants and molar absorptivities of complex species. SQUAD (84) introduces new functional units for (i) determination of the number of light-absorbing species, (ii) a rigorous fitness test, (iii) plotting three-dimensional graphs of a paraboloid minimum response-surface as a function of two selected parameters, and a graph of the fitted absorbance response-plane, (iv) simultaneous estimation of stoichiometric indices and stability constants, (v) simulation of an absorbance matrix data by loading with random errors related to the instrumental variance of the absorbance. A guide to experimental procedure and computational strategy for chemical model determination is given and nine diagnostic tools useful in finding the number of species present and their stoichiometry and stability constants by regression analysis of spectra are tested, by use of literature data.     

Microcomputer application of non-linear regression analysis to metal-ligand equilibria

A non-linear least-squares regression program is described which is suitable for PC-compatible microcomputers. The program is written in GWBASIC, but compiled to run with the Intel 8087 fast numeric processor. Subroutines which simulate functions are compiled separately from the main program. Parameters are optimized by a Gauss-Newton-Marquardt algorithm which can be provided with either analytically or numerically calculated partial derivatives. Multi-component potentiometric titrations are simulated and parameters optimized by using analytical derivatives. Spectrophotometric titrations are also simulated, but absorptivities are optimized by linear regression while stability constants are optimized non-linearly by using numerical derivatives. Provision is made for "global analysis" of parameters. The experimental points can be displayed on screen, along with the "best" fit and the speciation. The program is demonstrated here by the determination of the pK(a) values and stability constants of a hydroxypyridinone ligand and its complexes with Fe(III).     

Dissociation constants of some Schiff bases and stability constants of their copper,cobalt, nickel and zinc complexes

The stoichiometry and stability constant of metal complexes with 4-(3-methoxy-salicylideneamino)-5-hydroxynaphthalene-2,7-disulfonic acid monosodium salt (H₂L) and 4-(3-methoxysalicylideneamino)-5-hydroxy-6-(2,5-dichlorophenylazo)-2,7-naphthalene disulfonic acid monosodium salt (H₂L¹) were studied by potentiometric titration. The stability constants of H₂L and H₂L¹ Schiff bases have been investigated by potentiometric titration and u.v.–vis spectroscopy in aqueous media. The dissociation constants of the ligand and the stability constants of the metal complexes were calculated pH-metrically at 25 °C and 0.1 m KCl ionic strength. The dissociation constants for H₂L were obtained as 3.007, 7.620 and 9.564 and for H₂L¹, 4.000, 6.525, 9.473 and 10.423, respectively. The complexes were found to have the formulae [M(L)₂] for M = Co(II), Ni(II), Zn(II) and Cu(II). The stability of the complexes follows the sequence: Zn(II) < Co(II) < Cu(II) < Ni(II). The high stability of H₂L¹ towards Cu(II) and Ni(II) over the other ions is remarkable, in particular over Cu(II), and may be of technological interest. Concentration distribution diagram of various species formed in solution was evaluated for ligands and complexes. The formation of the hydrogen bonds may cause this increased stability of ligands. The pH-metric data were used to find the stoichiometry, deprotonation and stability constants via the SUPERQUAD computer program.     

Complexes of vanadyl and uranyl ions with the chelating groups of humic matter

The uranyl and vanadyl complexes formed with salicylic, phthalic and 3,4-dihydroxybenzoic acids have been studied by potentiometry in order to determine the stability constants of the M(m)L(n) species formed in solution, and the constants for the hydrolysis and polymeric complexes, at 25.0 degrees , in 0.10, 0.40 and 0.70M sodium perchlorate. MINIQUAD was used to process the data to find the best models for the species in solution, and calculate the formation constants. The uranyl-salicylic acid system was also studied by spectrophotometry and the program SQUAD used to process the data obtained. The best models for these systems show that co-ordination of the uranyl ion by carboxylate groups is easier than for the vanadyl ion, whereas the vanadyl ion seems to form more stable complexes with phenolate groups. Both oxo-cations seem to tend to hydrolyse rather than form complexes when the L:M ratios are greater than unity. Although the change in the constants with ionic strength is small, the activity coefficients of the salicylate and phthalate species have been calculated at ionic strengths 0.40 and 0.70M, along with the interaction parameters with Na(+), from the stability constants found for the species ML and H(2)L, according to the Br?nsted-Guggenheim expression.     

金属离子Bi(Ⅲ)络合物体系的极谱法研究

The B（i Ⅲ）-Ligand（pyridine-2-carboxylic acid）system was studied by the differential pulse polarography（DPP）at the fixed total-ligand to total-metal concentration ratio［LT］:［MT］and varied pH. An appropriate model for the metal complex system is designed according to polarographic peak parameters,that is,the observed shift in the peak potential and the change in the peak current with pH changes in the sample solution. The polarographic experimental complex formation curve（ECFC）and theoretical complex formation curve（TCFC）were used for checking the designed metal complex model as well as optimizing stability constants. The ECFC,in which experimental parameters（a shift in a peak potential and a variation in a peak current）are included,appears to be a characteristic function for a particular metal-ligand system. The TCFC is a theoretical curve calculated from massbalance equations for the designed metal-ligand model. Using the computer analytical program,stability constants of metal complexes and species distribution diagrams are obtained by solving the mass-balance equations written for the metal complex system and fitting the complex formation curve. Five bismuth complexes MHL,ML,ML2,ML3,ML3 （ OH）,and their stability constants as logβ 7.52±0.15,7.48±0.06,13.94±0.02,18.12±0.03 and 26.78±0.03,respectively,are finally reported while the TCFC fits best the ECFC.     

Interaction of distamycin-oligopeptides with the self-complementary (d-CACAAGCTTGTG)2 duplex in solution

The complex formation of the DNA groove binding drugs netropsin, distamycin-3 and distamycin-5 with the self-complementary duplex (d-CACAAGCTTGTG)₂ has been investigated using CD titration measurements. Employing a general curve fitting program for data analysis, binding constants of complex formation and stoichiometry numbers were evaluated for the two distamycin-type oligopeptides. The binding model assumes simultaneous occurrence of two kinds of complexes characterized by (1:1) - and (1:2) -stoichiometry (duplex:ligand). The binding constants obtained for the (1:1)-complexes for distamycin-3 as well as for distamycin-5 are in full agreement with those calculated for the duplex sequence by means of the single-base specific binding constants of the methylpyrole carboxamide group of Gursky et al. [33]. Obtained binding results are explained by a possible interaction mechanism of distamycin with the particular dodecamer duplex in solution.     

Stability of Copper(II), Nickel(II) and Zinc(II) Binary and Ternary Complexes of Histidine,Histamine and Glycine in Aqueous Solution

The binary and mixed-ligand complexes formed between ligands (histidine (His), histamine (Him) and glycine (Gly)) and some transition metals (Cu(II), Ni(II) and Zn(II)) were studied potentiometrically in aqueous solution at (25.0 ± 0.1) C and I = 0.10 M KCl in order to determine the protonation constants of the free ligands and stability constants of binary and ternary complexes. The complexation model for each system has been established by the software program BEST from the potentiometric data. The most probable binding mode for each binary species of histidine and for all mixed species was also discussed based upon derived equilibrium constants and stability constants related to the binary species. The ambidentate nature of the histidine ligand, i.e. the ability to coordinate histamine-like, imidazolepropionic acid-like and glycine-like modes was indicated from the results obtained. The stability of ternary complexes was quantitatively compared with their corresponding binary complexes in terms of the parameters, log K, log X and ₁₁₁₀. The concentration distributions of various species formed in solution were also evaluated. In terms of the nature of metal ion, the complex stability follows the trend Cu(II) > Ni(II) > Zn(II), which is in agreement with the Irving-Williams order of metal ions. Thus, the results obtained were compared and evaluated with those in the literature.     

Potentiometry, Stability and Thermodynamics of Diethyltin(IV) Dichloride with Some Selected Biomolecules

The interaction of diethyltin(IV), DET, with selected bioligands having a variety of model functional groups was investigated at 25 °C and ionic strength 0.1 mol·dm^?3 NaCl using the potentiometric technique. The hydrolysis constants of the DET cation and the formation constants of the complexes formed in solution were calculated using the MINIQUAD-75 program. The stoichiometry and stability constants for the complexes formed are reported. The results show the formation of 1:1 and 1:2 complexes with amino acids and DNA constituents. The dicarboxylic acids form 1:1 complexes. The peptides form both 110 complexes and the corresponding deprotonated amide species [Et₂Sn(LH_?1)] (11-1). The participation of different ligand functional groups in binding to organotin is discussed. The concentration distributions of the various complex species were evaluated as a function of pH. The standard thermodynamic parameters ΔH° and ΔS°, calculated from the temperature dependence of the equilibrium constants, were investigated for the interaction of DET with thymine as a representative example of DNA constituents. The effect of ionic strength and solvent on hydrolysis constants of DET, protonation equilibria of thymine and its complex formation with DET were investigated and discussed.     

Thermodynamic study of complex formation between Ni2+ and Co2+ cations and 1,13-bis(8-quinolyl)-1,4,7,10,13-pentaoxatridecane (kryptofix 5) in binary mixed nonaqueous solutions

Conductometric titrations were performed in pure and binary solvent solutions of ethyl acetate (EtOAc), methyl acetate (MeOAc) and methanol (MeOH) with acetonitrile (AN) at 288, 298, 308, and 318 K to determine the stoichiometry, the complex stability constants and the standard thermodynamic parameters for the complexation of nickel(II) and cobalt(II) cations with 1,13-bis(8-quinolyl)-1,4,7,10,13-pentaoxatridecane (kryptofix 5). The stability constants of the resulting 1:1 complexes formed between the metal cations and the ligand in different solvent mixtures were determined by computer fitting of the conductance-mole ratio data. The results revealed that the stability order of (kryptofix 5)-Ni²⁺ and (kryptofix 5)-Co²⁺ complexes changes with nature and composition of the solvent system. There is a non-linear relationship between the logK _f values of complexes and the mole fraction of acetonitrile in the mixed solvent system. In addition, the conductometric data show that the stoichiometry of the complexes formed between the nickel(II) and cobalt(II) cations with the acyclic ligand changes with the nature of the solvent. The enthalpy and entropy values for the 1: 1 [ML] complexation reactions were evaluated from the temperature dependence of the formation constants. Thermodynamically, in most of systems, the complexation processes of nickel(II) and cobalt(II) cations with kryptofix 5, are both enthalpy and entropy stabilized and the values of these parameters are influenced by the nature and composition of the binary mixed solvent solutions.     

Potentiometric Determination of the Stability Constants of Trimethyltin(IV) Chloride Complexes with Imino-bis(Methylphosphonic Acid) in Water and Dioxane–Water Mixtures

The interaction of trimethyltin(IV) (TMT) with imino-bis(methylphosphonic acid) (IDP), abbreviated as H₄L, was investigated at 25 °C and at ionic strength 0.1 mol⋅dm⁻³ (NaNO₃) using a potentiometric technique. The formation constants of the complexes formed in solution were calculated using the nonlinear least-squares program MINIQUAD-75. The stoichiometry and stability constants are reported for the complexes formed. The results show the formation of 110, 111, 112 and 11-1 complexes for the TMT–IDP system. The concentration distribution of the various complex species was evaluated. The effect of dioxane as a solvent, on both the protonation constants and the formation constants of trimethyltin(IV) complexes with IDP, is discussed. The thermodynamic parameters ΔH ^∘ and ΔS ^∘ calculated from the temperature dependence of the equilibrium constants were evaluated. The effect of ionic strength on the protonation constants of IDP is also discussed.     

A spectrophotometric and thermodynamic study of the charge-transfer complexes of iodine with 2-aminomethyl-15-crown-5 in chloroform and 1,2-dichloroethane solutions

Interaction of 2-aminomethyl-15-crown-5 (AM15C5) with iodine has been investigated spectrophotometrically in chloroform and 1,2-dichloroethane (1,2-DCE) solutions. The observed time dependence of the charge-transfer band and subsequent formation of I(3)(-) in solution were related to the slow transformation of the initially formed 1:1 AM15C5.I(2) outer complex to an inner electron donor-acceptor (EDA) complex, followed by fast reaction of the inner complex with iodine to form a triiodide ion. The pseudo-first-order rate constants were evaluated from the absorbance- and conductivity-time data. The stoichiometry and formation constants of the resulting EDA complexes have also been determined. Thermodynamic parameters, Delta H degrees and Delta S degrees , of the complexes have been determined from the temperature dependence of stability constants by Van't Hoff equation. The results indicate that iodine complexes of AM15C5 in both solvents are enthalpy stabilized but entropy destabilized. The influence of solvent properties on the kinetics and stability of the resulting charge-transfer complexes are discussed.     

Formation of Glycinate Complexes of Iron(II) at Different Ionic Strengths of Solution

Complexation of iron(II) in aqueous glycine (α-aminoacetic acid) solutions is studied in the pH range 1.0–8.0 at 298.16 K at various ionic strengths (NaClO₄). The stability constants of the resulting complexes go down as the ionic strength of the solution increases. The stability constants of the complexes are determined using the experimental “electromotive force of the system versus pH” curves by iterative fitting of the theoretical oxidation function to the experimental one with the Excel program. The correlation between the stability constants of complexes and ionic strength of the solution is also calculated using the Debye–Hückel equation and SigmaPlot 10.0 software. Statistical analysis of the calculated data is performed with P value of 0.95.     

Physical characterization of some complexes of organic nitrogen donor compounds

The stability constants of the complexes of cyanopyridine and aminoantipyrine with some divalent transition metal chlorides were determined at different temperatures. The thermodynamic parameters were calculated; the obtained data reveal that the complexation is an endothermic process. Also it was found that the type of bonding between the ligands and the corresponding metal ion is mainly ionic in nature. The stoichiometry of the formed complexes was found to be M:L and M:2L. The structure of some prepared complexes was confirmed by IR, ¹H NMR spectra as well as elemental analysis. Thermal stability was studied using TGA technique.     

Complexation of Cd2+, Ni2+, and Ag+ metal ions with 4,13-didecyl-l,7,10,16-tetraoxa-4,13-diazacyclooctadecane in acetonitrile-ethylacetate binary mixtures

Conductometric titrations have been performed in acetonitrile-ethylacetate (AN-EtOAc) binary solutions at 288, 298, 308, and 318 K to obtain the stoichiometry, the complex stability constants and the standard thermodynamic parameters for the complexation of Cd²⁺, Ni²⁺, and Ag⁺ cations with 4,13-didecyl-1,7,10,16-tetraoxa-4,13-diazacyclooctadecane (cryptand 22DD). The stability constants of the resulting 1: 1 complexes formed between the metal cations and the ligand were determined by computer fitting of the conductance-mole ratio data. There is a non-linear relationship between the logK _f values of complexes and the mole fraction of ethylacetate in the mixed solvent system. In addition, the conductometric data show that the stoichiometry of the complexes formed between the Cd²⁺, Ni²⁺, and Ag⁺ cations with the ligand changes with the nature of the solvent. The standard enthalpy and entropy values for the 1: 1 [ML] complexation reactions were evaluated from the temperature dependence of the formation constants. Thermodynamically, the complexation processes of the metal cations with the C22DD, is mainly entropy governed and the values of thermodynamic parameters are influenced by the nature and composition of the binary mixed solvent solutions.     

Complexation of 4′-nitrobenzo-15-crown-5 with Li+, Na+, K+, and NH 4 + cations in acetonitrile–methanol binary solutions

The complexation processes between Li⁺, Na⁺, K⁺ and NH ₄ ⁺ cations with macrocyclic ligand, 4′-nitrobenzo-15C5, were studied in acetonitrile–methanol (AN–MeOH) binary mixtures at different temperatures using conductometric method. The conductance data show that the stoichiometry of the complexes formed between the ligand and Li⁺, Na⁺, K⁺ and NH ₄ ⁺ cations is 1:1(M:L). Addition of 4′-nitrobenzo-15C5 to these cations solution, causes a continuous increase in the molar conductivities which indicates that the mobility of the complexed cations is more than the uncomplexed ones. The values of stability constants of the complexes were determined from conductometric data using GENPLOT computer program. The obtained results show that the selectivity order of the ligand for Li⁺, Na⁺, K⁺ and NH ₄ ⁺ cations changes with the nature and composition of the binary mixed solvent. The values of thermodynamic parameters (ΔH°_c, ΔS°_c) for formation of the complexes were obtained from temperature dependence of the stability constants using the van’t Hoff plot. The results show that the complexes are both enthalpy and entropy stabilized. A non-linear behavior was observed between the stability constants (log K _f ) of the complexes and the composition of the AN–MeOH binary solution.     

Interactions between Protonated Amine, Aza Crown Ether, and Cryptand with Dibenzocrown Ether Studied by a New Spectrophotometric Technique

The stability constants for the complexation of a diprotonated diamine, a diaza crown ether, and a cryptand with dibenzo-18-crown-6 and dibenzo-24-crown-8, have been studied in aqueous solution using a new spectrophotometric technique. Because of the complex formation, the solubility of the dibenzocrown ethers increases. Complex formation is possible between diamines and dibenzocrown ethers with both 1:1 and 2:1 stoichiometry. However, experimental data are insufficient to decide on the actual stoichiometry of the complexes formed. By computing the stability constants and comparing them with the corresponding results for monoamines, it is possible to decide on the actual stoichiometry of the complexes. Under the experimental conditions only 1:1 complexes with diamines are formed.     

Stability of copper(II) complexes with cellulose complexite in water-dioxane mixtures

The complexing properties of cellulose complexite with respect to copper(II) ions in water-1,4-dioxane (DO) mixtures are determined. The stoichiometry and stability constants of copper(II) complexes with the hydroxamic groups of the polymer are determined. The equilibrium speciation diagrams for the complexes at pH 2.0–6.0 are plotted; stereochemical configurations are suggested. The effect of the solvation parameters of the complexite on complex formation is analyzed.     

Synthesis and Thermodynamic Investigation of 4-Amino-6-Hydroxy-2-Mercapto Pyrimidine (AHMP) Complexes with Some Selected Divalent Metal(II) Ions

The synthesis and characterization of zinc(II), cadmium(II), lead(II), mercury(II) and phenylmercury(II) complexes of 4-amino-6-hydroxy-2-mercapto pyrimidine (AHMP) are reported. The stoichiometry of the complexes was found to be 1:2 except for the phenylmercury(II) complex where the ratio is 1:1. Characterization of these complexes was carried out by means of elemental analyses, IR and ¹H NMR measurements. In these complexes the ligand is bonded to the metal through its sulfur atom. The potentiometric results showed the formation of 1:1 and 1:2 complexes and the corresponding stability constants were determined for both Zn(II) and Cd(II) ions. The high insolubility of mercury(II), phenylmercury(II) and lead(II) complexes prevented the determination of their stability constants. The concentration distribution of the complexes in solution was evaluated. The effect of temperature on the dissociation constant of AHMP and the formation constants of both the Zn-AHMP and Cd-AHMP complexes were studied and the thermodynamic parameters were calculated.     

A Thermodynamic Study of Complexation of Iron Ions with Clarithromycin and Roxithromycin in Methanol Using a Conductometric Method

The formation constants K _ML of Clarithromycin (CLA) and Roxithromycin (ROX) with Fe(III) and Fe(II) ions in methanol have been determined at various temperatures using a conductometric technique. The interaction yields complexes with metal-to-ligand compositions of 1:1. The conductivity data were analyzed using a computer program based on 1:1 stoichiometry from which the stability constants and the limiting molar conductance were obtained. The stability of these complexes was found to increase with temperature. Compared with Fe(II), Fe(III) forms more stable complexes with ROX and CLA. The values of the thermodynamic parameters enthalpies (??H ^°), entropies (??S ^°), and the derived Gibbs energies (??G ^°) were deduced from the dependence of the formation constants on temperature. The positive values of ??H ^° and ??S ^° indicate that the complexation processes is enthalpically unfavorable but entropically favored. The negative values of ??G ^° show the ability of the studied ligand to form stable complexes and that the complexation process is favorable.