The pH scale usually runs from 0 to 14, but you can calculate a negative pH value for 12 M strong acid. Of course, calculating a negative pH is different from an aqueous solution actually having a negative pH. It turns out experimental verification of negative pH values is slightly complicated. You can't use a glass pH meter to get an accurate pH measurement under extremely acidic conditions. The glass meter will give a high reading for which you can't apply some standard correction. Even strong acids don't completely dissociate at high concentrations, so a concentrated strong acid might be expected to have a higher pH than you would calculate. On the other hand, the hydrogen ion activity is higher for a concentrated strong acid than for a more dilute solution, giving the pH a lower value than you would calculate. Which has more of an effect... the incomplete dissociation or the increased hydrogen ion activity? I'm not sure it's possible to say, but if the hydrogen ion activity wins out, the acid could have a negative pH.