The usual range of pH values runs from 0 to 14. If you are given the molarity of hydrogen ions of an acid that is greater than one, however, you'll calculate a negative pH value for the acid. Is it possible to have a negative pH value?

How Negative pH Works

It's definitely possible to calculate a negative pH value. But on the other hand, whether or not an acid actually has a negative pH value isn't something you can verify very well in the lab.

In practice, any acid that yields a concentration of hydrogen ions with a molarity greater than 1 will be calculated to have a negative pH. For example, the pH of 12M HCl (hydrochloric acid) is calculated to be -log(12) = -1.08. But, you can't measure it with an instrument or test. There isn't any special litmus paper that turns a color when the value is below zero. pH meters are better than pH paper, yet you can't just dip a glass pH electrode in the HCl and measure a negative pH. This is because glass pH electrodes suffer from a defect called 'acid error' which causes them to measure a higher pH than the real pH. It is very difficult to apply a correction for this defect to obtain the true pH value.

Also, strong acids do not fully dissociate in water at high concentrations. In the case of HCl, some of the hydrogen would remain bound to the chlorine, so in this respect, the true pH would be higher than the pH you would calculate from acid molarity.

To further complicate the situation, the activity or effective concentration of hydrogen ions in a concentrated strong acid is higher than the actual concentration. This is because there is so little water per acid unit. While pH commonly is calculated as -log [H+] (negative of the logarithm of the hydrogen ion molarity), it would be more accurate to write pH = - log aH+ (negative pf the logarithm of the hydrogen ion activity). This effect of the enhanced hydrogen ion activity is very strong and makes the pH much lower than you'd expect from the acid molarity.

Summary of Negative pH

In summary, you can't accurately measure extremely low pH with a glass pH electrode and it is difficult to tell whether the pH is lowered by the increased hydrogen ion activity more than it is raised by incomplete dissociation. Negative pH is possible and simple to calculated, but not something you can easily measure. Special electrodes are used to assess extremely low pH values. In addition to negative pH, it's also possible for pH to have a value of 0. The calculation also applies to alkaline solutions, in which the pOH value can extend beyond the typical range.