Oxalate ions, formation

Fig. 4.18 Decrease in distribution ratio of Be(II) as a function of oxalate ion (Ox ) concentration due to formation of aqueous BeOXo complexes. The extraction system is 0.03 M TTA in methyUsobutylketone and 1.0 M Na(0.5 Ox T CIO)). See Eq. (4.72) for ordinate function. (From Ref. 31.)...

In contrast with the sensors described elsewhere in this Chapter, the device proposed by the authors group uses no reagent, but photons, to induce a photochemical reaction, and involves electrochemical detection of the photochemical product, which allows one to continuously monitor the formation of the electroactive product. Kinetic monitoring increases the selectivity of determinations by eliminating matrix effects and the contribution of side reactions, whether slower or faster than the main reaction. The electrochemical system chosen for implementation of this special sensor was the Fe(II)/C204 couple, which was used for the kinetic determination of oxalate ion based on the following reaction ... 

An even more intimate mixture of starting materials can be made by the coprecipitation of solids. A stoichiometric mixture of soluble salts of the metal ions is dissolved and then precipitated as hydroxides, citrates, oxalates, or formates. This mixture is filtered, dried, and then heated to give the final product. 

This is a white crystalline solid obtained by treating polonium(IV) hydroxide or chloride with aqueous oxalic acid solubility studies indicate complex ion formation (11). 

Thorium has the oxidation state of (IV) in all of its important compounds. Its oxide, ThCL. and its hydroxide are entirely basic. The nature of the 10ns present in a number of solutions of the soluble compounds is not known with certainty. Complex ions involving sulfate are suggested by the increased solubility of the sulfate in solutions of the acid sulfates. Similarly, other complex ions are suggested by the solubility of the carbonate in excess alkali carbonate and of the oxalate in ammonium oxalate. Such ready complex ion formation is consistent with the high positive charge of the thorium-flV) ion. 

A similar but, perhaps, more interesting case is that of the ion pairs between Co(sep)3+ and oxalate ions. Excitation of the ion pair in the IPCT band leads to the formation of the Co(II) complex and of an oxalate radical which undergoes a fast decomposition reaction. Thus, the back electron transfer reaction is prevented and Co(sep)2+, which is a good reductant, can accumulate in the system. When colloidal... 

Determine the molar mass of calcium oxalate. Use the mass of calcium oxalate and its molar mass to find the amount (in mol) of calcium oxalate. Write the net ionic equation for the formation of calcium oxalate. From the coefficients in the net ionic equation, find the amount of oxalate ions (in mol). Calculate the mass of oxalate ions from the amount of oxalate ions (in mol) and the molar mass. Calculate the mass percent of oxalate ions in rhubarb leaves from the mass of the leaves and the mass of the oxalate ions present. 

In most cases, photo assisted electrochemical degradation processes are used. For instance, in the aforementioned Fenton method, formation of complexes of iron ions with some intermediates such as oxalate ions can reduce the effectiveness of the process. Then, photoelectro-Fenton methods involve the use of UV irradiation to produce photolysis of such Fe complexes and increase the rate of Fe" regeneration by means of the photo-Fenton reaction ... 

Now suppose we add some oxalic acid (H2C2O4) to the original solution. Oxalic acid ionizes in water to form the oxalate ion, C2O4, which binds strongly to the Fe ions. The formation of the stable yellow ion Fe(C204)3 removes free Fe " ions in so-Intion. Consequently, more FeSCN units dissociate and the equilibrium shifts from left to right ... 

From a knowledge of the solubility rules (see Section 4.2) and the solubility products listed in Table 16.2, we can predict whether a precipitate will form when we mix two solutions or add a soluble compound to a solution. This ability often has practical value. In industrial and laboratory preparations, we can adjust the concentrations of ions until the ion product exceeds K p in order to obtain a given compound (in the form of a precipitate). The ability to predict precipitation reactions is also useful in medicine. For example, kidney stones, which can be extremely painful, consist largely of calcium oxalate, CaC204 (K p = 2.3 X 10 ). The normal physiological concentration of calcium ions in blood plasma is about 5 mM (1 mM = 1 X 10 M). Oxalate ions ( 204 ), derived from oxalic acid present in many vegetables such as rhubarb and spinach, react with the calcium ions to form insoluble calcium oxalate, which can gradually build up in the kidneys. Proper adjustment of a patient s diet can help to reduce precipitate formation. Example 16.10 illustrates the steps involved in precipitation reactions. 

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 intermediate produced in the flash photolysis of copper(ii) oxalato complexes in deaerated aqueous solution has been identified as CUCO2. This species, which is also generated by pulse radiolysis of the Cu -oxalate-formate system, decays by first-order kinetics. A dependence of the rate constant on pH and on the concentrations of Cu and oxalate ions is established and this is interpreted in terms of competing reactions of CUCO2 [equations (7) and (8)]. 

MAR/RYA2] Marov, 1. N., Ryabchikov, D. I., Complex formation of Zr(lV) and Hf(IV) with chloride, nitrate, and oxalate ions, Russ. J. Inorg. Chem., 7, (1962), 533-539. Cited on pages 198, 266. 

Quantum yield determinations for the production of paraquat cation-radical were respectively 1 2 and 0 26 for benzilate and oxalate anions formate ion was somewhat less reactive and the reactions are assumed to proceed as indicated (192) for oxalate and... 

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