Plot a graph of pH versus ml of titrant. Find out equivalent point using it. At half equivalent point pH will be equal to pKa as per Henderson Hasselbach equation

In such case retention will be more. So for non polar compound go with normal phase and for polar compound go with non polar stationary phase.

Either from Pharmacopoeia or authentic source books- Merck index or certificate of analysis. Even you can determine in Lab by doing simple acid base titration using pH meter and apply Henderson Haselbach equation.

Log p is determined using octanol and water .

For weakly acidic or basic compound, start with C18

If a compound is weak acid or base, start with reverse phase that is C18 column, so compound retention time can be reduced

@@dimalshah9521 k sir, Example our compound is Arachidonic acid it's containing isomers, But C18 column in eluting single peak, and peak purity was failed and how to selection of column and how to move this arachidonic acid. Please explain sir this problem.

@@beejamchandrashekarreddy2952 As it is fatty acid, GC or Chiral separation in HPLC can be tried

Based of limit if RS, you can start with separation of RS with drug and optimization

To get reproducible and specific result

Presence of polar group in a compound can give idea

@@dimalshah9521 please give lecture on Preparative HPLC.

Presence of hetero atom and functional group will help to find out

@@dimalshah9521 can you explain by an example please

@@krish_krish354 presence of N, O, S, Halogens having higher electonegativity contribute in polarity