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). The same logarithmic compression that makes pH compact also makes it deceptive: small unit changes are dramatic. Stomach acid (pH 1.5) is 100,000 times more acidic than coffee (pH 5), and 30 million times more than pure water.

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 (neutral)
• Human blood: 7.35-7.45 (tightly regulated; deviation is medically critical)
• Sea water: 8.1
• Baking soda solution: 9
• Bleach: 12.5-13
• Lye: ~14

Why pH matters in cooking

Baking depends on pH in two ways. Chemical leavening: baking soda (sodium bicarbonate, NaHCO₃) needs an acidic ingredient (buttermilk, lemon juice, vinegar, brown sugar) to react and produce CO₂ gas. Baking powder bundles its own acid (cream of tartar or sodium acid pyrophosphate) so it works in any batter, even neutral ones. Use baking powder when the recipe has no acid; baking soda when it has one.

Browning chemistry is also pH-dependent. The Maillard reaction (the brown crust on bread, the seared edge on steak) accelerates as pH rises — alkaline conditions favour it. That’s why pretzels are dipped in a lye bath before baking: the alkaline surface produces the characteristic dark mahogany colour. The same trick on ramen noodles (kansui solution) gives them their yellow, springy character.

pH outside the kitchen

Garden soil pH determines what grows: blueberries want 4.5-5.5 (acidic); most vegetables 6.0-7.0 (slightly acidic to neutral); lavender and lilacs prefer 7.0-7.5 (slightly alkaline). Hydrangea flower colour shifts on pH: acidic soil produces blue blooms, alkaline soil pink.

Pool chemistry targets 7.2-7.6 — too acidic etches plaster and irritates eyes; too alkaline reduces chlorine effectiveness and clouds the water. Aquarium fish have species-specific pH preferences (freshwater tropicals usually 6.5-7.5; marine 8.1-8.4).

For the chemistry of browning specifically, see our Maillard reaction glossary entry.

Worked example

Mix 1 mL of 1 M hydrochloric acid into 1 L of pure water. The HCl dissociates fully: 1 mL × 1 mol/L = 0.001 mol H⁺. In a final volume of ~1.001 L: [H⁺] ≈ 10⁻³ mol/L, giving pH = −log₁₀(10⁻³) = 3. Now add 9 mL more of the same acid (10 mL total): [H⁺] ≈ 10⁻² mol/L, so pH = 2 — a tenfold drop in concentration but only one pH-unit drop in scale. To neutralise the original 1 mL of HCl, add 1 mL of 1 M NaOH and you’re back at pH 7. Note for cooks: a 1% lactic-acid solution (from buttermilk) sits at about pH 3.4; combined with 1.5 g of baking soda per 250 mL, it produces ~330 mL of CO₂ as it reacts — which is exactly the leavening behind a buttermilk pancake.

When and why it matters

Human blood maintains a pH of 7.35-7.45 — a ten-fold tighter range than weather forecasts get away with. A drop below 7.0 (acidosis) or rise above 7.7 (alkalosis) is medically life-threatening, which is why arterial blood gas measurements are reported to two decimal places. The body buffers via bicarbonate (kidneys) and CO₂ exhalation (lungs); diabetic ketoacidosis, sepsis, and chronic kidney disease all show up as small but dangerous pH excursions. Outside medicine: brewing, fermentation (sourdough at pH ~4 inhibits pathogens while letting Lactobacillus thrive), water-treatment chlorination effectiveness (chlorine is ~10× more potent at pH 6.5 than pH 8), and concrete curing (high pH protects rebar from corrosion) all depend on tight pH control. The cheapest pH meter that’s actually useful runs ~$40; cheaper strips drift ±0.5 pH units which is too coarse for any of these uses. Reference: IUPAC Gold Book — pH.