pH

pH is a logarithmic measure of the hydrogen ion concentration in a solution. The scale runs from 0 (most acidic) to 14 (most alkaline), with 7 as neutral. Each unit represents a 10× change in hydrogen ion concentration: a solution at pH 4 is 1000× more acidic than one at pH 7.

Formula: pH = −log₁₀[H⁺] where [H⁺] is the molar concentration of hydrogen ions. The notation comes from Søren Sørensen’s 1909 paper; “pH” is short for “power of hydrogen” (potens hydrogenii).

Reference points:

• Battery acid: ~0
• Stomach acid: 1.5-2
• Lemon juice: 2-3
• Coffee: 5
• Milk: 6.5
• Pure water at 25°C: 7
• Human blood: 7.35-7.45 (tightly regulated)
• Sea water: 8.1
• Baking soda solution: 9
• Bleach: 12.5-13
• Lye: ~14

Baking depends on pH: baking soda needs an acidic ingredient (buttermilk, lemon, vinegar) to react and produce CO₂; baking powder includes its own acid (cream of tartar) and works in any batter.