Stability constants apparent/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. 

From a plot of the internalisation flux against the metal concentration in the bulk solution, it is possible to obtain a value of the Michaelis-Menten constant, Am and a maximum value of the internalisation flux, /max (equation (35)). Under the assumption that kd kml for a nonlimiting diffusive flux, the apparent stability constant for the adsorption at sensitive sites, As, can be calculated from the inverse of the Michaelis-Menten constant (i.e. A 1 = As = kf /kd). The use of thermodynamic constants from flux measurements can be problematic due to both practical and theoretical (see Chapter 4) limitations, including a bias in the values due to nonequilibrium conditions, difficulties in separating bound from free solute or the use of incorrect model assumptions [187,188],... 

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. 

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

Kramer, C.3.M., 1986. Apparent copper complexation capacity and conditional stability constants in north atlantic waters and their relation to plankton activity. Mar. Chem. 17 (in press). 

Conditional (apparent) equilibrium constants - Equilibrium constants that are determined for experimental conditions that deviate from the standard conditions used by convention in - thermodynamics. Frequently, the conditional equilibrium conditions refer to - concentrations, and not to - activities, and in many cases they also refer to overall concentrations of certain species. Thus, the formal potential, i.e., the conditional equilibrium constant of an electrochemical equilibrium, of iron(II)/iron(III) may refer to the ratio of the overall concentrations of the two redox forms. In the case of complex equilibria, the conditional - stability constant of a metal ion Mm+ with a ligand L" refers to the overall concentration of all complex species of Mm+ other than Conditional equilibrium... 

Our results show that conditional stability constants for binding of copper by natural organic ligands are highly dependent on the pH at which the measurement was conducted and the portion of the titration curve which was examined. Published stability constants for fulvic acids often vary considerably and Cheam (22) has reported that these apparent differences can be reasonably... 

Figure 3. The urea concentration dependence of the natural logarithm of the apparent residue stability constants. For clarity a single line is shown for groups of residues that exhibit similar behavior. These calculations simulate the energetic conditions at 25°C where higher resolution between protection factors is expected (adapted from (Hilser Freire, 1996a)).

Knowledge of the stability constants of complexes is not enough to predict their masking possibilities. The stability constants of complexes are apparent constants which vary with the pH and with the concentration of other species capable of complexation (competitive reactions).The effect of these factors is taken into account in the conditional stability constants of complexes [91]. 

Kramer, C. J. M. (1986), Apparent Copper Complexation Capacity and Conditional Stability Constants in North Atlantic Waters, Mar. Chem. 18, 335-349. 

An 1,8-naphthyridine q-aminonitrile moiety serves both as an effective donor-acceptor array for complexation of creatinine and as an intrinsic chromophore and fluorophore. In the pH range of 4.1-4.6 the monoprotonated form apparently predominates in 70 % aqueous methanol, producing the absorption spectrum shown in Figure 14. Under these conditions creatinine exists as a mixture of protonated and unprotonated forms, since its pK is approximately 4.2 in this solvent mixture. Such proton-transfer equilibria complicate the calculation of specific stability constants, but under buffered conditions absorption and emission changes result only from complexation, not from proton transfer. As shown in Figure 14, addition of creatinine to a buffered solution decreases the intensity of the 442 nm absorption band attributed to the protonated receptor. Creatinine complexation also quenches the yellow-green fluorescence of the protonated receptor and titration experiments in progress may yield the effective stability constant of the complex. This receptor exemplifies the manner in which intrinsic chromophores and fluorophores may be incorporated into hosts for reversible complexation of clinically important analytes (26). 

Kmi is the apparent, conditional, or effective stability constant. It is not a real constant since it depends on experimental conditions (here pH). From eqn [5],... 

In these computations we have assumed a constant dry deposition velocity, although it is actually dependent, among other factors, on wind speed and atmospheric stability. In particular, the dry deposition velocity is typically smaller in stable conditions. However, the loss of pollutant from the plume depends also on the vertical concentration distribution. For stable conditions, the plume is substantially more shallow dry deposition therefore causes an increased decrease of concentration at larger distances, compared with the corresponding case in neutral stability. The apparent convergence of the concentration curves for the two stability classes is fortuitous. 

Apparent or conditional stability constants, j3 , are defined in terms of the equilibrium between a metal complex and its components, except that free ligand concentration is replaced by total concentration of all ligand species not actually complexed to the metal, and the free metal ion term includes hydrolysed metal ion and metal ion bound to other complexing species. The relation is... 

Often, stability constants are determined for experimental conditions that deviate from the standard conditions used by convention in thermodynamics and refer to concentrations, but not to activities. Such conditional (apparent) stability constants are used whenever the activity coefficients and concentrations of the species are not known or not accessible, or when these simplifications are sufficient to treat certain equilibrium. As outlined next, 0 values expressed in concentration terms, that is. 

Because soil organic substances are extremely heterogeneous, stability constants for Fe and Al are most often reported as apparent or conditional stability constants that are functions of pH, ionic strength, and concentrations of metal and organic material (Weber 1988 Stevenson and Vance 1989). Our understanding of complexation is limited not only because of the complicated structure of humic substances, but also because they contain multiple complexing sites. 

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