Have you ever wondered whether a negative pH value is possible in chemistry? The answer is yes! The pH scale usually runs from 0 to 14, but if the molarity of hydrogen ions in a strong acid is greater than one you’ll calculate a negative pH value. For example, the calculated pH of 12M HCl (hydrochloric acid) is:
pH = -log[H+]
pH = -log[12]
pH = -1.08
How Negative pH Works
Of course, calculating a negative pH is different from an aqueous solution actually having a negative pH. The pH equation assumes complete dissociation of a strong acid into its ion. It turns out highly concentrated strong acids don’t completely dissociate, so a concentrated strong acid might have a higher pH than the calculated value. On the other hand, the hydrogen ion activity is higher for a concentrated strong acid than for a more dilute solution. This is because the solution contains very little water per unit of acid. This tends to make the true pH lower than the calculated value. The key is to figure out which has a greater effect: the incomplete dissociation or the increased hydrogen ion activity. If the hydrogen ion activity wins out, the acid could have a negative pH.
The pH of a 12M strong acid is negative using the pH equation. The dissociation of the acid and its hydrogen ion activity determine whether the true pH is negative. It’s also possible to have a pH of 0. For strong bases, it’s possible to have a pH greater than 14.
How to Measure Negative pH
It turns out experimental verification of negative pH values is slightly complicated. There isn’t a special litmus or pH paper that changes color at a negative value. Glass pH meters don’t give accurate pH measurement under extremely acidic conditions. A glass meter gives a high reading called an “acid error” that can’t be fixed using the standard correction. In real-world situations with negative pH, special correction factors are applied to glass electrode measurements. The Pitzer approach and MacInnes assumption are the names of two methods used to scale measurements. One place to find negative pH is in the water from metal sulfide mining operations.
References
- Bates, Roger G. (1973). Determination of pH: theory and practice. Wiley.
- Lim, Kieran F. (2006). “Negative pH Does Exist”. Journal of Chemical Education. 83 (10): 1465. doi:10.1021/ed083p1465
- Nordstrom, D. Kirk; Alpers, Charles N. (March 1999). “Negative pH, efflorescent mineralogy, and consequences for environmental restoration at the Iron Mountain Superfund site, California”. Proceedings of the National Academy of Sciences of the United States of America. 96 (7): 3455–62. doi:10.1073/pnas.96.7.3455