The Kilcher Homestead is a 600 acre Pioneer Homestead that has been protected for perpetuity via a ConservatIon Easement. The Kilcher Family has also preserved the Historic Cabin and site as a LIving Museum.
Yes, negative pH is possible, but not in water as a solvent. The range of pH in water is 0 to 14 (to be more precise, it is -1.7 to 15.7, but for simplicity's sake, we start with the usual values.) Thus, what pH indicates is the ability of the acid to form H3O+, which can then donate a proton.
The reason NaCl doesn't change the pH of a solution is because the cation (Na+) and anion (Cl-) that are produced when NaCl is dissolved are really weak acids and bases. We know this because their conjugate acid and base partners are really strong acids and bases. The conjugate base of Na+ is NaOH and the conjugate acid of Cl- is HCl. Look how tiny those dissociation constants are. Those dissociation constants tell us that the reaction will lie very far to the left.