-
- Posts: 2
- Joined: Mon Aug 11, 2014 4:57 pm
The question is about a famous equation about retention factor and solvent strength.
log k = log kw - sφ
And when some books describe this equation like this;
'For small molecules, the value of S is approximately 3,
and this predicts that a 10% change in organic modifier concentration will produce about a three-fold change in retention as measured by k.'
(quoted from http://books.google.co.kr/books?id=jn0e ... A6&f=false)
My local language textbook also said so(though they say S = 4, but I think that makes no much difference).
But I can't understand why 10% decrease in B(organic) makes three-fold retention increase when S is about 3-4.
As I thought, if there is concentration A and concentration B that is 10% increase to A, so is B = (A+0.1), the equation about two concentrations are
log Ka = log Kw - sA
log Kb = log Kw - sB = log Kw - sA - 0.1s
(Ka is retention factor when φ is A, and Kb also)
and to substract each side, the equation can change like
log(Ka/Kb) = 0.1s
If 10% increase of organic solvent make three-fold increase of k, the left side of equation will be approximately log 3, 0.4771. But the right is 0.3 when s = 3.
And I think 0.3 is closer to log 2 than log 3. If s = 4, still 0.4 ≃ log 2.5.
I am not sure about my thoughts for now, and need someone to check my bias. Please would anyone give me a enlightenment?
And another trivia question; is 'that' logarithm really common, not natural?