Does hydrochloric acid change in concentration over time? I have new bottles of 1 N hydrochloric acid from Fisher. The acid comes standardized from the company:


  Standardized at 25 °C to pH 8.2 against sodium hydroxide that has been standardized against NIST™ potassium acid phthalate.


Will it remain standardized after opening the bottle, or do we have to standardize it regularly?
First of all, check the expiry date. In general HCl solutions are secondary standards. 1 N is just a approximation. You need to standardize it every time with a primary standard such as sodium carbonate. On the other hand, constant boiling HCl is indeed a primary standard. It is not very well known, perhaps because modern analytical chemists have become lazier.

Constant-boiling hydrochloric acid
John L. Sheldon
J. Chem. Educ., 1935, 12 (8), p 377
https://pubs.acs.org/doi/abs/10.1021/ed012p377