Stability constant, conditional

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. 

Concentrated acids D. of strength, (ti) 296 Concentration of aqueous solutions common acids and ammonia, (T) 829 Concentration cells 63, 549 overpotential, 506 polarisation, 506 e.m.f. of, 506 Condenser current 595 Conditional stability constant 59 Conductance 519... 

K3 is the conditional stability constant for the association of plutonium with the solid adsorber. 

Figure 10.5 Effect of pH on the conditional stability constants at 25 °C of Fe(m) and Fe(n) chelates of aminopolycarboxylic acids [20]...

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... 

This conditional stability constant, K, is related to the thermodynamic stability constant (KM]J by (133) ... 

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. 

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. 

Conditional stability constants have been determined for cadmium binding to humic acid in freshwater, log Kk 6.5 [27], which may be comparable to binding to humic acid coated particles. The experiments demonstrated the importance of cadmium uptake from particles rather than from the dissolved phase. The authors recognised that the overall conclusion was similar to previous studies [28], but there remain inconsistencies in the uptake levels which may be related to the heterogeneity of the systems. Uptake from the intestine into the mucosal cells was not investigated. It was presumed that the material was digested extracellularly by hydrolytic enzymes and the released metal was taken up by facilitated diffusion. 

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. 

These constants have the rank of conditional stability constants. For exact considerations we need to correct by a coulombic term for electrostatic interaction (see Chapter 4). 

In order to answer these questions, laboratory experiments were compared with field measurements in a local river (Muller and Sigg, 1990). Laboratory experiments with the centrifuged particulate matter, isolated from a river, were used. The results obtained by titrating a metal solution with a suspension of the centrifuged particles were interpreted in terms of binding capacities and conditional stability constants (Fig. 11.5). 

Van den Berg, C. M. G., and J. R. Kramer (1979), "Conditional Stability Constants for Copper Ions with Ligands in Natural Waters", in E. Jenne, Ed., On Chemical Modeling Speciation, Sorption, Solubility and Kinetics in Aqueous Systems, ACS Symp. Series. 

This value should be treated with caution, as it is a conditional stability constant determined in a medium modeling blood plasma (Martin, R. B. Savory, J. Brown, S. Bertholf, R. L. Wills, M. R. Clin. Chem. 1987, 33, 405-107). 

Figure 10.11 Conditional stability constant graphs for (a) ethylenediaminedisuccinic acid [(S,S)-EDS], (b) ethylenediaminetetraacetic acid (EDTA), (c) iminodisuccinic acid (IDS), (d) glutamic acid N,N-diacetic acid (CLDA), and (e) methylglycinediacetic acid (MGDA).

Having determined the metal ion profile, the question is, which metal ions are important enough to the process to warrant control The role of metal ions and the performance of chelants will vary markedly with pH and also with temperature, so it is vital to know the conditions under which a chelant will be used and how this changes with time and/or the reactions taking place. Here reference can be made back to the conditional stability constant graphs (Figure 10.11) to determine if a chelant will be effective at a particular pH. Other chemicals or materials can also interact with metal ions, introduce metal ions into a system, or react with metal ions, so it is important to know what else is in the reaction/ system. Can the chelant cope with the full range of conditions to which it will be exposed ... 

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... 

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. 

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