Oxalic acid lattice

In addition to the stable compounds mentioned below, the dl Crv ion is readily detected by esr spectroscopy as a transient species in the reduction of CrVI, as well as in certain oxide lattices and catalysts. The reduction of Crvl by oxalic acid, citric acid, isopropanol, and various other organic reductants gives esr-detectable Crv... 

Oxalic acid is a precipitation agent for rare earth ions. The solubility of rare earth oxalates range from 10 to lO" mol in neutral solutions. The precipitate usually contains coordinated and/or lattice water molecules, RE2(C204)3 n H2O, where = 10 for lanthanum to erbium and yttrium while n = 6 for holmium, erbium, thulium, ytterbium to lutetium and scandium. 

Sparingly soluble in cone, mineral acids, except hydrofluoric acid when freshly precipitated, soluble (withformationof a complex) in a mixture of H3PO4, oxalic acid and cone. HaS04. An acid-soluble, white pero compound is formed when a cold suspension of the phosphate is reacted with an NaOH-Na a solution and then digested at 70°C. ZrPaO crystallizes in the cubic K 61 lattice. 

As an example of a molecular lattice in which cohesion is also effected predominantly by dipole forces, oxalic acid dihydrate may be discussed here, because it allows of a remarkable insight into the role of the water of crystallization. 

Fig. 36b shows the distances between the center of gravity of a water molecule and the three nearest oxygen atoms of oxalic acid residues one sees that they lie between 2.52 and 2.87 A. It appears, therefore, that the hydrogen atoms of the acid residue and those of the water in the lattice play the same role and are arranged between the oxygen atoms of the water and the oxalic acid residue. In this sense, the crystal is best regarded as oxonium oxalate with the formula... 

Fig. 36b. Arrangement of oxalic acid molecules in the lattice (hydrogen atoms...

The crystal structure of many compounds is dominated by the effect of H bonds, and numerous examples will emerge in ensuing chapters. Ice (p. 624) is perhaps the classic example, but the layer lattice structure of B(OH)3 (p. 203) and the striking difference between the a- and 6-forms of oxalic and other dicarboxylic acids is notable (Fig. 3.9). The more subtle distortions that lead to ferroelectric phenomena in KH2PO4 and other crystals have already been noted (p. 57). Hydrogen bonds between fluorine atoms result in the formation of infinite zigzag chains in crystalline hydrogen fluoride... 

The survey of the investigations and results covers the release of water from salts and hydroxides, the calcination of carbonates and oxalates, the reactions of metallic oxides and carbonates with SO2, and reactions on the surface of carbon. The application of the non-isothermal method to the thermal decomposition of carboxylic acids and polymeric plastics as well as to the pyrolyses of natural substances, in particular bituminous coal, is explained. Finally, chemical reactions in a liquid phase, the desorption of gases from solids, annihilation processes in disturbed crystal lattices and the emission of exo-electrons from metallic surfaces are discussed. 

With feldspars and layer silicates (Brady and Walther, 1992) the points of attack of the protons are the O atoms that interlink the Al-oxide groups with the Si-oxide structures. The protonation causes a slow detachment of Al from the crystal lattice, which is coupled with the subsequent detachment of Si(OH)4 species. Oxalates, diphenols, and citric acid—similar substances that occur in soils as by-products of biological decomposition and root exudates—can also accelerate the dissolution of Al silicates. 

Figure 13.10. Schematic representation of the oxide dissolution processes [exemplified for Fe(III) (hydr)oxides] by acids (H ions), ligands (example oxalate), and reductants (example ascorbate). In each case a surface complex (proton complex, oxalato and ascorbato surface complex) is formed, which influences the bonds of the central Fe ions to O and OH on the surface of the crystalline lattice, in such a way that a slow detachment of a Fe(III) aquo or a ligand complex [in case of reduction an Fe(ll) complex] becomes possible. In each case the original surface structure is reconstituted, so that the dissolution continues (steady-state condition). In the redox reaction with Fe(III), the ascorbate is oxidized to the ascorbate radical A . The principle of proton-promoted and ligand-promoted dissolution is also valid for the dissolution (weathering) of Al-silicate minerals. The structural formulas given are schematic and simplified they should indicate that Fe(III) in the solid phase can be bridged by O and OH.

Despite some crystallographic similarities with the acid salt (see below), the water molecules in barium oxalate dihydrate are accommodated differently in the lattice [140]. The extent of water loss from BaC204.2H20 varied with pQi20). The products were the anhydrous salt below 4 Torr, the hemihydrate between 4 and 100 Torr, and the monohydrate above 100 Torr. The rate of water evolution at 353 K exhibited Smith-Topley behaviour and the changes in rate occur at values of pQyD) (5 and 100 Torr) which are close to those characteristic of the stabihty range of the hemihydrate. 

---