Stability constants potentiometric measurements

The stability constants of zinc complexes of 4,6-dimethyl-2-thiopyrimidine have been determined by potentiometric measurements. The crystal structure shows infinite zigzag chains of ZnL2 units with each zinc coordinated by an N3S2 donor set in a trigonal-bipyramidal geometry.853... 

Borrok et al. (2004a) used potentiometric titration to measure Cd sorption by different bacterial consortia, and a surface complexation approach to determine thermodynamic stability constants. When the data were modeled by adopting a single set of stability constants, a similar sorption behavior was shown by a wide range of bacterial species. Further, current models that rely on pure strains of laboratory-cultivated bacterial species appear to overestimate the extent of metal biosorption in natural systems. 

Potentiometric titrations can also be used to determine stability constants for complexes in some cases, which can be useful as a substitute or complementary approach to spectrophotometric titrations (56). By starting at a low pH, where the relative amount of complex formation is low, and then measuring the pH upon addition of standardized base, one can determine the relative amounts of the species in solution or how many equivalents of H+... 

To conduct meaningful mechanistic and kinetic studies in alcohol media reliable and simple measurement and control of the solution jjpH is essential. Potentiometric titration is the method of choice for obtaining acid dissociation constants or metal ion complex stability constants and in favorable cases the speciation of mixtures of metal-ion-containing complexes in solution can be proposed.20 Titrations in non-aqueous solvents are not nearly as widely reported as those in aqueous media, particularly in cases with metal ions21 and determination of pH in a non-aqueous solvent referenced to that solvent is complicated due to the lack of a way to relate the electrode EMF readings to absolute jjpH (see footnote and ref. 6) so non-aqueous solvents are generally inconvenient to use22 for detailed studies of reaction mechanisms where pH control is required. 

Numerous investigations have shown the existence of the heptamolybdate, [Mo7024]6 , and octamolybdate, [Mo8026]4, ions in aqueous solution. Potentiometric measurements with computer treatment of the data proved to be one of the best methods to obtain information about these equilibria. Stability constants are calculated for all species in a particular reaction model, which is supposed to give the best fit between calculated and experimental points. In the calculations the species are identified in terms of their stoichiometric coefficients as described by the following general equation for the various equilibria... 

Methods used successfully for stability constant determinations are calorimetry (14—16), potentiometric measurements with ion-selective electrodes (3, 17), and certain spectroscopic techniques where favorable spectral properties are found (13, 18). Values of log K have also been deduced indirectly from conductance (19, 20) and potential measurements on phospholipid bilayers (21). Descriptions of these procedures... 

The stability constants of the formed chloro-cadmium complexes in aqueous solution [135] and water-2-butanone mixtures [136] were also determined, using potentiometric measurements. The influence of hydrogen bonding of the solvent on the stability of Cd(II)-ethylenediamine complexes in water-DM SO mixtures was analyzed using pH and calorimetric measurement [137]. In five water-acetonitrile mixtures, the stability constants of Cd(II) and Zn(II) cyanide complexes were determined [138]. 

Narula and Gupta76 have measured the thermodynamic stepwise stability constants for the formation of complexes of dialkyltin(TV) ions, R2Sn2+ (R = Me or Bu ), with hydroxamic acids, RCONHOH (R = Ph, Me2CH, Me) potentiometrically in 75% dioxane-water media. This has been followed by determination of formation constant of Af-arylhydroxamic acids-dialkyltin(IV) ions.77... 

The protonation constants of the ligands L9, L11, L14, and L1S (Scheme 5), having one, two or three aminoethyl or three aminopropyl pendant arms, respectively, on the [9]aneN3 framework, and the thermodynamic stabilities of their mononuclear complexes with Cu11 and Zn11 have been investigated by potentiometric measurements in aqueous solutions (Table 1) [51],... 

In the use of potentiometry for the evaluation of stability constants for complex ions, the expressions can become extremely complicated if multiequilibria are present. For a simple one-to-one complex a direct potentiometric titration curve again provides die most satisfactory route to an accurate evaluation of the constant. The curve looks similar to that for an acid-base titration, and the appropriate point to pick is the half-equivalence point. If the complex is extremely stable, then die amount of free metal ion at this point on die dtration curve (ligand titrated with metal ion) is sufficiently low that it can be disregarded. If not, it must be handled in a way similar to the first point on the titration curve for phosphoric acid. Assuming that it is a stable complex, at the first half-equivalence point the concentration of complexed metal ion will be equivalent to that of the free ligand. The potential will give a direct measure of the free metal ion and allow the stability constant for the complex to be evaluated at the half-equivalence point ... 

For multistep complexation reactions and for ligands that are themselves weak acids, extremely involved calculations are necessary for the evaluation of the equilibrium expression from the individual species involved in the competing equilibria. These normally have to be solved by a graphical method or by computer techniques.26,27 Discussion of these calculations at this point is beyond the scope of this book. However, those who are interested will find adequate discussions in the many books on coordination chemistry, chelate chemistry, and the study and evaluation of the stability constants of complex ions.20,21,28-30 The general approach is the same as outlined here namely, that a titration curve is performed in which the concentration or activity of the substituent species is monitored by potentiometric measurement. 

Fig. 4.14. Stability constants of Ln(III) coronates versus the ratio D /Dc ratio (assuming an average coordination number of 9) as determined in propylene carbonate at 298 K by potentiometric measurement (counterion CIO ). Redrawn from J.-C.G. Biinzli et al., Inorg. Chem. 28, 2638, 1989.

Potentiometric measurements have also been used in such studies. As an example we mention only the work of Larson and Iwamoto [257] on Cu(II) hydration in nitromethane, that of Luehrs coworkers [258, 259] on stability constants of silver(I) with DMF, DMSO, and HMPA, and the studies of interaction of sodium(I) with different protic and aprotic solvents and acetonitrile media carried out by Izutsu and coworkers [260]. 

Takacs-Novak et al. (14) used a potentiometric assay in combination with UV spectroscopy and NMR to study the acid-base properties and protonation equilibria for lomefloxacin and other quinolone antibacterials. Okabayashi et al. used potentiometric measurements to determine the stability constants of metal ions with several quinolones including lomefloxacin (20). 

Diamines. Chromatography has been used to isolate three isomers of trans- and cis-[Co(CN)2 (RR)-cyclohexane-l,2-diamine 2] and five isomers of the corresponding propylenediamine complexes. Mer- and /ac-isomers of tris(meso-pentane-3,4-diamine)cobalt(iii) have been prepared and separated using column chromatography. The rates of aquation of three isomers of [CoCl(tmd)(dien)] and one isomer of [CoCl(tmdXdpt)] have been measured and the kinetic parameters calculated [dpt = NH2(CH2)3NH(CH2)3NH2, tmd = NH2(CH2)3NH2]. The interaction of [Co(dien)2] with sulphate, thiosulphate, sulphite, selenite, tellurite, and carbonate ions has been studied potentiometrically and stability constants determined for the outer-sphere complexes. The i.r. spectrum of octahedral... 

M thallium solutions, containing 1 M NaC104 and 3 M HCIO4 as ionic medium, the obtained stepwise stability constant, = 0.3(1) is not higher than the detection limit given by Ahrland et al. in their potentiometric study (91, 92). It is also in acceptable agreement with K5 = 0.8(2) M obtained by spectrophotometric measurements... 

In a recent study the acid dissociation constants of substituted l-amino-4-aryl-2(li/)-pyrimidinethiones of the type 1 and stability constants of their silver complexes have been measured pH-potentiometrically in a 75% (v/v) mixture of dioxane/water11. The influence of the substituents R1 and R2 on pKa and lgf n values has been discussed11. 

For a given aqueous solution containing known concentrations of a metal ion and ligand L, it may have been found that only one coordination complex of known formula is present in solution. If this is the case, then the stability constant for this complex can be obtained directly from a determination of the concentration of uncomplexed L or complexed in that solution. Such determinations can be made by polarographic or potentiometric measurements (if a suitable reversible electrode exists), by pH measurements (if the ligand is the conjugate base of a weak acid), or by ion-exchange, spectrophotometric (i.e. observation of electronic spectra and use of the Beer-Lambert Law), NMR spectroscopic or distribution methods. 

Examples of linked cyclic amine systems have been known for a considerable time. For example, the s3mthesis of the bis-linked system (1 n = 2), consisting of two potentially tridentate macrocycles, was reported in 1977 (8). Potentiometric measurements have been employed to obtain the stability constants for the 1 1 complexes of this ligand system with Mn(II), Ni(II), Co(II), Zn(II), and Cd(II). All these metals are bound strongly, with the binding constants var5dng from a log Aml value of 15.0 for the Mn(II) species to 21.5 for the Ni(II) species. Although the respective structures are not... 

Potentiometric H2O f = 30.0 02 I = 0.01 (KCl) N2 atmosphere Doluisio JT and Martin AN, Metal complexation of die tetracycline hydrochlorides, /. Med. Chem., 6, 16-20 (1963). NB Metal-free solutions of the tetracycline were titrated with standard NaOH solution and the pH measured. No details given of the pH meter calibration. Metal stability constants determined from identical titrations in the presence of varying concentrations of nickel(II), zinc(II) or copper(II) ions. Foye 1 N K Avery W G... 

In a typical stability constant determination, an acidified mixture is titrated with alkali after each addition, the pH is measured to produce a so-called potentiometric titration curve. 

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