Chromium complexes oxalic acid

In the reactions of each of the oxalato complexes with cerium(IV) it is clear that no dichromate is formed until all the oxalate has been consumed. This observation is reasonable, for while the cerium(IV)-Cr(OH2)6+3 reaction is moderately rapid and while dichromate ion reacts sluggishly with even free oxalic acid, chromium(IV), a probable intermediate in the cerium(IV)-chromium(III) reaction, might be expected to behave very much like cerium(IV) toward the oxalato complexes. 

The classical preparation16 of this salt involves the reaction of oxalic acid, dipotassium oxalate, and potassium dichromate, in which the reduction Cr(VI) — Cr(IH) is accompanied by complexation. The method described below has chromium in the +3 state in the highly reactive starting material. 

Malonic acid CH2(C02H)2 (H2mal) (209) has a coordination chemistry with chrommm(III) closely resembling that of oxalate. Malonic acid is a slightly weaker acid than oxalic acid and slightly more labile complexes are formed. The tris complex is the most extensively studied, prepared by the reduction of chromate solutions or the reaction of chromium(III) hydroxide with malonate.917,918 919 The cis and trans diaqua complexes may be prepared by the reduction of chromate with malonate the isomers are separated by fractional crystallization. The electronic spectrum of the tris complex is similar to that of the tris oxalate and a detailed analysis of these spectra has appeared.889... 

The formation and decomposition of Crv in aqueous and non-aqueous media during the oxidation of organic substrates such as oxalic acid and ethylene glycol by potassium dichromate has been recognized for some time. No study resulted in the isolation of a stable, well-characterized chromium(V) complex until 1978 when potassium bis(2-hydroxy-2-methylbutyrato)oxochromate(V) monohydrate was prepared from chromium trioxide and the tertiary a-hydroxy acid in dilute perchloric acid according to equation (91). The Crv, which is... 

Oxalic acid is used in excess to ensure a rapid oxidation of the alcohol and to destroy the excess chromic acid when the cooxidation process is over. Part of the oxalic acid is consumed by chromium(III) to form oxalatochromium(III) complexes. 

The malonato complexes of chromium(III) are analogous to the oxalate complexes of chromium(III). Since malonic acid is a weaker acid than oxalic acid, the malonato complexes are expected to be more labile than the oxalato complexes. The dicarboxylate complexes of chromium(III) form a group of anionic complexes which are suitable for the study of octahedral complex reactivity. 

Some of the oxalato-complexes of chromium(III) are also of interest. Potassium trioxalatochromate(III), K3 Cr(C204)3 SHgO, is obtained by adding potassium oxalate to the solution obtained by reducing KgCrgO with oxalic acid ... 

The diphenylcarbazide method is almost specific for chromium(Vl). Interferences result only from Fe, V, Mo, Cu, and Hg(II) present at much higher concentrations than the chromium. Iron(lll) can be masked by phosphoric acid or EDTA. Iron(III) can also be separated as Fe(OH>3, after chromium has been oxidized to Cr(VI), or by extraction. Vanadium can be separated from Cr(VI) by extraction as its oxinate at pH -4. Molybdenum is masked with oxalic acid, and Hg(II) is converted into the chloride complex. 

It has been known for some time that tolerance towards high levels of both essential and toxic metals in a local soil environment is exhibited by species and clones of plants that colonize such sites. Tolerance is generally achieved by a combination of exclusion and poor uptake and translocation. Some species can accumulate large quantities of metals in their leaves and shoots at potentially toxic levels, but without any harmful effects. These metal-tolerant species have been used in attempts to reclaim and recolonize metal-contaminated wastelands. More recently such species have attracted the attention of inorganic chemists. There is abundant evidence that the high metal levels are associated with carboxylic acids, particularly with nickel-tolerant species such as Allysum bertolonii. The main carboxylic acids implicated are citric, mahc and malonic acids (see refs. 30 and 31 and literature cited therein). Complexation of zinc by malic and oxalic acids has been reported in the zinc-tolerant Agrostis tenuis and oxalic acid complexation of chromium in the chromium-accumulator species Leptospermum scoparium ... 

The resistance offered by the different chromium complex salts to this change depends on the nature of the complex radicles—chloride, sulphate, formate, oxalate, etc. With longer ageing, or heating, a more drastic, irreversible, internal change occurs, which, according to D. Balanyi, results in the formation of oxy-salts, a change which makes the mother-liquor more acidic, thus ... 

Moles of acetone are produced per equivalent of chromium(vi) used, and the decay of the intermediate complexes involving Cr " has been followed using spectrophotometric and e.s.r. techniques. In the reaction with oxalic acid, the rate law ... 

Surface complexation description of the dissolution of chromium(III) hydrous oxides by oxalic acid, Inorg. Chem. Vol. 36, 6423-6430, 0020-1669. 

Other common transition metal corrosion products typically monitored at various sites within the plant include iron, copper, nickel, zinc, and chromium. More than 80% of BWR plants analyze for iron, nickel, copper, and zinc in reactor water, and nearly all of the BWR plants determine these metals in feed water. In addition, zinc is also an additive used in many plants to control the shutdown radiation dose rate. Nickel and chromium are corrosion products in BWR plants fi-om stainless-steel piping. The best selectivity and sensitivity for achieving low to submicrogram/Liter detection limits for transition metals can be obtained by separating transition metal complexes using pyridine-2, 6-dicarboxylic acid (PDCA) or oxalic acid as chelators in the eluent, followed by postcolumn derivatization with 4-(2-pyridylazo)resorcinol (PAR) and absorbance detection at 520 nm (see Section 8.2.1.2). This approach was successfully used to determine trace concentrations of iron, copper, nickel, and zinc in BWR and PWR matrices [197]. Figure 10.113 compares the chromatograms from the... 

In the presence of oxidizable metal ions, however, e.g. Ce or Mn ) there is always the possibility of trapping the intermediates in competition with the ligands reacting to reduce the Cr. Chromium(v) plays an important role in oxidations of this type and a long-lived Cr intermediate has been identified in the reaction with oxalic acid. Using both e.s.r. and difference absorbance spectra, it has been shown that there is a significant build-up of a complex in moderately acidic media. The data... 

Calcium forms stable insoluble salt with oxalic acid (see Section 10.2.3.2). In plant cells with higher concentrations of oxalic add, caldum oxalate can be actually present in the form of crystals. Some plants have been shown to bind metals in mixed complexes. For example, chromium can be bound in an oxalate-malate complex, and nickel and zinc can form a dtrate malate complex. Citric add has been proven to be a low molecular weight zinc ligand in human milk, and in casein micelles it binds calcium. It is also used as a food additive (acidulant, synergist to antioxidants and sequestrant), so great attention has been paid to the formation of its complexes with metal ions. The addition to cereal products leads to increased solubihty of naturally present iron, due to its release from phytic acid salts (phytates). 

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