Question

9. What is von Weimarn ratio? Define the terms associated with this ratio. Explain Weimarn theory for selection of optimum conditions for precipitation giving suitable example.

10. Discuss the method of separation of the precipitate from its mother liquior.

Answer 1

von Weimarn's theory states that the size of the particle (precipitate)is inversly proportional to the relative supersaturation state(RSS). *Relative supersaturation(von Weimarn Ratio)=(Q-S)/S *Q=concentration of mixed reactants before precipitation. *S=solubility of precipitate at equilibrium. *The lower the relative supersaturation the better the precipitation: *If high, get excessive nucleation, produce lots of small crystals with high surface area. *If low,get large crystals and fewer small crystals with low surface area.   Favorable condotions for Precipitation. we want to keep Q low and S high during precipitation. Minimize relative supersaturation and obtain large crystals: 1) Precipitate from dilute solution.(Q low) 2)Add dilute precipitating reagents slowly,with effective stirring(Q low and prevent excess of reagents). 3) precipitate from hot solution.(High - S). 4)Precipitate at low pH as possible(adjust the pH).(High-S). 5)Add a little excess of the precipitating agent -check completeness of the precipitation. Digestion(Oswald Ripening) *A process in which a precipitate is heated for an hour or more in the solution from which it was formed(the mother liquor) *Digest the precipitate to make larger and more pure crystals.The large crystals grow at the expense of the small one.   

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