Crystallinity dissolution temperature effects

Effects of the original crystallization temperature and of the dissolution temperature in relationship to the crystalline melting point of PEO (in equilibrium with solvent) T may be summarized in two rules ... 

Cobalt(Il) dicobalt(Ill) tetroxide [1308-06-17, Co O, is a black cubic crystalline material containing about 72% cobalt. It is prepared by oxidation of cobalt metal at temperatures below 900°C or by pyrolysis in air of cobalt salts, usually the nitrate or chloride. The mixed valence oxide is insoluble in water and organic solvents and only partially soluble in mineral acids. Complete solubiUty can be effected by dissolution in acids under reducing conditions. It is used in enamels, semiconductors, and grinding wheels. Both oxides adsorb molecular oxygen at room temperatures. 

Water is without effect on the element even at a red heat,3 but the combined action of water and ozone produces telluric acid at the ordinary temperature.4 The action of hydrogen peroxide upon tellurium is influenced considerably by the physical state of the element colloidal tellurium is readily oxidised, but crystalline tellurium is not readily attacked and has first to be dissolved in an aqueous solution of alkali hydroxide, when oxidation becomes possible with formation of tellurate.5 Hydrogen peroxide of 60 per cent, strength reacts very slowly with tellurium at a temperature of 100° C., but with increasing amount of telluric acid formed, the rate of dissolution increases. Amorphous tellurium as ordinarily prepared behaves in a similar manner to the crystalline variety, but if it is dried by treatment with alcohol and ether instead of by heating at 105° C. it will dissolve readily in a concentrated solution of hydrogen peroxide.6... 

Louis Pasteur was the first scientist to study the effect of molecular chirality on the crystal structure of organic compoimds [23], finding that the resolved enantiomers of sodium ammonium tartrate could be obtained in a crystalline form that featured nonsuperimposable hemihedral facets (see Fig. 9.1). Pasteur was quite surprised to learn that when he conducted the crystallization of racemic sodium ammonium tartrate at temperatures below 28 °C, he also obtained crystals of that contained nonsuperimposable hemihedral facets. He was able to manually separate the left-handed crystals from the right-handed ones, and foimd that these separated forms were optically active upon dissolution. More surprising was the discovery that when the crystallization was conducted at temperatures exceeding 28 °C, he obtained crystals having different morphologies that did not contain the hemihedral crystal facets (also illustrated in Fig. 9.1). Later workers established that this was a case of crystal polymorphism. 

We have now said everything necessary about activities and standard states, but the overall effect for the newcomer is often one of confusion at this stage. To try to draw the various threads together we consider in Figure 12.5a a hypothetical three-phase equilibrium at temperature T and pressure P. A solid crystalline solution of B in A is in contact with an aqueous solution of A(aq) and B(a<7), which is in turn in contact with a vapor phase containing A(v) and B(v) in addition to water vapor. We can suppose the dissolution of (A,B)(5) to be stoichiometric so that the ratio of A to B is the same in all three phases, but this is irrelevant to our development as we consider only component A. Let s say that for a solid solution composition of A"a = 0.5, Vr = 0.5, the concentration of A aq) at equilibrium rri/ ) is 10 molal, and the fugacity of A in the vapor (/a) is 10 bars. Assuming activity coefficients in the solid and liquid... 

Hydrates of this type contain metal ion coordinated water, and the major concern with these is the effect of the metal-water interaction on the structure of crystalline hydrates. The metal-water interaction can be quite strong relative to the other bonding in a molecular crystal, so that dehydration takes place only at very high temperatures [13], Drugs with solubility, dissolution, or handling problems are most often recrystallized as Na(I), K(I), Ca(II), or Mg(II) salts and are often hygroscopic to some degree [16],... 

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