Conditional Stability Constants
The introduction of conditional stability constants permits us, from a theoretical standpoint, to treat a complexation equilibrium to which are superimposed other equilibria of any nature as if it were alone, at least in some conditions. Conditional stabi
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Conditional Stability Constants
The introduction of conditional stability constants permits us, from a theoretical standpoint, to treat a complexation equilibrium to which are superimposed other equilibria of any nature as if it were alone, at least in some conditions. Conditional stability constants were introduced by G. Schwarzenbach 1957 to take into account acido-basic phenomena occurring simultaneously with complexation phenomena. The first case for which conditional stability constants were introduced concerned the titration of some metallic ions with EDTA tetraanion. The scope of their application scope was extended to other parasitic phenomena by A. Ringbom 1959.
26.1
Species Existing in Solution When a Metallic Ion Is Titrated with EDTA
In a typical titration of a metal cation with EDTA tetraanion, the solution in the reaction vessel, among other species, contains the formed complex [MEDTA](n−4)+ , free remaining metallic ions Mn+ , hydroxo complexes resulting from their hydrolysis, and some complexes formed with a possibly present buffer. Usually, the latter are those formed with ammonia and with acids-alcohols (see Chap. 28). We have become aware of the complexity of the titrand solution’s composition and of the difficulty in calculating the different species’ concentrations after all the reactions in the reaction vessel have taken place. It indeed appears to be a true challenge. Fortunately, the formation of polynuclear complexes, which would complicate the calculations even further, may be neglected since the concentrations of these derivatives are usually very weak close to the equivalence point. This is due to the strong complexing power of EDTA. But the titrand solution also contains other species, which this time come from the addition of the titrant. They are the protonated forms of EDTA: HEDTA3− , H2 EDTA2− , H3 EDTA− , and H4 EDTA. The complex under study, [MEDTA](n−4)+ , may possibly capture a proton or a hydroxide ion, therein exhibiting a basic or acidic character. Finally, we shall see that a secondary complexing agent may be added (see Chap. 28). This once more complicates our calculations.
J.-L. Burgot, Ionic Equilibria in Analytical Chemistry, DOI 10.1007/978-1-4419-8382-4_26, © Springer Science+Business Media, LLC 2012
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26 Conditional Stability Constants grading : EDTA4–, HEDTA3–, H2EDTA2–, H3EDTA–, H4EDTA
NH4+/NH3
M, [M(OH)], [M(OH)2], etc. [M(NH3)], [M(NH3)2], etc. [MEDTA], [M(H)EDTA], [M(OH),EDTA]
Fig. 26.1 Various species that may be present in the titration of a metallic ion M with EDTA or with its anions (the possible presence of a secondary complexing agent is not mentioned)
26.2
Conditional Constants, Parasitic Reaction Coefficients, and Apparent Concentrations
Using conditional constants permits us easily and rapidly to quantitatively predict the course of complexes’ formation reactions that would occur in the experimental conditions depicted in the previous section. However, the prediction can be performed only at the cost of some approximations, which can
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