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Is a negative pH possible?

When you are given the molarity of an acid solution, and asked to determine the pH, a molarity greater than 1 produces a negative pH. Is this possible?
Barry Bosco

It's possible. If the molarity of hydrogen ions is greater than 1, you'll have a negative value of pH. For example, you might expect a 12 M HCl solution to have a pH of -log(12) = -1.08.

Why don't you hear more about negative pH? There are some complications in high molarity acid solutions that make pH calculations from acid molarity inaccurate and difficult to verify experimentally:

  1. Even strong acids don't dissociate completely at high concentrations. Some of the hydrogen remains bound to the chlorine, making the pH higher than you'd expect from the acid molarity.
  2. Because there are so few waters per acid formula unit, the influence of hydrogen ions in the solution is enhanced. We say that the effective concentration of hydrogen ions (or the activity) is much higher than the actual concentration. The usual general chemistry text definition of pH as -log [H+] (negative the logarithm of the hydrogen ion molarity) is better written as pH = - log aH+ (negative the logarithm of the hydrogen ion activity). This effect is very strong, and makes the pH much lower than you'd expect from the acid molarity.
  3. If you were to dip a glass pH electrode into the 12 M HCl solution to actually measure the pH, you would get a pH that was higher than the true pH. This well-known defect in glass pH electrode measurements is called the "acid error"; it is sensitive to experimental conditions and difficult to correct for.

Author: Fred Senese senese@antoine.frostburg.edu

General Chemistry Online! Is a negative pH possible?

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Last Revised 02/15/10.URL: http://antoine.frostburg.edu/chem/senese/101/acidbase/faq/negative-pH.shtml