Question

In a reserved-phase column, a solute was found to have a retention time of 31.3min while an unretained species required 0.48min to elute when the mobile phase was 30% (by volume) methanol and 70% water.

Find
a) k'
b) a water/methanol composition that will make the k' value to be 5.

Answer 1

k =(31.3-0.48)/0.48=64

To obtain Pˊ for the mobile phase we substitute polarity indexes for methanol and water from to give

P ˊ = polarity index.

P ˊ = Ø_AP_{A ˊ}+ Ø_BP_B

P ˊ =0.30×5.1+0.70×10.2=8.7

substitution of this result into Equation

REVERSE-phase separation=K^"₂ /K"₁=10^(P2-P1)/2

=5 /4=10^(P2-8.7)/2

Taking the log of both sides of equation gives P₂=6.5

Letting x be the volume fraction methanol in the new solvent mixture and substituting again into Equation we find 6.5= x ×5.1+(1-x) 10.2 x =0.73 or 73%

Thus, a 73% methanol / 27% water mixture should provide the desired value of k