[aqhl]

I have a small question. Why and when pH meters display negative value ?

[sdekivit]

pH = -log [H+], so when will the pH-meter give a negative value ?

[Borek]

Calculate [H⁺] for pH = -0.1 - that's your answer.

Just remember that reality is much more complicated, as pH is not based on concentration, but on ion activity, which - in concentrated solutions - can be very different from concentration. Thus it is not easy to observe negative pH values, and even more difficult to measure them reliably, as pH electrodes are usually outside of their working range.

sdekivit was faster, but I am posting anyway