WHY POTASSIUM HYDROGEN PTHALATE IS USED FOR PERCHLORIC ACID?
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Answer / shrinivas rao chirivella
1) Dissolution is the process in which a solute is dissolved in solvent to give solution. For solids,the solute has its crystalline structure disintegrated as separate ions, atoms, and molecules form. For liquids and gases, the molecules must be adaptable with those of the solvent for a solution to form.
2) For some separations the effect of changing pH is minimal. However for acids and bases, the effect of even a very small change in pH is very significant. Changing the pH changes the degree of ionisation of molecules in solution. It thus affects their polarity, and as a consequence it changes their retention times in an HPLC separation.
Even more of a problem is that it changes the retention times of different components in the sample mixture to different extents. Hence it affects the degree of selectivity. It can mean that peaks become further apart, but it can also mean that at certain pH’s they co-elute, and then the peak elution order will change.
It is therefore important that for this type of separation, the pH is controlled using a pH buffer.
3) Perchloric acid is a strong acid, whereas potassium hydrogen phthalate (KHP) is a weak acid. Therefore, perchloric acid will protonate KHP to form phthalic acid. In other words, even though both are considered acids, KHP is more basic than perchloric acid (when you compare their pKa or Ka, the true measure of acid strength).
KHP is the primary standard used for the standarization of perchloric acid, usually 0.1M HCLO4 in concentration. It is the recommended primary standard in the pharmaceutical industry for analytical testing using perchloric acid for non-aqueous titration determinations. The protonation of KHP to phthalic acid when reacted with perchloric acid can be determined stoichiometrically. Using a dried KHP standard of known purity allows the determination of HCLO4.
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