This question already has an answer here:

Why can't there be more hydrogen ion concentration in acids or lesser hydrogen ion concentration in bases? Why should there even be any hydrogen ion concentration in alkalis? (I know pH is calculated by the negative log of H+ ion concentration.)


marked as duplicate by Linear Christmas, Mithoron, Loong Feb 11 '18 at 19:19

This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.

  • $\begingroup$ From wikipedia, "For concentrated solutions of acids, especially strong acids for which pH < 0, the $H_0$ value is a better measure of acidity than the pH." $H_0$ value is the Hammett acidity function. I don't know many more details to write an exact answer. I would suppose the $pH$ scale doesn't work very well for very strong acids. We might have to use the activity of $\ce{H+}$ ions in solution instead of their concentration (as we do in ionic equilibrium calculations). Though I still don't know the complete answer. $\endgroup$ – Gaurang Tandon Feb 11 '18 at 11:46
  • 2
    $\begingroup$ I am voting to close this question as a duplicate of Is a negative pH level physically possible?. The question is based on a misconception; the definition of $\pu{pH}$ does not limit itself to the closed interval $[0,14]$. The proper definition is also based on activity, not amount concentration, molality etc. It is difficult to measure highly acidic and basic solutions with usual glass electrodes (due to acid and base errors, among other things) but this is a metrological issue. $\endgroup$ – Linear Christmas Feb 11 '18 at 13:06

It isn't. We're just used to working in the lab where solutions are generally 1 M or less in concentration. So we get no more than 1M (solvated) hydrogen ions where pH is 0, or at the other end no more than 1 M hydroxide ions corresponding to pH 14.

These 1 M limits are, however, just for our convenience and not any law of nature. Right in our own labs we likely have concentrated acid stock solutions, well above 1M, where if we attempted to measure pH we would get less than zero. Acid pickling solutions in the steel industry are concentrated enough to do the same thing, as manufacturers are more interested in getting hot mill scale cleaned off than in a convenient pH scale. Bases like sodium hydroxide also have concentrated stock solutions with pH above 14.


Not the answer you're looking for? Browse other questions tagged or ask your own question.