Oxalic acid stability constants

Table XIX contains stability constants for complexes of Ca2+ and of several other M2+ ions with a selection of phosphonate and nucleotide ligands (681,687-695). There is considerably more published information, especially on ATP (and, to a lesser extent, ADP and AMP) complexes at various pHs, ionic strengths, and temperatures (229,696,697), and on phosphonates (688) and bisphosphonates (688,698). The metal-ion binding properties of cytidine have been considered in detail in relation to stability constant determinations for its Ca2+ complex and complexes of seven other M2+ cations (232), and for ternary M21 -cytidine-amino acid and -oxalate complexes (699). Stability constant data for Ca2+ complexes of the nucleosides cytidine and uridine, the nucleoside bases adenine, cytosine, uracil, and thymine, and the 5 -monophosphates of adenosine, cytidine, thymidine, and uridine, have been listed along with values for analogous complexes of a wide range of other metal ions (700). Unfortunately comparisons are sometimes precluded by significant differences in experimental conditions.

The measurement of stability constants of complexes of yttrium, lanthanide, and actinide ions with oxalate, citrate, edta, and 1,2-diaminocyclohexanetetra-acetate ligands has revealed that there is a slight increase in the stability of complexes of the /-electron elements, relative to the others. A series of citric acid (H cit) complexes of the lanthanides have been investigated by ion-exchange methods and the species [Ln(H2cit)]", [Ln(H2cit)2] , [Ln-(Hcit)], and [Ln(Hcit))2] were detected. Simple and mixed complexes of dl- and jeso-tartaric acid have been obtained with La " and Nd ions, and the stability constants of lactate, pyruvate, and x-alaninate complexes of Eu and Am " in water have been determined. 

Catalytic decarboxylation processes occur in aliphatic keto acids in which the keto group is in an a-position to one carboxyl group and in a P-relationship to another. Thus, the normal decarboxylation of a p-keto acid is facilitated by metal coordination to the a-keto acid moiety. The most-studied example is oxaloacetic acid and it has been shown that its decarboxylation is catalyzed by many metals following the general order Ca2+ < Mn2+ < Co2+ < Zn2+ < Ni2+ < Cu2+ < Fe3+ < Al3"1".66 67 The overall rate constants can be correlated with the stability constants of 1 1 complexes of oxalic acid rather than oxaloacetic acid, as the uncoordinated carboxylate anion is essential for the decarboxylation. The generally accepted mechanism is shown in Scheme 15. Catalysis can be increased by the introduction of x-bonding ligands, which not only increase the... 

Solution equilibrium studies410 have identified both 1 1 and 1 2 complexes found between V02+ ions and oxalic acid, but only 1 1 complexes for phthalic, maleic, succinic, and adipic acids. Equilibrium constants for the formation and subsequent hydrolysis of these complexes have been determined. VO(H2L) (H5L = trihydroxy-glutaric acid) is the principal complex species in mixed aqueous acetone solutions but minor amounts of (VO)3(H2L)2 are also formed 411 the stability of VO(H2L)-increases with increasing acetone content. Solvent effects are also evident in vana-dium(iv)-quercetin system 412 the protonated ligand RH2 forms VOR+ and VR2 + in methanol at pH 3 but only VOR+ in aqueous methanol. 

Various fluorides may be precipitated from aqueous solution for use as constituent powders in solid state reactions. Co-precipitation offers very elegant access to intimate mixtures, but the actual products are strongly dependent on the fluoride ion activity within the solution but also on the stability constants of the respective metal complexes. Accordingly, not only anhydrous fluorides are obtained, but also hydrated fluorides or hydroxide fluorides, which may be very difficult to convert to pure fluorides. As noted already [3], reactive compounds, e.g. carbonates, acetates, oxalates, hydroxides etc., which quite easily dissolve in acidic HF solutions, are the preferred starting materials for fluoride syntheses. In contrast, many oxides which have been heated to rather high temperature are frequently unreactive and may not dissolve at all. To enhance reactivity but also improve crystallinity of the product, it has proved useful to perform reactions above the boiling point of water in adapting the hydrothermal method, which has already been shown to be useful in the recrystallisation of materials which are more or less insoluble at ambient temperatures and pressures. Up to about 240°C even PTFE vessels may be used. A number of selected examples with respective reaction conditions are listed in Table 3. 

Carbonate Complexes. Of the many ligands which are known to complex plutonium, only those of primary environmental concern, that is, carbonate, sulfate, fluoride, chloride, nitrate, phosphate, citrate, tributyl phosphate (TBP), and ethylenediaminetet-raacetic acid (EDTA), will be discussed. Of these, none is more important in natural systems than carbonate, but data on its reactions with plutonium are meager, primarily because of competitive hydrolysis at the low acidities that must be used. No stability constants have been published on the carbonate complexes of plutonium(III) and plutonyl(V), and the data for the plutoni-um(IV) species are not credible. Results from studies on the solubility of plutonium(IV) oxalate in K2CO3 solutions of various concentrations have been interpreted to indicate the existence of complexes as high as Pu(C03) , a species that is most unlikely from both electrostatic and steric considerations. From the influence of K2CO3 concentration on the solubility of PuCOH) at an ionic strength of 10 M, the stability constant of the complex Pu(C03) was calculated (10) to be 9.1 X 10 at 20°. This value... 

Mixed oxalate-salicylate complexes of Cd of the stoicheiometry [Cd(oxXsal)] and [Cd(ox)(sal)2] have been detected stability constants for these, [Cd-(ox) ] " (n = 1—3), and [Cd(sal) ] "" (n = 1 or 2) have also been determined. ° In oxalate solution, Hg forms the mixed complexes [Hg(C204)X ]" ( = 1, X = SCN = 1 or 2 X = OAc, NO2, Br, or tartrate). The complexation of Cd by tartrate has also been investigated. Stability constants for mixed complex formation between piperidine and a variety of zinc and cadmium mono- and dicar boxylic acids have been reported. " ... 

Few data are available on the concentration of dicarboxylic acid anions in subsurface waters. C2 through C q saturated acid anions have been reported in addition to maleic acid (cz5-butenedioic acid) (5. 15-16L Oxalic acid (ethanedioic) and malonic acid (propanedioic) appear to be the most abundant. Reported concentrations range widely from 0 to 2540 mg/1 but mostly are less than a few 100 mg/1. Concentrations of these species in formation waters are probably limited by several factors, including the very low solubility of calcium oxalate and calcium malonate (5), and the susceptibility of these dicarboxylic acid anions to thermal decomposition (16). This paper will focus on the monocarboxylic acids because they are much more abundant and widespread, and stability constants for their complexes with metals are better known. We do recognize that dicarboxylic acid anions may be locally important, especially for complexing metals. 

The greater difference in acidity constants K i and Kaj) of adipic acid than those of oxalic acid is due to the structural differences of these two acids. Adipic acid has four CH2 groups between the two carboxylic groups and this will cause a greater electron delocalization which stabilizes the carboxylic anions. As a result, the two protons on the adipic... 

It should be carefully noted that the term ligand does not apply to all of the organic material present, but only to that part of it which is in the appropriate ionic form for combining with the metal cation. For ethylenediamine, glycine, and oxalic acid, the ligands are the molecule, the monoanion, and the dianion, respectively. Hence, when stability constants are used to compare the relative... 

Due to the anionic nature of rhamnolipids, they are able to remove metals from soil and ions such as cadmium, copper, lanthanum, lead and zinc due to their complexation ability [57-59], More information is required to establish the nature of the biosurfactant-metal complexes. Stability constants were established by an ion exchange resin technique [60], Cations of lowest to highest affinity for rhamnolipid were K+ < Mg + < Mn + < Ni " " < Co " < Ca2+ < Hg2+ < Fe + < Zn2+ < Cd2+ < Pb2+ < Cu2+ < M +. These affinities were approximately the same or higher than those with the organic acids, acetic, citric, fulvic and oxalic acids. This indicated the potential of the rhamnolipid for metal remediation. Molar ratios of the rhamnolipid to metal for selected metals were 2.31 for copper, 2.37 for lead, 1.91 for cadmium, 1.58 for zinc and 0.93 for nickel. Common soil cations, magnesium and potassium, had low molar ratios, 0.84 and 0.57, respectively. 

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