Conditional Stability Constants Of Complexes Research Articles

Pyro- and Tripolyphosphates and Citrates as the Complexing Agents for Micronutrients in Liquid Fertilizers

The conditional stability constants of tripolyphosphate, pyrophosphate, and citrate complexes of Zn2+, Mn2+, Co2+, and Fe2+ as function of pH have been determined. The polarographic method was employed in these studies, and the ionic strength of the solution used was 0.5 mol/dm3. P3O105- complexes with the respective micronutrients were found to show higher conditional stability constants more often than P2O74- and citrate complexes. In comparison with complexes containing other cations, conditional stability constants of complexes with zinc studied in this work showed higher values. Distinct from other micronutrients, Cu2+ ions were observed as significantly accelerating the hydrolysis process of P3O105- in a range of pH from 4.5 to 10.0. Hence, the degree of hydrolysis increased together with the pH value of their solutions. In the case of other micronutrients, the degree of hydrolysis of P3O105- was lower for higher pH values of their solutions. Additionally, it was stated that the structure of Fe2+ tripolyphosphate complexes differs from complexes with other central ions. The preparation conditions of liquid fertilizers stable over long periods of time, containing macroelements and micronutrients in the complexing form by polyphosphates, have been elaborated. Stability of the solutions depended on the content of individual components, pH, and kind of complexing agent. It remained closely related to stability constants of complexes.

Conditional stability constants of metal complexes of organic ligands in sea water: past and present, and a simple coordination chemistry model

The conditional stability constants of complexes of organic ligands with trace metals (Cu and Zn) that have been determined by solution chemistry techniques since 1980 are summarized. The present common understanding on the chemical speciation of trace metals is that > 99% of copper dissolved in sea water is complexed with natural organic ligands; more than strongly binding organic ligand exists in sea water. A simple coordination model, including two non-specific discrete strong organic ligands such as EDTA, is introduced to understand the complex features of recent metal speciation. The model, consistent with the result of the ligand titration, permits the wide range of conditional stability constants determined by metal titrations and the present concentration levels of several trace metals in sea water to be explained. These findings suggest that organic metal complexes play a significant role in the geochemical cycle of reactive trace metals.

Comparison of differential pulse anodic stripping voltammetry and spectrofluorometry for determination of complexes between copper and organic matter in interstitial waters extracted from marine sediments

Differential pulse anodic stripping voltammetry (DPASV) and fluorescence quenching are methods that allow measurement of complexing capacities and conditional stability constants of complexes in natural waters. Whereas the electrochemical measured signal reflects the presence of labile copper in solution, spectrofluorometry leads to an estimation of the uncomplexed fluorescent ligands. Interstitial waters extracted from estuarine and unpolluted marine sediments, as well as from sediments near urban outfall, contain high concentrations of organic matter, and their complexing capacity, as found by DPASV, often exceeds 1 μmol l −1. Furthermore, stripping curves are complex and the determination of stability constants is possible only under specific conditions. We have compared both methods by working on fulvic acid solutions and natural interstitial waters. Some correlation between the results of the two methods has been found, and it has been concluded that these methods are favourable for complementary study of interstitial waters with high organic matter content. Also, this gives a better interpretation of spectrofluorometric results versus dissolved organic carbon in natural waters.

Variation of the metal complexing ability of humic acids with coal rank

The variation of the metal complexing ability of humic acids with the rank of the parent coal was investigated by determining the conditional stability constants of complexes formed between cupric ions and humic acids from several coals. Humic acids were extracted from four Spanish coals varying in rank from peat to subbituminous coal. The stability constants were determined at pH 5.0 and μ = 0.1 by a modified version of Schubert's ion exchange method. The stability of the metal-humic acid bonds decreases with the rank of parent coal. The stoichiometry of the complexes tends to 1:1 with increasing coalification of humic acids. The influence of oxidative conditions of extraction on the conditional stability constants of complexes was also examined and it was found that they do not affect significantly the elemental composition or the metal complexing ability of regenerated humic acids.

Determination of the complexing capacity and conditional stability constants of complexes of copper(II) with natural organic ligands in seawater by cathodic stripping voltammetry of copper-catechol complex ions

A new method is proposed for the determination of complexing capacities and conditional stability constants for complexes of copper(II) with dissolved organic ligands in seawater. This method is based on ligand competition by the added ligand catechol for free metal ions. The concentration of copper-catechol complex ions is measured with great sensitivity by cathodic stripping voltammetry. The concentration of the free copper ion is calculated from the concentration of copper-catechol complex ions. Ligand concentrations and conditional stability constants are obtained from a titration of the ligands with copper. Two techniques for treatment of the data are compared. A seawater sample, originating from open oceanic conditions, is analysed and two complexing ligands were detected, having concentrations of 1.1 × 10 −8 and 3.3 × 10 −8 M, and conditional stability constants (log K′ CuL) of 12.2 and 10.2, respectively.

Formation and conditional stability constants of complexes formed between heavy metals and bacterial extracellular polymers

The strength and nature of the binding of heavy metal ions to bacterial extracellular polymers has been investigated. The conditional stability constants ( K t ) for complexes formed between extracted Klebsiella aerogenes polymer and copper, cadmium, cobalt and nickel were determined, using a gel chromatographic technique, and log K t values of 7.69, 5.16, 5.48 and 5.49 respectively were obtained. Adsorption isotherms constructed for copper, cadmium and cobalt indicated that metal uptake occurred after the initial complexation capacity had been exceeded, suggesting the presence of more than one binding site, but nickel adsorption ceased when the complexation capacity was reached. Nickel was found to be associated predominantly with the soluble form of polymer, and copper and cadmium with the colloidal fraction when metals were added to the polymer simultaneously, rather than individually. The overall specific metal uptake by polymers extracted from activated sludge was approximately ten times higher than that by K. aerogenes polymer.

Complex formation of cyclic phosphate anions with bivalent cations

Complex formation of four kinds of cyclic phosphate anions, trimeta-, tetrameta-, hexameta- and octametaphosphate, with calcium, strontium, zinc and cobalt(II) ion has been investigated. Conditional stability constants of complexes of these cyclic phosphate anions with the metal ions have been determined by a cation exchange method. For the calcium complexes, thermodynamic stability constants have been estimated by extrapolating the conditional stability constants determined at various ionic strengths to infinite dilution. From variations in the constants with the anion charges it is considered that the complexes are formed as a consequence of electrostatic interactions. Fuoss' and Bjerrum's equations based on electrostatic theory have been used to estimate interionic distances in the calcium complexes.

Photometric determination of composition and conditional stability constants of complexes A mB n

The method of proportional absorbances has been generalized and applied both to the determination of absolute values of complex composition A mB n and of its conditional stability constant. The method of continuous variation or of molar ratio is suitable for determination of absolute values of the composition of the very stable complex A mB n.