A new graphical method for determining stability constants of weak and polynuclear complexes

A graphical method is proposed for differentiating mono- and poly-nuclear complexes as well as for determining the stability constants of weak complexes. The method is based on the effect of dilution on the degree of dissociation of the complex. The precision for log K is ±0.02 for a degree of dissociation between 30 and 70%. The greatest values for log K that can be determined are 4.3, 12.4, 7.5 and 10.7 for 1:1, 2:2, 2:1 and 3:1 ligand: metal complexes, respectively. If log K₁ ? 5.7, the method permits log K₂ to be determined if simultaneous complexes are formed.     

Stability,spectra and structure of the cobalt(II) chloride complexes in hexamethylphosphoramide solution at 25°C

Cobalt(II) chloride complexes were investigated spectrophotometrically in hexamethylphosphoramide (HMPA) solutions. It has been established that four consecutive tetrahedral chloro complexes of cobalt(II) are formed in the Co(ClO₄)₂-CoCl₂ and CoCl₂-LiCl-HMPA solutions. The formation constants of the complexes have been calculated: log K₁ = 9.0 (±0.5), log K₂ = 6.5 (±0.5), log K₃ = 3.05 (±0.05) and log K₄ = − 1.42 (±0.03). The high stability of the dichloro-complex results in the fact that this species is the only complex of cobalt(II) existing in HMPA solutions of CoCl₂.     

4-carboxy-1,2-cyclohexanedionedioxime complexes of nickel(ii) in alkaline media

The reactions of 4-carboxy-1,2-cyclohexanedionedioxime and nickel(II) were studied in alkaline media. Spectrophotometric studies indicate the presence of a 1:1 complex ion, NiD^-. Magnetic susceptibility measurements on a series of solutions of varying ratios of vic-dioxime and nickel(II) showed that the 1:1 complex ion was diamagnetic and that two paramagnetic complexes, probably NiD₂^4- and NiD₃^7-, are present in solution. The stability constants for the three complexes were calculated from spectrophotometric and magnetic susceptibility data. The log K values were found to be log K₁ = 28.74 ± 0.60, log K₂ = 0.76 ± 0.15, and log K₃ = 3.67 ± 0.73, respectively.     

Iron(III)‐Complexes Engaged in the Biochemical Processes in Seawater. II. Voltammetry of Fe(III)‐Malate Complexes in Model Aqueous Solution

Characteristics of iron(III) complexes with malic acid in 0.55 mol L^?1 NaCl were investigated by voltammetric techniques. Three iron(III)‐malate redox processes were detected in the pH range from 4.5 to 11: first one at ?0.11 V, second at ?0.35 V and third at ?0.60 V. First process was reversible, so stability constants of iron(III) and iron(II) complexes were calculated: log K₁(Fe^III(mal))=12.66±0.33, log β₂(Fe^III(mal)₂)=15.21±0.25, log K₁(Fe^II(mal))=2.25±0.36, and log β₂(Fe^II(mal)₂)=3.18±0.32. In the case of second and third reduction process, conditional cumulative stability constants of the involved complexes were determined using the competition method: log β(Fe(mal)₂(OH)_x)=15.28±0.10 and log β(Fe(mal)₂(OH)_y)=27.20±0.09.     

A spectrophotometric determination of the stability constants of the complexes of titanium(III) with acetylacetone

The stability constants of the consecutive complexes of titanium(III) with acetylacetone were determined by means of spectrophotometric methods. The values are log /gb₁ = 10.4, log β₂ = 18.8, and log β₃ = 24.9. Because of the fact that K₁K_a = 10^1.5 a high concentration of acetylacetone results in a considerable formation of TiL even at pH 0.     

Complex formation of 1,4,7,10-tetraoxacyclododecane with alkali metal ions studied by 13C spectroscopy

Complex formation of 1,4,7,10-tetraoxacyclododecane (12-crown-4) with lithium, sodium and potassium salts in methanol solution was investigated. The strong influence of complexing on the chemical shift of the single ¹³C NMR line permitted titration of the ligand with alkali metal salts. Concentration stability constants of the complexes were obtained by a computerized iterative least squares method. Na⁺ and K⁺ form both 1: 1 and 1: 2 complexes, log K₁ = 2.1 and log K₂ = 3.8, log K₁ = 1.7 and log K₂ = 2.4 respectively. Li⁺ is complexed weakly. Assuming 1: 1 stoichiometry the complex stability constant is estimated to be < 1.     

Copper(II) complexing with 2′,2′-diethylheptanohydrazide

Acid-base properties of 2′,2′-diethylheptanohydrazide (DEHH) and its complex formation with copper(II) ions in water-isopropanol solutions were studied. Cationic complexes with [Cu²⁺] : [DEHH] ratios of 1 : 1 and 1 : 2 were found to be formed in weak acid media, and an uncharged complex with a [Cu²⁺] : [DEHH] ratio of 1 : 2 was found to be formed in a basic medium. log K _st values were determined. The data obtained were compared with the stability constants of copper(II) 2′,2′-diethylbenzohydrazide complexes.     

The determination of the ionisation constants of polymaleic acid and the stability constants of its complexes with copper

The ionisation constants of unfractionated polymaleic acid, H₃A, a synthetic analogue of fulvic acid, and the stability constants of its copper complexes were determined by differential pulse polarography. The ionisation constants are: pK₁ 5.16, pK₂ 7.04 and pK₃ 9.82 at 20° C and ionic strength 0.12 M. The stability constant of the CuHA complex, which is the predominant species at pH 5–9, lies in the range log (CuHA) = 7.15–8.7; the mean value is 8.17.     

A POTENTIOMETRIC STUDY OF METAL CHELATES WITH TETRAETHYLENEPENTAAMINEHEPTA-ACETIC ACID

Abstract

The protonation constants of tetraethylenepentaamineheptaacetic acid, TPHA, were determined by potentiometric titration in aqueous solution at an ionic strength of 0.10 M KNO₃ and at 25°C. The formation constants of various metal-TPHA complexes were also obtained by titrating mixtures of metal to ligand in molar ratios of 1 :1 and 2:1. Calculations were performed with the computer program BEST. Individual stability constants are reported for Co(II). Ni(II), Cu(II), Zn(II), Cd(II), Hg(II) and Pb(II) with TPHA as well as their related pro-tonated species. The stabilities of the 1:1 complexes parallel to those of similar complexes with DTPA and TTHA. However the 2: 1 complexes have significantly larger log K _ML's than their TTHA counterparts. The extra stability of the 2:1 metal-TPHA complexes is explained in terms of ligand denticity and steric effects. Mercury(II)-TPHA complexes exhibited the highest formation constants and the copper-TPHA complexes had slightly higher log K _ML's than those for Co(II), Ni(II), Zn(II), Cd(II) and Pb(II).     

A potentiometric study on complex formation of cadmium(II) ion with 2-mercaptoacetic and 2-mercaptopropionic acids

Complex equilibria between cadmium ions and 2-mercaptoacetic acid (H₂maa) or 2-mercaptopropionic acid (H₂mpa) have been studied in aqueous solutions containing 3 mol dm^?3 LiClO₄ as a constant ionic medium at 25°C by potentiometric titration. Formation constants of mono-η and bis-2-mercaptoalkanoato)cadmium complexes were found to ge log K₁₁ = 4.34 and log K₁₂ = 2.15 for the cadmium-H₂maa complexes, and log K₁₁ = 5.66 and log K₁₂ = 2.85 for the cadmium-H₂mpa complexes, respectively. The protonated complexes, CdHmaa⁺ and CdHmpa⁺, and a mixed ligand complex, Cd(maa)(mpa)²- were also detected.     

Protonation equilibria and complex formation ability of 2-amino-3-aminomethyl-4,6-disubstituted pyridines

The protonation equilibria of 2-amino-3-aminomethyl-4,6-disubstituted pyridines were studied by potentiometric titrations and by UV and ¹³C NMR spectrometry. Protonation constants log K₁ and log K₂ potentiometrically determined for 4-methoxymethyl-6-methyl (8.959 and 5.778), 6-methoxymethyl-4-methyl (8.964 and 5.407), and 4,6-dimethyl derivative (9.141 and 6.604), were compared with the values obtained by the spectrometric methods. Complex formation of these compounds with Cu(II), Ni(II), Zn(II) and Co(II) was investigated in solution whereby the outstanding effect of the pH value was established and the presence of the species with 1 : 1 and 2 : 1 ligand to metal ratios confirmed. Stability constants obtained by potentiometric titrations show a decrease of the complex stability in the order Cu > Ni > Zn with respective log β₁ values of 5.85, 3.56 and 2.93. Series of complexes isolated in the crude state were characterized by elemental analyses and spectrometric data, the latter serving also as the basis for making an attempt at the discernment of the coordination mode and sites.     

Formation of protonated salicylate complexes of Cd(II) and Zn(II)

The formation constants of the protonated complexes of Cd(II) and Zn(II) with salicylic acid (H₂Sal) and the equilibrium constants for the extraction of these complexes into cyclohexane containing tributyl phosphate (TBP) have been determined. For the complex formation:

log K₁₁ is 1.9 for Cd(II) and 1.4 for Zn(II) at 30°C and 0.1 M NaClO₄. For the extraction:

log K_ex is ?1.6 for Cd(1I) and ?2.2 for Zn(II).     

Mercaptokomplexe von Indium(III)

The complexes of In(III) with thioglycolate (HO? CH₂? CH₂? S^?) have been investigated by pH-metric methods. Only the mononuclear species In(SR)_n(3-n)+ (n = 1, 2, 3, 4) are formed (Fig. 1): It was difficult to obtain an accurate value for the first stability constant K₁ because of the high stability of the 1:1-complex and therefore mixtures of KCl-KNO₃ were used to keep up the ionic strength of μ = 0.1. In presence of chloride, the constants K_n are smaller because of the formation of chloro complexes and mixed complexes In(SR)_nCl_i(3-n-1)+ and the stability constants of these have also been determined. The constants for the complexes without coordinated chloride can finally be obtained by an extrapolation (Fig. 2). The problem of the influence of increasing charge on the stability of various complexes of the isoelectronic 4d¹⁰ cations is discussed (Fig. 3).     

Affinity capillary electrophoretic study of K+/Na+ selectivity of hexaarylbenzene-based polyaromatic receptor

Affinity capillary electrophoretic (ACE) study has proved the selectivity of hexaarylbenzene-based polyaromatic receptor (R) for K⁺ ion over Na⁺ ion. The apparent binding constants of the R complexes with K⁺ and Na⁺ ions were determined from the dependence of effective electrophoretic mobility of R on the concentration of the above alkali metal ions in the background electrolyte using a non-linear regression analysis. The apparent binding constants (K_b) of the K-R⁺ and Na–R⁺ complexes in methanolic medium were evaluated as log K_b = 3.20 ± 0.22 for the K–R⁺ complex, and log K_b??0.7 for the Na–R⁺ complex.     

Potentiometric investigations of the equilibria between caffeic acid and copper(II), zinc(II), iron(II) and hydrogen ions in aqueous solution

Interactions in aqueous solution of caffeate with copper(II), zinc(II), iron(II) and iron(III) have been investigated. Virtually instantaneous and complete reduction of iron(III) was observed. Glass electrode potentiometry was used to determine the speciation and corresponding formation constants of caffeate with each of the other three metal ions named. Conditions were: temperature, 25°C; ionic strength, 0.100 mol dm⁻³ with respect to chloride. Values obtained for the logarithms of the stepwise protonation constants of the singly protonated dianion of caffeate (L²⁻) are 8.72 and 4.41. The titration data carried out in the presence of the three metal(II) ions can be explained by postulating the major complexes: LCu, logβ₁₁₀=6.02; LCuH₋⁻₁, logβ_11-1=0.25; L₃Cu₂H₋₃⁻⁵, logβ_32-3=0.97; LZnH₋₁⁻, logβ_11-1 = −3.03; L₃ZnH₋₂⁶⁻, logβ_31-2 = −5.51; LFe, logβ₁₁₀ = 3.86; LFeH₋₁⁻, logβ_11-1 = −3.83; L₃FeH₋₂⁶⁻, logβ_31-2 = −6.14, together with a variety of minor species. Complexation in the major species involves, predominantly, chelation by the catecholic site of caffeate whereas coordination to the carboxylate group together with catechol-type chelation featured amongst the minor species. The tendency of copper(II) to form oligonuclear complexes is evident. A single dinuclear iron(II) complex was also found amongst the minor species.     

The Association Constants of H<Superscript><Emphasis Type="Italic">+</Emphasis></Superscript> and Ca<Superscript><Emphasis Type="Italic">2 +</Emphasis></Superscript> with 2-Keto-D-Gluconate in Aqueous Solutions

2-Keto-D-gluconate (kG) is naturally produced in soils, sediments and rock faces through the microbial oxidation of glucose. Studies have qualitatively shown kG to enhance the dissolution of soil minerals. However, quantitative information, such as the log K values for the formation of metal–kG complexes, are not available. This paper presents the results of potentiometric titration studies that employ H⁺ and Ca²⁺ ion selective electrodes (ISEs) to determine the conditional ion association constants (log Q values) for the protonation and deprotonation of kG and the formation of Ca–kG complexes. The experimentally-determined log Q values were then converted to the corresponding ion association constants (the zero ionic strength condition; log K values) by employing a modified Davies equation for charged species and the Setchenów equation for neutral species. The log K values were determined by potentiometric titrations at constant kG concentration, varied ionic strengths, 25 or 22 ^∘C, and in the absence of CO₂. The computer model GEOCHEM-PC was used to determine the aqueous speciation of ions other than kG and the computer model FITEQL was used to estimate conditional log Q values for reactions in the various chemical models. Based on our evaluations, equilibrium constants for the following reactions were determined: H⁺+ kG^– ⇌ HkG⁰, log K_a1 = (3.00 ± 0.06), kG^–⇌ H_–1kG^2–+ H⁺, log K_a–1 = –(11.97 ± 0.41), and Ca²⁺+ kG^–⇌ CakG⁺, log K₁₀₁ = (1.74 ± 0.04).     

Radiometric studies on the reaction of some metals with potassium pentacyanonitrosyl manganate

The nature and composition of complexes formed by the reaction of Fe(III), Cr(III), Zn(II), and Co(II) with potassium pentacyanonitrosyl manganate K₃ [Mn(CN)₅NO] has been investigated by radiometric method. The metals form 1∶1 complexes with K₃ [Mn(CN)₅NO], the optimum pH for maximum precipitation being 3.6 for Fe(III), 7.3 for Cr(III), 5.4 for Zn(II), and 8.3 for Co(II). The solubility of the complexes as computed from activity at maximum precipitation point follows the order: chromium complex > iron complex > cobalt complex > zinc complex. The radiometric titration curves also show the formation of colloidal precipitates with dilute Zn(II) solutions.     

A study of the chelation reaction of gallium and indium with Arsenazo 1

The chelation reactions of gallium and indium with Arsenazo 1, i.e., 3-[(o-arsonophenyl)azo]-4,5-dihydroxy-2,7-naphthalene disulfonic acid, have been studied in detail. The nature and number of complexes formed was studied and it was found that gallium and indium form only one complex with Arsenazo 1. The composition of the complexes formed was found by various methods and it was found that both the complexes have composition 1:1 (metal:reagent). The values of log K were calculated by different methods at four different fixed values of ionic strength. For both the complexes, the value of thermodynamic stability constant has been obtained by plotting values of log K against ionic strength and extrapolating the curves to zero ionic strength. A tentative suggestion about the structure of the chelate ring has been made. The studies were further extended to the analytical aspects of the complexes and a suitable procedure has been recommended for the spectrophotometric determination of gallium and indium using this reagent.     

Supramolecular assembler based on cucurbit[8]uril: Photodimerization of a styryl dye in water

Photolysis of aqueous solutions of styryl dye 1 in the presence of cucurbit[8]uril (CB[8]) has been studied by optical spectroscopic methods for the molar ratios n = c _CB[8]/c ₁ in the range of 0 ≤ n ≤ 6. It has been found that the inclusion complexes (1)₂@CB[8] dominate in the solution at n ≤ 0.5, whereas the complexes 1@CB[8] dominate at n ≥ 1. The stability constants have been determined for the 1: 1 (log K ₁ = 6.2 (L mol^?1)) and 2: 1 (log β = 11.9 (L² mol^?2)) complexes. The fluorescence decay kinetics of dye 1 in the presence of CB[8] is two-exponential, with the average lifetime increasing substantially at n ≥ 1. It has been shown that the system can operate in the cyclic mode as an assembler (or supramolecular catalyst) in the photodimerization reaction of dye 1 to form cyclobutane derivative 2. The stability constant of the complex 2@CB[8] (log K ₃ = 5.9 (L mol^?1)) and the quantum yield of cycloaddition (? ≈ 0.07 at n ≈ 0.5) have been determined.     

Binding studies on substituted benzoic acids: Part I. 3,5-dinitrosalicylic acid—acid dissociation and complexation with nickel(II) and cobalt(II)

The values of pK₁ and pK₂ for 3,5-dinitrosalicylic acid (DNSA, H₂L) have been determined at 25.0°C and 0.1 M ionic strength (sodium perchlorate) as 0.25 and 7.20. respectively. The binding of nickel(II) and cobalt(II) has been investigated over the pH range 1.0–7.0 at the same temperature and ionic strength. Values of log K_ML for the formation of the unprotonated complexes are 4.05±01 and 3.82±01 for nickel(II) and coball(ll) respectively. Values for log K_MHL are close to 11 mol^-1 for both metals; it seems probable that these species are not inner-sphere complexes, and possible reasons for this are suggested. The stability constants obtained are compared with others in the literature, and the possibility of using DNSA as a metal-ion indicator is discussed briefly.