The pH scale runs from 0 to 14 and measures hydrogen ion concentration:

• pH 0-6 is acidic — more H+ ions. Battery acid sits near 0, lemon juice at 2, coffee at 5
• pH 7 is neutral — pure water
• pH 8-14 is basic (alkaline) — more OH- ions. Baking soda is 9, ammonia is 11, drain cleaner approaches 14
• Each whole number is a tenfold change in H+. pH 3 is ten times more acidic than pH 4, and a hundred times more acidic than pH 5

Your blood stays between pH 7.35 and 7.45, and pushing it past roughly 7.0 or 7.8 becomes life-threatening.