I'm wondering which of the two phenomenae grows in importance and which one becomes irrelevant when the proportion of impurities in a product being purified become increasingly small:

The common ion effect will favor the solubility of the purified substance to the detriment of impurities that have lower solubility values if these impurities share a common ion with the substance being purified.

The higher free energy of impure crystals vs pure crystals on the other hand, will favor the dissolution of impurities over that of the pure product.

Is it reasonable to say that, as the purity of a more soluble desired substance increases the common ion effect loses relevance vs the effect imparted by the difference in free energy of pure vs impure precipitated particles?

My basic reasoning comes from the idea that the common ion effect decreasing the solubility of a lower solubility impurity with a common ion will almost double going from 50% to 99% purity, but has a plateauing influence as the purity grows, while on the other hand I can't see why the other effect would plateau with increasing purity.

In other words: Is the common ion effect of interest for the purpose of purification only in early (bulk) stages, while recursive recrystalisation would forever decrease a stable proportion of insolubilities (provided ideally pure solvent & glassware)?


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