Stability potentiometry

The complexation of other mixed oxa aza macrocycles has been studied, and protonation and stability constants of the zinc complexes of macrocycles l,4,10,13-tetraoxa-7,16-diazacycloocta-decane-7,16-bis(malonate), the -7-malonate derivative and -7,16-bis(methylacetate) derivative have been determined by potentiometry at a 1 1 ligand-to-metal ratio.730... 

Complexation of Cd with a series of polyamine macrocycles, but also related open-chain polyamines, comprising or attached to the 2,2 -bipyridine (bipy) and 1,10-phenanthroline (phen) moieties, has been studied by combined UV/vis spectrometry and potentiometry.24 Formation constants and distribution diagrams of the species present have been evaluated. As a result the thermodynamic stabilities, i.e., the formation constants, are lower for the bipy- and phen-contain-ing ligands than those for Cd complexes with aliphatic oligoaza macrocycles containing the same number of N donors. The probable reason is loss of flexibility of the ligands caused by the size and stiffness of the inserted heteroaromatic moieties. 

Stepwise formation constants have been determined in the system Hg2+/Cl-/diethylenetriamine (dien) and related systems by potentiometry. Thermodynamic parameters have been calculated and the contribution of the entropy term to complex stability discussed.208... 

In order to determine the stability constants for a series of complexes in solution, we must determine the concentrations of several species. Moreover, we must then solve a rather complex set of equations to evaluate the stability constants. There are several experimental techniques that are frequently employed for determining the concentrations of the complexes. For example, spectrophotometry, polarography, solubility measurements, or potentiometry may be used, but the choice of experimental method is based on the nature of the complexes being studied. Basically, however, we proceed as follows. A parameter is defined as the average number of bound ligands per metal ion, N, which is expressed as... 

These equilibria have been studied by various methods of which potentiometry and 51V NMR spectroscopy proved to be particularly useful for the identification of species and the determination of stability constants (8-10,13-16,18,20,22,24). Values obtained for stability constants by different research groups under the same conditions usually show good agreement, e.g., for 0.60 M NaCl medium (Table II). In some cases so-called Bronsted (or mixed) constants (25) are... 

In this work, we have chosen several systems stabilized through hydrogen bonds. The homopolymer is a polybase, i.e. PEO, PVME or PVP, and the copolymer is polyacrylic acid with various degrees of neutralization a, in which the acrylates are the non active groups. Complex formation is studied by potentiometry (because complexation induces a variation of the solution pH) and by viscometry and polarized luminescence which respectively give information about the macroscopic and local structure of the complex in solution. The influence of parameters such as the degree of neutralization of PAA a, the concentration ratio r - [polybase]/[PAA], the concentration and the molecular weight of polymers is examined. 

In the Ba2+ complex with (145), two anions coordinate to the cation in different ways (Figure 32b). The metal ion sits primarily in a cavity provided by one of the anions and is six-coordinated by two ether, two hydroxy, one keto and one carboxylate oxygen atoms. A nine-fold coordination is completed by further coordination to two oxygen atoms from the second anion and a water molecule. 73 A review of the structures of polyether antibiotic complexes is available and includes a compilation of structural data.372 The stoichiometries of alkali and alkaline earth complexes of (145) in methanol, have been determined potentiometri-cally and show 1 1 neutral complexes for the alkali metal cations, and high stability 1 1 (charged) and 1 2 (neutral) complexes for the alkaline earth cations.574... 

Acetonitrile interacts with the d10 metal ions Cu1 and Ag1 to form solvated species of marked stability. This stability has been used in potentiometry where the Ag, 0.01 M AgN03 couple in acetonitrile has been recommended as a reversible reference electrode.154... 

Maltodextrins (dextrose equivalent (DE) 4.0-7.0, 13.0-17.0 and 16.5-19.5) are proposed as novel chiral selectors for the construction of EPMEs for S-captopril assay [36]. The EPMEs can be used reliably for the assay of S-captopril as raw material and from pharmaceutical formulations as Novocaptopril tablets, using direct potentiometry. The best response was obtained when maltodextrin with higher DE was used for the electrode s construction. The best enantioselectivity and stability in time was achieved for the lower DE maltodextrin. L-Proline was found to be the main interferent for all proposed electrodes. The surface of the electrodes can be regenerated by simply polishing, obtaining a fresh surface ready to be used in a new assay. 

Cuillerdier compared the radiolytic stability of several malonamides [(C4H9(CH3) NCO)2CHRy ] in t-butylbenzene in contact with nitric acid as a function of the central alkyl chain R" (3.3 kGy h1, 40°C) (215). The order of stability, established on the total concentration of amide functions measured by potentiometry in nonaque-ous medium, was H < C2H5 < C2H4OC6H13 = C2H4OC2H4OC6H13. The authors then proposed the following tendencies ... 

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 ... 

Twenty years ago the main applications of electrochemistry were trace-metal analysis (polarography and anodic stripping voltammetry) and selective-ion assay (pH, pNa, pK via potentiometry). A secondary focus was the use of voltammetry to characterize transition-metal coordination complexes (metal-ligand stoichiometry, stability constants, and oxidation-reduction thermodynamics). With the commercial development of (1) low-cost, reliable poten-tiostats (2) pure, inert glassy-carbon electrodes and (3) ultrapure, dry aptotic solvents, molecular characterization via electrochemical methodologies has become accessible to nonspecialists (analogous to carbon-13 NMR and GC/MS). 

The oxidation of propylene oxide on porous polycrystalline Ag films supported on stabilized zirconia was studied in a CSTR at temperatures between 240 and 400°C and atmospheric total pressure. The technique of solid electrolyte potentiometry (SEP) was used to monitor the chemical potential of oxygen adsorbed on the catalyst surface. The steady state kinetic and potentiometric results are consistent with a Langmuir-Hinshelwood mechanism. However over a wide range of temperature and gaseous composition both the reaction rate and the surface oxygen activity were found to exhibit self-sustained isothermal oscillations. The limit cycles can be understood assuming that adsorbed propylene oxide undergoes both oxidation to CO2 and H2O as well as conversion to an adsorbed polymeric residue. A dynamic model based on the above assumption explains qualitatively the experimental observations. 

Stability constant — is the - equilibrium constant of formation of a - complex. The reciprocal quantity is called instability constant. Extensive compilations of various kinds of stability constants are available [i—iv]. Stability constants are reported for a given ionic strength (or extrapolated to zero ionic strength), and pH. In many cases, stability constants can be determined with the help of electrochemical techniques, e.g., -> chronopotentiometry, -> potentiometry, -> polarogra-phy, and - voltammetry, provided that the systems exhibit -> electrochemical reversibility, and some other prerequisites are fulfilled [v]. See also -> conditional equilibrium constants. 

In this section it will be useful to discuss some common experimental methods used in the determination of stability constants of rare earth complexes. Ethylenediamine tetraacetate anion (EDTA) is a hexadentate and forms complexes with trivalent rare earth ion readily. The pioneering studies of Schwarzenbach [4] on the determination of stability constants of rare earth EDTA complexes by potentiometry and polarography can be considered to illustrate the principles involved in the determination of stability constants. 

Solvent extraction, potentiometry, and calorimetry have been used to determine the thermodynamic parameters of the formation of the monofluoride complex of the trivalent lanthanide ions at 25°C. and an ionic strength of IM (NaClOj ). The enthalpies were all endothermic, ranging from 4.0 to 9.5 Kcal./mole consequently, the large, positive entropies, ranging from 25 to 48 cal./°C./mole, explain the high stability constants. This large entropy results from the decrease in overall water structure when the fluoride ion is complexed. The difference in the enthalpies of formation of LnF and LnAc " can possibly be explained by a difference in covalence for Ln-F and Ln-O bonds. 

Stability constants of 1 1 and 2 1 complexes with diethylenetriamine (dien) have been determined by potentiometry. ... 

It should be noted that some data on complexation of transuranium elements with various N-donor ligands, such as azide ion, isothiocyanate ion, amine-N-polycarboxylic and heterocyclic acids, 8-hydroxyquinoline and its derivatives, were published as early as 1950-1970 [42-77], The quantitative parameters of complexation (number and composition of complexes, stability constants, thermodynamic parameters) were determined using accessible at that time methods (spectrophotometry, potentiometry, ion exchange, solvent extraction). It was shown that transuranium elements in different oxidation states can form complexes with N-donor ligands. However, only presumptive conclusions regarding structure of complexes were drawn at that time,... 

The EDTA analog, N,N,N ,N -tetrakis(2-pyridylmethyl)ethylenediamine (TPEN), forms stable complexes with lanthanides Ln(TPEN) as shown in the work [111]. The stability constant for Am complex of the similar composition was calculated from potentiometry titration in the work [33]. The [M(TPEN)] complex is the major and the [M(OH)] is the minor components from the... 

Octadentate anion DTPA forms stable complexes with Np in solution. In contrast to EDTA, the absorption band of [Np(DTPA)] with maximum at 983 nm is very broad and has low intensity. Based on the spectrophotometry stability constant logy5i=30.33 0.12 was calculated [47,74]. In the work [122], the similar value, logy5i=29.29 0.02, was obtained using the same technique. Ion exchange measurement yields the value logy5i=30.96 [53]. Potentiometry titration gives very similar value for Pu(lV), logy5i=29.49 0.10 [123]. 

The complexation of pentavalent neptunium with pyridinecarboxylic acids was investigated by different methods including spectrophotometry, potentiometry and solvent extraction [18,42,101,127-130]. Depending on the technique, complexation model used in calculations and experimental conditions, different values of stability constants were obtained (Table 8). 

Complexation of Np with NCS has been extensively studied using different experimental techniques. In the work [57], stability constants were calculated from spectrophotometry and potentiometry data for ionic strength 1=9 (Table 12). The stability constant logy0i=O.32 was obtained using solvent extraction technique 1=2 M) [60], Since the isothiocyanate ion is able to reduce Np(V), the special attention was given in this study to maintain the Np oxidation state. 

GOx Enzyme-linked field effecl transistor (ENFET) PAMAM(G4)-Pt LbL Potentiometry vs SCE 10 mM Phosphate buffer and 100 mM NaCl at pH 7.4 Linear range - 0.25-2.0 mM Detection limit 0.15 mM Response time = 200s Repetability for 5 measurements storage stability measured for 30 days 12.5 mM/mV [133]... 

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