Ostwald s rule

Compound Physical Form Ostwald s Rule of Stages... 

Metastable crystalline phases frequently crystallise to a more stable phase in accordance with Ostwald s rule of stages, and the more common types of phase transformation that occur in crystallising and precipitating systems include those between polymorphs and solvates. Transformations can occur in the solid state, particularly at temperatures near the melting point of the crystalline solid, and because of the intervention of a solvent. A stable phase has a lower solubility than a metastable phase, as indicated by the solubility curves in Figures 15.7a and 15.7/ for enantiotropic and monotropic systems respectively and,... 

Formation of trimetaphosphate anions from those of Graham s salt also occurs in solution. In water this process is not associated with hydrolytic cleavage of the P—0—P bonds, as was at first assumed (10, 322), but is independent of it (361). Thus polyphosphates of long-chain amines which are soluble in benzene form trimetaphosphate quantitatively on warming the anhydrous solution (359). This transformation follows Ostwald s rule for successive transformations, since trimetaphosphates are more stable than polyphosphates (322). 

These reactions will proceed without heating under nitrogen, where the driving force is the more favorable iron oxide precipitate. Following Ostwald s rule of stages, the hydroxide would be the kinetic product and the oxide would be the thermodynamic product. 

Also,if the solutions had an initial Si02/Al20 ratio of 73,S formed but no Y, while if this initial ratio was 102,P formed but no S. This behaviour suggests caution in interpreting all crystallisation sequences as examples of Ostwald s rule of successive transformations. The rule states that in a crystallisation sequence the new phases replace each other in the order of a step by step descent of a ladder of increasing thermodynamic stability. An example in a hydrothermal system is ( ) ... 

The optimum check of Ostwald s rule would, as in the above sequence, involve parent gel and successive phases all of the same composition. This condition is not met in many crystallisation sequences. 

Threafall, T. (2003), Structural and thermodynamic aspects of Ostwald s rule, Org. Proc. 

Increasing the sublimation and desublimation temperature (I d 200 °C) causes the transformation of initial primary product, jS-SeCL into a-SeCLi. Thus crystalUzation from the gas phase obeys Ostwald s rule. a-SeCU is also formed... 

Selenium Tetrabromide. Selenium tetrabromide is mainly prepared by the reaction of the elements, either pure or dissolved in CS2, using an excess of bromine. It occurs in two crystalline modifications, the thermodynamically stable Q -SeBr4 and the metastable jS-SeBr4. The latter forms first, in accordance with Ostwald s rule. 

Ostwald (1897) was aware of the fact that his rule was tenuous, since it was not based on a very large set of observations. In addition, if the metastable region were to shrink to a vanishingly small value, then sufficient time would not be allowed for crystallization of Form I to appear, and the rule would be invalidated. In fact, this does happen in many cases. Nevertheless, Ostwald s Rule has remained in the lexicon of crystal chemists, probably because it is generally observed that if a succession of polymorphic forms is obtained, those which appear later are generally more stable than those whieh appear earlier. 

Fig. 1.4 Example of Ostwald s Rule of Successive Reactions. 2,4-dibromoacetanilide initially crystallizes from alcoholic solutions as small needle-shaped crystals, forming the voluminous mass in (1). Successive photos (2,3,4) of the same crystallization vessel, taken at two-day intervals show the transformation to the more stable chunky rhombic crystals (from Findlay 1951, with permission).

There are significant flaws in Ostwald s conclusion that led to his rule. For instance, when a crystallization experiment yields only a single form there is the question of whether it contradicts the Rule or whether the material is simply not polymorphic. Moreover, there is no way of answering this question. However, a sufficient number of cases of successively crystallizing polymorphic forms have been observed to warrant considering the principles behind Ostwald s Rule as guidelines for understanding the phenomenon of the successive crystallization of different polymorphic phases. 

Ojala etal. (1998) reported on the crystallization of two conformational polymorphs (Bernstein 1987) (see Chapter 5) of acetone tosyUiydrazone 3-XX. A triclinic form and a monoclinic form are both obtained from anhydrous ethanol—sometimes together. If the crystallizing solution is allowed to evaporate completely, only the monoclinic form is obtained, suggesting that it is the thermodynamically preferred form at room temperature. This is consistent with Ostwald s Rule of Stages and McCrone s (1965) test for relative stability of polymorphs according to which the more stable polymorph... 

S-transferase from rat liver (Fig. 3.4). Day and McPherson (1991) reported crystallization of two crystalline forms in stages in accord with Ostwald s Rule for cytochrome c from Valida membran-aefaciens (Fig. 3.5). In every case of crystallization they obtained arrays of thin triclinic plates (Fig. 3.5a), some of which grew up to 0.5 mm in the largest physical dimension (Fig. 3.5b). In some cases, some of these dissolved (in accord with Ostwald s Rule of Stages (Section 2.3)) to give rectangular prisms (Fig. 3.5c), which turned out to be orthorhombic. 

In accordance with Ostwald s rule, the cooling of melts of polymorphic substances ordinarily yields the least stable modification, which subsequently rearranges into the stable modification in steps. Since the... 

This is supported by the empirical Ostwald s rule of stages, which states that the most stable state does not necessarily form when an unstable solution crystallises." ... 

Closely allied with the concepts of metastable equilibrium and suspended transformations is Ostwald s rule (Ostwald s step rule or law of successive reactions). Essentially Ostwald s rule states that in all processes it is not the most stable state with the least amount of free energy that is initially obtained but the least stable state lying nearest to the original state in free energy (Ostwald 1897). It is easy to see how this rule and the concept of suspended transformations can explain the production of a metastable polymorph through crystallization from a melt or solution. 

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