Oxalate, formation constants with

C18-0033. Zinc oxalate, Zn(C2 O4), is sparingly soluble in water (Zjp = 1.4 X 10 ). The Zn ion forms a tetrahedral-shaped complex with ammonia. The formation constant for the complex is 4.1 X 10. How many moles of zinc oxalate will dissolve in 1.0Lof0.200M aqueous ammonia ... 

Admixture of the reactants caused the pink colour of NpO to change to yellow, suggesting complex formation, and one mole of oxalic acid consumed four moles of oxidant to give Np(V) as the final product, identified optically, k is independent of ionic strength and is 0.012 + 0.001 sec at 25 °C ( = 1 M) with E = 15.5 kcal.mole . Breakdown of an oxalatoneptunium(VI) complex of low formation constant is presumably the mechanism. 

Estimate the variation of surface charge of a hematite suspension (same charac-teristics as that used in Example 7.2) to which various concentrations of a ligand H2U (that forms bidentate surface complexes with the Fe(III) surface groups, FelT such a ligand could be oxalate, phtalate, salicylate or serve as a simplified model for a humic acid we assume acidity constants and surface complex formation constants representative for such ligands. The problem is essentially the same as that discussed in Example 5.1. We recalculate here for pH = 6.5. 

Following earlier studies of the oxidation of formic and oxalic acids by pyridinium fluoro-, chloro-, and bromo-chromates, Banerji and co-workers have smdied the kinetics of oxidation of these acids by 2, 2Tbipyridinium chlorochromate (BPCC) to C02. The formation constant of the initially formed BPCC-formic acid complex shows little dependence on the solvent, whilst a more variable rate constant for its decomposition to products correlates well with the cation-solvating power. This indicates the formation of an electron-deficient carbon centre in the transition state, possibly due to hydride transfer in an anhydride intermediate HCOO—Cr(=0)(0H)(Cl)—O—bpyH. A cyclic intermediate complex, in which oxalic acid acts as a bidentate ligand, is proposed to account for the unfavourable entropy term observed in the oxidation of this acid. 

To take into account the effect of pH on the free ligand concentration in a complexation reaction, it is useful to introduce a conditional or effective formation constant. Such constants are pH-dependent equilibrium constants that apply at a single pH only. For the reaction of Fe with oxalate, for example, we can write the formation constant Ki for the first complex as... 

Write conditional formation constants for 1 1 complexes of Fe III) with each of the ligands in Problem 17-7. Express these constants in terms of the a value and the formation constant and in terms of concentrations, as in Equation 17-20. 17-9. Write a conditional overall formation constant for Fe(Ox)3 in terms of aj for oxalic acid and the j3 value for the complex. Also express the conditional constant in terms of concentrations as in Equation 17-20. 

The formation constant, /3, for Ga -bipy complex is almost independent of [H ], but for the analogous phen species, /3i, and 2 increase at lower [H ], probably owing to the formation of mixed hydroxoamine complexes." Mixed complexes of Ga" with bipy or phen have been detected for a variety of acido-ligands (anions of oxalic, tartaric, or malonic acids etc.). ... 

The following sequence of dipositive metal ions shows a decreasing effect on the rate of decarboxylation of oxaloacetic acid Cu(II), Zn(II), Co(II), Ni(II), Mn(II), Cu(II) (91). The rate constants for these decarboxylations approximately parallel the formation constants of the corresponding metal oxalates. A similar result was found in the decarboxylation of acetonedicarboxylic acid in the presence of certain transition metal ions the decarboxylation rates paralleled the formation constants of the metal malonates (170). These parallelisms indicate that the effectiveness of a metal ion in these decarboxylation reactions depends on its ability to chelate with the oxalate ion and the malonate ion, which resemble the transition states of the oxaloacetic and acetonedicarboxylic acids, respectively. 

The formation constants Kma = [MA]/[M ][A ] do not correlate with the Rma values but there is a quite good linear free energy relationship between log Icma and the formation constants of the corresponding oxalato complexes. Fig. 7. It is argued that the transition state for decarboxylation should more closely resemble the enolic intermediate which is "oxalate-like" in character. 

The tendency of divalent No to form complexes with citrate, oxalate, and acetate ions in aqueous solution has been studied by McDowell et al. using solvent extraction techniques [58]. In general, the complexing tendency of nobelium is between that of Ca and Sr, being more like Sr. Formation constants for the 1 1 complexes of 151.9 18.5,48 5.6, and — 5 51 mol" for citrate, oxalate, and acetate were reported. 

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