Complex conditional stability

Ring substituents show enhanced reactivity towards nucleophilic substitution, relative to the unoxidized systems, with substituents a to the fV-oxide showing greater reactivity than those in the /3-position. In the case of quinoxalines and phenazines the degree of labilization of a given substituent is dependent on whether the intermediate addition complex is stabilized by mesomeric interactions and this is easily predicted from valence bond considerations. 2-Chloropyrazine 1-oxide is readily converted into 2-hydroxypyrazine 1-oxide (l-hydroxy-2(l//)-pyrazinone) (55) on treatment with dilute aqueous sodium hydroxide (63G339), whereas both 2,3-dichloropyrazine and 3-chloropyrazine 1-oxide are stable under these conditions. This reaction is of particular importance in the preparation of pyrazine-based hydroxamic acids which have antibiotic properties. 

In equation (q) only the fully ionised form of EDTA, i.e. the ion Y4 , has been taken into account, but at low pH values the species HY3, H2Y2, H3 Y and even undissociated H4Y may well be present in other words, only a part of the EDTA uncombined with metal may be present as Y4. Further, in equation (q) the metal ion M"+ is assumed to be uncomplexed, i.e. in aqueous solution it is simply present as the hydrated ion. If, however, the solution also contains substances other than EDTA which can complex with the metal ion, then the whole of this ion uncombined with EDTA may no longer be present as the simple hydrated ion. Thus, in practice, the stability of metal-EDTA complexes may be altered (a) by variation in pH and (b) by the presence of other complexing agents. The stability constant of the EDTA complex will then be different from the value recorded for a specified pH in pure aqueous solution the value recorded for the new conditions is termed the apparent or conditional stability constant. It is clearly necessary to examine the effect of these two factors in some detail. 

To predict the complexation behavior of ligands at physiological pH, the protonation constants of the ligand have to be considered by using conditional stability constants. Conditional stability... 

Correlation between complex stability and in vivo toxicity has always been an important question in contrast agent development. Conditional stability constants were often correlated to the selectivity of a given ligand for Gdm over endogenous metals, such as Zn11 or Cu11 (134), and therefore they are widely used to compare the behavior of different complexes at physiological pH. 

Table XIX contains stability constants for complexes of Ca2+ and of several other M2+ ions with a selection of phosphonate and nucleotide ligands (681,687-695). There is considerably more published information, especially on ATP (and, to a lesser extent, ADP and AMP) complexes at various pHs, ionic strengths, and temperatures (229,696,697), and on phosphonates (688) and bisphosphonates (688,698). The metal-ion binding properties of cytidine have been considered in detail in relation to stability constant determinations for its Ca2+ complex and complexes of seven other M2+ cations (232), and for ternary M21 -cytidine-amino acid and -oxalate complexes (699). Stability constant data for Ca2+ complexes of the nucleosides cytidine and uridine, the nucleoside bases adenine, cytosine, uracil, and thymine, and the 5 -monophosphates of adenosine, cytidine, thymidine, and uridine, have been listed along with values for analogous complexes of a wide range of other metal ions (700). Unfortunately comparisons are sometimes precluded by significant differences in experimental conditions.

Ruzic [278 ] considered the theoretical aspects of the direct titration of copper in seawaters and the information this technique provides regarding copper speciation. The method is based on a graph of the ratio between the free and bound metal concentration versus the free metal concentration. The application of this method, which is based on a 1 1 complex formation model, is discussed with respect to trace metal speciation in natural waters. Procedures for interpretation of experimental results are proposed for those cases in which two types of complexes with different conditional stability constants are formed, or om which the metal is adsorbed on colloidal particles. The advantages of the method in comparison with earlier methods are presented theoretically and illustrated with some experiments on copper (II) in seawater. The limitations of the method are also discussed. 

Ion-selective electrodes have been used to determine the stability constants for the complexation of copper II ions with soil fulvic acids [4], Two classes of binding sites were found with conditional stability constants of about 1 xf 06 and 8xl03. 

It is operationally difficult to distinguish between dissolved and colloidally dispersed substances. For example, colloidal metal-ion precipitates occasionally have particle sizes smaller than 100 A, sufficiently small to pass through a membrane filter, and organic substances can exist as a stable colloidal suspension. Information on the types of species encountered under different chemical conditions (type of complexes, their stabilities, rate of formation) is a prerequisite to better understanding of the transformation in properties of toxic chemicals in a water body. 

Rudd, T., Sterritt, R. M. Lester, J. N. (1984b). Formation and conditional stability constants of complexes formed between heavy metals and bacterial extracellular polymers. Water Research, 18, 379-84. 

Finally for the formation of the complex ML under practical conditions we can define an overall conditional stability constant by... 

Stone writes this reaction as occurring with Ni2+ ions, but it is more likely that if it takes place, the small number of Ni3+, which are almost certainly present, are involved.) This extraction reaction would become rate-determining at higher temperatures, say, > 160°. Under the conditions used by Schwab, the irreversible formation of (COl /n7)(er/D7) by direct reaction of gaseous CO would be rate-determining and would be influenced in the way observed by him if the complex were stabilized by neighboring impurity centers. This explanation of the discrepancy is not impossible but... 

To illustrate one type of speciation research, i.e. the determination of the apparent complexation capacity for copper (CCqu) and the conditional stability constant (K1), examples are given for three marine areas, viz. the Scheldt estuary, the Southern Bight of the North Sea and the open north Atlantic Ocean. A hypothetical model is presented giving the complexation capacity across the land-sea boundery from river to ocean. 

Complexation capacity and conditional stability constants in marine waters. 

Complexation Capacity (CC), conditional stability constant (K1), temperature, pH, oxygen and suspended matter content in the river Scheldt estuary at different salinities. 

The conditional stability constants calculated for the different salinity ranges are given in Table 5. It looks as if in the more saline samples complexes with lower K are formed. 

Fig. 5. A) Apparent copper complexation capacity (CC U) in nM Cu2+ B) Conditional stability constants (K ) presented as log K. ...

Mean values of complexation capacity (CC), conditional stability constants (K1) and rate constants (kf ) for different area in north Atlantic waters. 

The conditional stability constants found for the different marine areas are not very different. A trend can be observed of K increasing towards the open ocean. Thus relative stronger complexes are formed in the open ocean than in estuarine and coastal waters. Comparable values have been observed by other authors, using the same technique. Application of different techniques however, can result in several orders of magnitude higher K (Kramer, 1986). This cannot only be attributed to geographical differences. Another factor is, that different techniques have different sensitivities for the various species of the element concerned. 

Hirose, K., Dokiya,Y. and Sugimura, Y., 1982. Determination of conditional stability constants of organic copper and zinc complexes dissolved in seawater using ligand exchange method with EDTA. Mar. Chem., 11 343-354. 

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