Hydroxides formation constants

Lind, C. J. Polarographic determination of lead hydroxide formation constants at low ionic strength. Environ. Sci. Technol. (in press). 

Determine the stoichiometry of the Pb-hydroxide complex and its formation constant. 

Hexa.cya.no Complexes. Ferrocyanide [13408-63 ] (hexakiscyanoferrate-(4—)), (Fe(CN) ) , is formed by reaction of iron(II) salts with excess aqueous cyanide. The reaction results in the release of 360 kJ/mol (86 kcal/mol) of heat. The thermodynamic stabiUty of the anion accounts for the success of the original method of synthesis, fusing nitrogenous animal residues (blood, horn, hides, etc) with iron and potassium carbonate. Chemical or electrolytic oxidation of the complex ion affords ferricyanide [13408-62-3] (hexakiscyanoferrate(3—)), [Fe(CN)g] , which has a formation constant that is larger by a factor of 10. However, hexakiscyanoferrate(3—) caimot be prepared by direct reaction of iron(III) and cyanide because significant amounts of iron(III) hydroxide also form. Hexacyanoferrate(4—) is quite inert and is nontoxic. In contrast, hexacyanoferrate(3—) is toxic because it is more labile and cyanide dissociates readily. Both complexes Hberate HCN upon addition of acids. 

Hydrolysis reactions. As the system under investigation contains not only carbonate ions but also hydroxide ions of considerable concentration, it is quite plausible that the reactions of hydrolysis and carbonate complex formation compete with each other. Since the hydrolysis reaction is not investigated separately in this experiment, the magnitude of this reaction as a function of pH is evaluated on the basis of the formation constants available in the literature (18), which are reproduced... 

K and formation constant of each hydroxide species 6m., such as °P Hi 7... 

Fig. 8. Correlation between Pearson s hardness parameter (7P) derived from gas-phase enthalpies of formation of halide compounds of Lewis acids (19), and the hardness parameter in aqueous solution (/A), derived from formation constants of fluoride and hydroxide complexes in aqueous solution (17). The Lewis acids are segregated by charge into separate correlations for monopositive ( ), dipositive (O), and tripositive ( ) cations, with a single tetrapositive ion (Zr4+, ). The /P value for Tl3+ was not reported, but the point is included in parentheses to show the relative ionicity of Tl(III) to ligand bonds. 

The complex Cu(II)2(0-BISTREN) is much more acidic than the free Cu2+ ion, by a factor of more than three log units. This is primarily due to the presence of two Cu(II) ions, because the formation constant of the Cu2(OH)+ complex is not much less than that for the Cu2(0-BISTREN) complex with hydroxide. This is not a good indication of how well two free Cu2+ ions would bind hydroxide compared to the Cu2(0-BISTREN) complex, however, since one must take into account the dilution effect operative in the chelate effect to make the comparison more realistic (90). Thus, the formation constant for the Cu2OH+ complex above applies for the standard reference state of 1 M Cu2 +. In contrast, in 10 6 M Cu2+, for example, the pH at which Cu2(OH) + would form is raised from pH 5.6 to 11.6, ignoring the fact that Cu(OH)2(s) would precipitate out long before this pH as reached. By comparison, the acidity of the Cu2(0-BISTREN) complex is not affected by dilution and would still form the hydroxide complex at pH 3.9 if present at a 10"6 M concentration. 

These data comparisons were used by Hummel Berner (2002) to estimate values for the missing formation constants of the mixed carbonate hydroxide complexes of U4+, Np4+ and Pu4+, and for various missing com-plexation constants of Np3+ and Pu3+. The estimated constants were used as limiting values in performance assessment but are not included in the Nagra/PSI TDB 01/01. 

Gallium hydroxide is amphoteric, and is a much stronger acid than aluminum hydroxide. For Ga(OH)3 the first acid dissociation constant is 1.4 x 10-7 [for Al(OH)3 the value is 2 x 10-11].1 Polymerization occurs in aqueous Ga3+ solutions to which OH- is added,533 but this tendency is less than in the case of aluminum solutions (Section 25.1.5.1). The formation constants of mononuclear hydroxo complexes of Ga, including Ga(OH)4, and the hydrolysis constants of gallium ions have been measured by a competing ligand technique.534... 

In Section 12-3 we had no auxiliary complexing ligand and we implicitly assumed that aM + = 1. In fact, metal ions react with water to form M(OH) species. Combinations of pH and metal ion in Section 12-3 were selected so that hydrolysis to M(OH) is negligible. We can find such conditions for most M2+ ions, but not for M3+ or M4+. Even in acidic solution, Fe3+ hydrolyzes to Fe(OH)2+ and Fe(OH)(,17 (Appendix I gives formation constants for hydroxide complexes.) The graph shows that aFe., is close to 1 between pH 1 and 2 (log c[Pg.240]

This equation shows that not only a high metal-ion concentration, but also a high pH, often favors the formation of higher polynuclear species, since y generally increases more rapidly than x. For many aqua metal ions, however, the precipitation of insoluble hydroxides sets an upper pH limit, so that in practice it is possible to study the oligomerization reactions only within a narrow pH region defined by the magnitude of the first acid dissociation constant of the monomeric aqua ion and the pH at which insoluble hydroxide formation occurs. 

Initially studies of metal ion-promoted hydrolysis were centred on simple monoamin esters.36,44,43 However, many of the initial investigations led to rather conflicting results. Th reactions are difficult to study due to the low formation constants of the active complexes. Mor recent measurements46 48 have provided rate constants (Table 4) which show only order c magnitude agreement however, it has been possible to establish that hydroxide ion is th predominant nucleophile at pH values of ca. 5. Higher pH values lead to precipitation of meti hydroxides. Evidence for nucleophilic attack by water has also been obtained.46"48... 

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