Why is perchloric acid standardized using potassium hydrogen phthalate since both are acidic?

Answer 1

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).

I'm not entirely sure what you mean by "standardized" however. Do you mean doing a titration? Often a solution of KHP is used to calibrate a pH meter because it's pH in solution is very stable. I would be surprised if a titration was done with perchloric acid and KHP however, but I'm not sure what else you could mean by asking why an acid is standardized using KHP.

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.