Hydroxide, formation constants with

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... 

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. 

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]

Neutral and Polymeric Aluminum and Iron. The association constants and enthalpies of aluminum and iron hydroxides have been evaluated by comparing the critically selected data of Baes and Mesmer (51) with that of R. M. Siebert and C. L. Christ (personal communication, 1976). Differences between the two data sets are negligible and the final selection was from Baes and Mesmer (51) because data on more complexes are found there. Important new species added to tjjie model are the polynuclear complexes Fe2(0H)2 and Fes(OH). Some controversy has arisen over the existence of Fe(0H) and A1(0H)3. Baes and Mesmer (51) have indicated that although the formation constant of A1(0H)3 is only known from one measurement (52) and has a large uncertainty, it is real, with a log K < -15.0 for the reaction... 

The similarities between Ga, In and Fe " are manifest in vivo by the binding of all three ions to the serum protein transferrin, Tf, normally used for iron transpQit. The formation constant for the Ga -Tf complex has been found to be and Welch has calculated values for the equilibrium constants for the exchange of trivalent metal ions between EDTA or DTPA and Tf as shown in Table 19. These figures show that only the DTPA complex of Ga is stable with respect to metal exchange with Tf. Table 19 also shows values for the equilibrium exchange reaction between Tf and hydroxide ion. These indicate that, while the indium-Tf complex should be stable to hydrolysis in vivo, in the long term the insoluble Ga(OH)3 should form from the... 

Equation (5.2-2) is a useful form for the EDTA complexes of metal ions such as magnesium and calcium that are rather strong bases, and therefore have little tendency to form hydroxides. However, many other metal ions that can be titrated with EDTA will often form hydroxy complexes, and these are usually titrated in the presence of complexing agents that keep hydroxide formation at bay. In that case, the expression for the conditional formation constant must take such complex formation of the metal ion into account as well, and then reads Kf = KfaY0 aM0, where aM0 is the fraction of... 

Any metal ion that has an EDTA formation constant higher than calcium or magnesium will interfere. Cyanide complexes strongly with copper, cobalt, nickel, zinc, and ferrous iron. Hydroxylamine or ascorbic acid is added to reduce iron to the ferrous state. If the solution is buffered to pH 10 before the indicator is added, then iron will not interfere because it precipitates as the hydroxide before it can react with the indicator or the EDTA. 

The quadridentate ligands A,iV-dihydroxyethylglycine and N-hydroxyethyliminodiacetate(HIMDA) form 2 1 chelate zirconium complexes which are stable with respect to hydroxide precipitation even up to pH 10. These quadridentate ligands involve the bonding of the alkoxide groups at the higher pH values. The formation constant for the 1 1 HIMDA-hafnium complex in 0.123 M HCIO4 is log A = 14.6 (170, 305). 

This reference is an extensive monographic Ph.D. thesis on the formation of aqueous carbonate complexes and carbonate solids with tetravalent metals (Ce(lV), Th(lV), Hf(IV) and Zr(IV)). It includes detailed cryoscopic, conductometric, potentiometric and ion exchange data as well as solubility measurements of the metal hydroxides in the presence of carbonate and a solvent extraction study. The combined information from the various methods and the large amount of supplied data made it possible to identify Zr(C03)4 as the main complex formed and to derive a conditional formation constant, which was then extrapolated to zero ionic strength with the help of approximated SIT... 

Figure XI-5 Speciation schemes calculated with the selected formation constants for ThCCOj) and the ternary Th(lV)-hydroxide-carbonate complexes. Left side 0.1 and 0.5 M NaHC03-Na2C03 solutions of varying ionic strength (/ = [NaHCOs] + 3[Na2C03]). Right side NaHC03-Na2C03-NaCl (or NaC104) solntions of the same carbonate concentration C t = 0.1 and 0.5 M), bnt at constant ionic strength (/ = 0.5 and 2.0 M, respectively.)...

Figure XI-5 Speciation schemes calculated with the selected formation constants for Th(C03)5 and the ternary Th(lV)-hydroxide-carbonate complexes.363...

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