Citric acid crystal structure

IR spectra, 2,469 protonation, 2, 465 zinc transport, 6, 672 Citric acid, fluoro-absolute configuration, 2, 478 Citric add, hydroxy-crystal structure, 2, 478 Clathrates amines, 2,25... 

Citric acid ionization, 465 metal complexes, 476, 963 crystal structure, 476 NMR, 467... 

In subsequent experiments, using other crystal systems, such as ferrous sulfate and sodium hydrogen phosphate, it was similarly observed that the first crystallization product to form was the one most closely resembling the structure of the solvent (Nyvlt, 1995). For the case of citric acid, this is the monohydrate, which more closely resembles the aqueous structure. As the temperature of the solution is increased, the structure of the solvent, as well as the solubility of the crystal, changes, resulting in a more thermodynamically stable anhydrous product. This conversion between the kinetic and thermodynamic product occurs at a critical transition temperature, below which the structure of the solution favors the formation of the hydrated product. As the transition temperature is surpassed, the anhydrous product becomes favored. 

Ferric Ammonium Citrate, Green, occurs as thin, transparent green scales, as granules, as a powder, or as transparent green crystals. It is a complex salt of undetermined structure, composed of iron, ammonia, and citric acid. It is very soluble in water, but is insoluble in alcohol. Its solutions are acid to litmus. It may deliquesce in air and is affected by light. 

The X-ray crystal structure of A1 -tetrahydrocannabinolic acid B is reported,362 and one of the previously reported (Vol. 4, p. 75) dihydrobenzofurans from the citric acid-catalysed condensation of orcinol with menth-4-en-3-ol is shown to be (256) by X-ray analysis.363 G.c.-m.s. assay of A -THC-OMe allows the detection of 1 ng mP1 plasma of A -THC.364 The mass spectral fragmentation of A -THC, A6-THC, and some isomeric cannabinoids to the prominent m/e 231 ion has been examined.365 Miniaturized syntheses of 32 natural, or potentially natural, cannabinoids are reported in connection with their chromatographic analysis.366... 

Hydroxycarboxylic acids, which include citric acid, malic acid, lactic acid, etc., are benign to the environment and very convenient for the solution processing. Moreover, since these reagents can form stable complexes with other cations, they rarely yield a precipitate. For several complexes single crystals of well-defined composition suitable for the X-ray structural analysis were isolated. Thus, these water-soluble titanium complexes of hydroxycarboxylic acids are promising precursors for the synthesis of ceramics from an aqueous solution and their industrial utilization is expected in the future. In this chapter we decribe the method of synthesis, structural analysis, and stability of these complexes. The examples of multicomponent oxide materials preparation using these compounds are presented. 

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. 

Citric acid monohydrate occurs as colorless or translucent crystals, or as a white crystalline, efflorescent powder. It is odorless and has a strong acidic taste. The crystal structure is orthorhombic. 

The procedure is discussed in detail by Delmon et al.(82-84). The crucial step appears to be the rapid dehydration of the starting solution before any of the components can crystallize out of solution separately. Delmon(85) suggests that a rotary vacuum evaporation would be an effective method of drying the precursor. The actual structure of the precursor is not well defined, but appears to require at least one equivalent of citrate ion per mol of metal ion(83), as presumably the citrate complexes all the metal species in so Iution. The resulting powder patterns, after annealing, indicated no contamination. Delmon(J3) suggests that any multifunctional acid containing at least one carboxyl and one hydroxyl function may be effective. Experiments with tartaric acid on the iron/chromium system produced results similar to citric acid a calcination temperature of 500°C was necessary before crystallization occurred. 

In contrast, citric acid binds to citrate synthase through seven hydrogen bonds (Figure 13.12, the present discussion is based on the structure determination 3CTS [66]). Several crystal structures of citric acid, both as free acid and complexed to metal ions, are available [67]. In Figure 13.13 these crystal conformations are superimposed and shown in projection (right). They are compared with the conformation at the receptor site of citrate synthase (left). Three different torsion angles have to be considered. 

Fig. 13.13. Left Conformation of citric acid at the binding site of citrate synthase (3CTS) Right superposition of conformations of citric acid both as free acid and complexed to metal ions in small-molecule crystal structures...

Probably the most important phenomenon for controlling the crystal size distribution is nucleation. By controlling nucleation, the desired crystalline structure can be attained. In many food products, the nucleation rate curve increases initially as driving force increases, passes through a maximum and then decreases as viscosity limitations inhibit the rate of nuclei formation. An example of this nucleation behavior was observed for citric acid by Mullin and Led (1969). At very low temperatures, fewer crystals were formed and this correlated with the strong increase in viscosity at these conditions. This behavior has been described (Mullin 1993 Walton 1969 Van Hook and Bruno 1949) by ... 

---