[crazypakithug]

Can anyone help me answer this question.
Why does potassium hydrogen phthalate have a purity slightly higher than 100% when using a standard acid?

[Arkcon]

When titrated against a standard base, by experts, under very carefully controlled conditions, they found that it neutralized a slightly greater amount of base.  Probably, it contains a slight impurity of another acid, another compound, or a breakdown product, I'd guess phthalic acid.  So when you use this batch of  potassium hydrogen phthalate, you'll use that purity, and get the correct titer value for your base.

This happens often, sometimes a trace impurity has an effect on the bulk properties much greater than it's concentration.  In the example above, phthalic acid is a diprotic acid, so it's effect, on a base neutralization, is twice what it's concentration is.  As another example, purified enzymes, or hormones may have activities a fraction of a percent above what their purity suggests, that could be a trace of another, more active hormone, or the removal of an inhibitor.

[azmanam]

Could it also have something to do with the industrial scale mono-deprotonation either not going to completion or going too far and starting to deprotonoate the other acid?  that would cause and incorrect titration, too.  Or is that kinda what you were saying, too.

[Borek]

Note: KHP is used as a primary substance not without a reason. It can be realtively easy obtained as a very pure substance, without impurities. Than, it can be easily dried to constant mass. Wtill, it is always possible that your bottle has been contaminated by some moron

I wonder what this question really asks about? What does "when using standard acid" mean? For me - nothing. No such thing as standard acid AFAICT. Perhaps the question is really about titration error and indicators?

[Arkcon]

No such thing as standard acid AFAICT. Perhaps the question is really about titration error and indicators?

Yes, absolutely.  It is more likely the label says something like, "Purity (by phenolpthalein titration) -- 100.05%."  It could equally likely say, "Purity (by Total Organic Carbon) -- 99.97%," if they wanted to highlight another impurity to put into your standard calculations.  Again, like Borek said, it is as much a known limitation of the analytical method, as a real quantification of the impurities in the jar.