salts in buffer prepration of hplc analysis

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mohd
mohd's picture
salts in buffer prepration of hplc analysis

dear sir
hi 
can u tellwhy we will always use sod/pot/ammonium di hydrogenorthophospahte  for the prepration of buffer solutions in hplc anaalysis

Dr. Analytical
Dr. Analytical's picture
We use buffers in HPLC mobile

We use buffers in HPLC mobile phases to control the pH, which will control the ionization form of the acid or base.

HA + H2O -> A- + H3O+

The neutral form (HA) usually has more retention.  The ionic form (A-) is more polar, and more water soluble, and has less retention on a reversed phase column.  So, we control the pH to control which form is present, depending on whether we want retention, or do not want retention of the acid (or base).  A pH is chosen to be at least 2 pH units different from the pKa of the acid.  A lower pH means the neutral form is present.  A higher pH means the ionic form is present. 

Most weak acids have a pKa of about 4 - 6, so a pH of 2-3 is typical to produce retention, or a pH of 7 - 8 to put the acid in its ionic form.  Then, you must choose a buffer that has sufficient buffering capacity at the desired pH.

The phosphate buffers are common because they are not expensive and can be used in three different pH ranges, but there are many other options also.