Sorrel

Crystallizes from water in large colourless prisms containing 2H2O. It is poisonous, causing paralysis of the nervous system m.p. 101 C (hydrate), 189°C (anhydrous), sublimes 157°C. It occurs as the free acid in beet leaves, and as potassium hydrogen oxalate in wood sorrel and rhubarb. Commercially, oxalic acid is made from sodium methanoate. This is obtained from anhydrous NaOH with CO at 150-200°C and 7-10 atm. At lower pressure sodium oxalate formed from the sodium salt the acid is readily liberated by sulphuric acid. Oxalic acid is also obtained as a by-product in the manufacture of citric acid and by the oxidation of carbohydrates with nitric acid in presence of V2O5. 

Oxalic acid is poisonous and occurs naturally in a number of plants including sorrel and begonia It is a good idea to keep houseplants out of the reach of small children who might be tempted to eat the leaves or berries... 

Potassium Hydrogen Oxalate. Potassium acid oxalate [127-95-7], KHC2O4, mol wt 146.15, exists as a monohydrate [6100-03 ]. It is of historical interest because it is the salt of sorrel found in vegetation and the first oxalate isolated. 

D. W. Wendland and W. R. Matthes, Visualicyation of Automotive Catalytic Converter Internal Flom, SAE 861554,1986 D. W. Wendland, P. L. Sorrell, and J. E. Kreucher, Sources of Monolithic Catalytic Converter Pressure Eoss, SAE 912372, Society of Automotive Engineers, Warrendale, Pa., 1991. 

SORREL, s. E., Paper Base Laminates, Cleaver-Hume Press, London (1950)... 

Chemical Designations - Synonyms Potassium acid oxalate Salt acetosella Salt of sorrel Chemical Formula KHC2O4. 

Ampfer, m. sorrel, dock (Rumex). amphichroitisch, a. amphichroic. amphoter, ampboterisch, a. amphoteric. Ampulle, Ampulla, /. ampulla (Pharm.) am poule. 

Fuchs, m. fox trestle, jack flue sorrel horse freshman fluke (Metal.) piece of iron that... 

Hasen-klee, m. rabbit-foot clover, hare s-foot trefoil, -kohl, m. wood sorrel, -ohrlein, n. hare s-ear (Bupleurum). -schwanz, m, hare s-tail grass. 

Klee-salz, n. salt of sorrel (potassium hydrogen oxalate), -samenol, n. cloverseed oil. kleesauer, a. of or combined with oxalic acid, oxalate of. 

Sauer-ampfer, m. sorrel, sour dock (Rumex). 

SM.uerling, m. acidulous mineral water, sparkling mineral water sparkling mineral spring sour wine , cheese from sour milk clover sorrel, wood sorrel. 

Sorrelsalz, n. salt of sorrel (potassium hydrogen oxalate). 

Another class of AB cement, the oxychloride cements of zinc and magnesium, are also formulated in aqueous solution and retain substantial amounts of water on setting (Sorrell Armstrong, 1976 Sorrell, 1977). 

In the cements of this type a number of phases are known to be present. For example, in the zinc oxychloride cement two discrete phases, corresponding to the composition ZnO. ZnCl. H O in the ratios 4 1 5 and 1 1 2 respectively, are known to occur (Sorrell, 1977). Similarly, in the magnesium oxychloride cement, phases corresponding to Mg(OH)a. MgClj. HjO in the ratios 5 1 8 and 3 1 8 have been shown to exist and have been studied by X-ray diffractometry (Sorrell Armstrong, 1976). 

Sorrell, C. A. Armstrong, C. R. (1976). Reactions and equilibria in magnesium oxychloride cements. Journal of the American Ceramic Society, 59, 51. ... 

The most important characteristic of the magnesium oxide powder used in these cements is its reactivity (Glasson, 1963). Magnesium oxide needs to be calcined to reduce this, otherwise the cement pastes are too reactive to allow for placement. Surface area and crystal size are important and relate to the calcination temperature (Eubank, 1951 Harper, 1967 Sorrell Armstrong, 1976 Matkovic et ai, 1977). The lower reactivity of calcined magnesium oxide relates to a lower surface area and a larger crystallite size. 

By contrast, the acidity of the metal salts used in these cements has a less clear origin. All of the salts dissolve quite readily in water and give rise to free ions, of which the metal ions are acids in the Lewis sense. These ions form donor-acceptor complexes with a variety of other molecules, including water, so that the species which exists in aqueous solution is a well-characterized hexaquo ion, either Mg(OH2)g or Zn(OH2)g. However, zinc chloride at least has a ternary rather than binary relationship with water and quite readily forms mixtures of Zn0-HCl-H20 (Sorrell, 1977). Hence it is quite probable that in aqueous solution the metal salts involved in forming oxysalt cements dissolve to generate a certain amount of mineral acid, which means that these aqueous solutions function as acids in the Bronsted-Lowry sense. 

Holland, by contrast, reported the existence of three well-defined phases in this system, corresponding to ZnO ZnClj HjO ratios of 5 1 8, 1 1 1 and 1 1 2 respectively (Holland, 1930). More recently it was pointed out that these claims lack any unequivocal support such as X-ray characterization of the phases, so that their validity must remain in doubt (Sorrell, 1977). 

Droit s original 4 1 5 phase has been studied by X-ray diffraction (Nowacki Silverman, 1961, 1962) and found to have a rhombohedral layer structure. The 1 1 1 phase was also found to have a layer structure, which consisted of pseudohexagonal layers of zinc atoms separated by ordered layers comprising oxygen and chlorine atoms (Feitknecht, Ostwald Forsberg, 1959). This fundamental structure was apparently found for both of the crystalline modifications in which this phase has been found to occur, namely the monoclinic and the orthorhombic (Sorrell, 1977). 

X-ray analysis of the various samples that were produced indicated that the system ZnO-ZnClj-HjO includes four crystalline phases, two of which, ZnO and ZnClj. l HjO, are essentially the starting materials. Sorrell also found the 4 1 5 phase, reported by Droit, with an identical X-ray powder diffraction pattern to that reported by Nowacki Silverman (1961, 1962), and a 1 1 2 phase. Since neither the 1 1 2 nor the 4 1 5 phase lost or gained weight on exposure to air at about 50% relative humidity and 22 °C and no changes developed in the X-ray diffraction pattern following this exposure, he concluded that the previously reported 1 1 1 phase cannot be formulated from mixtures of ZnO and aqueous ZnCl,. 

The cementition reaction between zinc oxide powder and aqueous zinc chloride was found to be both rapid and extremely exothermic. Although at least four days equilibration was allowed before examining any of the cements in detail, Sorrell found evidence that reaction was complete within 20 to 30 minutes and occurred without observable development of intermediate phases. He also found that, as the concentration of reactants was increased, so reaction rate increased until, at sufficiently high concentrations, reaction occurred too quickly to allow proper mixing of the reactants. Preheating the zinc oxide at 900 °C for 16 hours was found to slow the reaction down, but only slightly. 

Sorrell showed that the two discrete phases could be readily and reversibly interconverted. For example, the 1 1 2 phase was found to react... 

Figure 7.1 Phase relationships in the ZnO-ZnCl2-H20 system at room temperature (Sorrell, 1977).

The 4 1 5 phase was shown by thermogravimetric analysis to dissociate at about 160 °C to zinc oxide and the 1 1 2 phase, a process which was verified using X-ray diffraction (Sorrell, 1977). Once the 1 1 2 phase was formed it underwent characteristic dissociation at temperatures above 160 °C. 

The equilibrium relationships found by Sorrell (1977) were valid only for room temperature (22+2 °C) and, because samples were allowed to cure in sealed containers, for equilibrium water vapour pressures determined by the assembly of phases present. The phases which exist under such conditions were quite unequivocally found to be 4 1 5 and 1 1 2. However Sorrell pointed out that it is entirely possible that lower hydration states of either phase could be stable at higher temperatures or lower humidities. In particular the 4 1 4 phase (Feitknecht, 1933) may well be such a phase, particularly as one of the five waters of hydration is known to be held only loosely in the structure. Indeed, Sorrell reported that he observed a slight shoulder on the larger dehydration peak of the DTG curve of the 4 1 5 phase that might be assigned to the loss of this first water molecule. He did not, however, succeed in isolating or characterizing a 4 1 4 phase. 

Similarly Sorrell (1977) had no success in attempts to isolate the 1 1 1 phase reported by Forsberg Nowacki (1959), though he conceded that it, too, might exist as a lower hydration state of his well-defined 1 1 2 phase. Unfortunately Forsberg and Nowacki did not provide details of the alleged 1 1 1 phase, so comparison of their results with Sorrell s was not possible. 

In very dilute HCl solutions, specifically those with a pH above 5-48, the 4 1 5 phase was found to be insoluble. By contrast, addition of concentrated HCl to the 4 1 5 phase was shown to lead to formation of the 1 1 2 phase (Sorrell, 1977). Below 35wt% HCl, the 4 1 5 phase was found to dissolve congruently. Since the 1 1 2 phase was also found to dissolve congruently in hydrochloric acid solutions with concentrations above 23 wt %, it follows that there is a range of concentrations over which both phases are soluble in aqueous HCl. This behaviour explains why the zinc oxychlorides have proved to be unsatisfactory in attempts to use them as dental cements. The preparation of such cements from concentrated aqueous solutions of ZnClj results in the formation either of the 1 1 2 phase alone or of mixtures of the 4 1 5 and 1 1 2 phases, neither of which is stable in the presence of water. Preparing dental cements from less concentrated solutions also results in the formation of mixed phases, unless the bulk composition has excessive amounts of ZnO present. In these latter cases the cement stability is acceptable but it lacks both a workable consistency and a reasonable working time. 

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