pka value

Basic questions from students; resources for projects and reports.

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what is pka value? How it help us to develop an analytical method in case of acid/base/neutral compound. please explain.
pka value is described in many places. For a start, I will suggest: http://en.wikipedia.org/wiki/Acid_dissociation_constant.

There are many ways that the pKa is used, both for sepatation as well as characterization of chemical species.

(And this discussion probably belongs under student projects?)
The pKa value is one method used to indicate the strength of an acid. pKa is the negative log of the acid dissociation constant or Ka value. A lower pKa value indicates a stronger acid.
The pKa is the pH where 1/2 of a weak acid is undissociated and 1/2 of it is ionized in aqueous medium.

This is the pH where the buffering capacity (ability to resist changes in pH, acid or base) of the weak acid/conjugate base is a maximum.
… so it's of relevance all over the place, but I'll describe for reverse phase chromatography only. In reverse phase, hydrophobic things travel most slowly. If your analyte is charged, it will be more hydrophilic, and travel faster.
If you want to get more retention, then amines, which are bases, may be better retained in alkaline conditions where they're -NH2 groups rather than -NH3(+). The other way round, acids may be better retained in acidic condition where they're -COOH groups and not -COO(-).
If your system isn't ideal, and temperatures aren't perfectly equilibrated, then the outside of a column may be at a slightly different temperature to the inside. Since chemical equilibria depend on temperature, this means the position of the equilibrium H + A <--> HA for your analyte may be different in different parts of the column! This creates the horrible situation that some analyte is moving faster than other analyte, and the peak gets broad and misshapen. To avoid this, it's a good idea to use running solvents whose pH isn't too close to the pKa of your analytes. That way, the analyte will all be A or HA, and not some variable mix of the two.
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