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How to decide whether Kd = [NH3(aq)] / [NH3(CHCl3)] or Kd  =  [NH3(CHCl3)] / ...

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How to decide whether Kd = [NH3(aq)] / [NH3(CHCl3)] or Kd  =  [NH3(CHCl3)] /  [NH3(aq)] when either upper layer/lower layer or density is not given?In short when i have no idea on the density how can i decide what is the upper layer and what is the lower layer?

I need a General answer which i can apply to any question.


Please help me on this.

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No general convention exists regarding whether the concentration in the organic phase or aqueous phase should be placed in the numerator to determine K_d. One should always specify which way the partition coefficient is being expressed.

e.g. K_d=C_(water)/C_(CHCl3)=2

However, while purifying a particular compound from a mixture through liquid-liquid extraction, the impure mixture is dissolved in a small volume of solvent A first, then the desired compound is extracted with solvent B, in which the desired compound is more soluble. In that case, the convention is to express the partition co-efficient as (C_B)/(C_A). In other words, concentration of the compound in the extracting solvent, or the solvent that is used repeatedly in large quantities, is placed at the numerator.

Choice of procedure based on a knowledge of relative solubility, rather than density is the key here.


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