[kizm0]

Say I have a bottle of 32% hydrochloric acid.  How can I purify it or dry some of it to get up to 36-37% hydrochloric acid.

[sjb]

Why do you need such a precisely defined concentration of acid?

[Arkcon]

Probably it's just distilled, the vapor captured, and redissolved in water.  That's an industrial application, not for a smaller lab.  Hydrogen chloride may not be soluble in water, at all temperatures, at that concentration.  For example, constant boiling HCl, used for quantitative analysis, is an azeotrope of ~20 % HCl.  It's actual concentration varies, and is determined first, then it's sealed in an ampule.  You break the seal, and use it promptly, because the HCl fumes away.  Concentrating it by a few percent in the lab will result in you losing more during the manipulation.