- measure of hydrogen ion concentration in a solution
- scale runs from 0 (strongly acidic) through 7 (neutral) to 14 (strongly alkaline)
- defined as pH = -log10([H+])
- each unit represents a tenfold change in ion concentration
- blood pH held at 7.35-7.45 by buffer systems; deviation is life-threatening
- soil pH determines nutrient availability for plants
- stomach acid operates at pH 1.5-3.5, enabling protein digestion
- measured by pH meters, litmus paper, universal indicators
- ocean acidification: rising carbon dioxide lowers ocean pH, threatening marine calcium carbonate structures