The pH Scale
pH measures the acidity of a solution on a logarithmic scale from 0 (most acidic) to 14 (most basic), with 7 being neutral (pure water at 25°C).
pH Formulas
pH = -log₁₀[H⁺]
[H⁺] = 10^(-pH) (mol/L)
pOH = -log₁₀[OH⁻]
pH + pOH = 14 (at 25°C)
Strong acid (HCl 0.01 M):
[H⁺] = 0.01 = 10⁻² → pH = 2Weak Acid (Ka)
pH = ½(pKa - log[HA])
= ½(pKa + log C) (approximate)
Acetic acid (Ka=1.8×10⁻⁵, pKa=4.74), 0.1M:
pH ≈ ½(4.74 - log 0.1) = ½(4.74+1) = 2.87Common pH Values
- Gastric acid: 1.5–3.5
- Lemon juice: ~2.2
- Coffee: ~5.0
- Milk: ~6.5
- Pure water: 7.0
- Baking soda: ~8.3
- Bleach: ~12.5
Calculate pH: Free pH Calculator
pH Quick-Reference Table
| Substance | pH | Classification |
|---|---|---|
| Battery acid (H₂SO₄) | ~0 | Strongly acidic |
| Lemon juice | 2.0–2.5 | Strongly acidic |
| Vinegar | 2.5–3.5 | Acidic |
| Black coffee | 5.0 | Mildly acidic |
| Pure water (25°C) | 7.0 | Neutral |
| Blood | 7.35–7.45 | Slightly alkaline |
| Baking soda solution | 8.3 | Mildly alkaline |
| Household bleach | 11–13 | Strongly alkaline |
| Drain cleaner (NaOH) | ~14 | Strongly alkaline |
How pH Works
pH = −log₁₀[H⁺], where [H⁺] is the hydrogen ion (proton) concentration in mol/L. pH 7 at 25°C is neutral (pure water); below 7 is acidic; above 7 is alkaline (basic). Because the scale is logarithmic, each unit change represents a 10× change in [H⁺]: pH 4 has 10× more H⁺ than pH 5, and 100× more than pH 6. The full expression is pH + pOH = 14 (at 25°C), where pOH = −log₁₀[OH⁻].
pH control is critical in chemistry (reaction rates depend on pH), biology (enzyme activity peaks in narrow pH ranges — pepsin works at pH 2; trypsin at pH 8), agriculture (soil pH affects nutrient availability), water treatment, swimming pool maintenance, pharmaceutical manufacturing, and food preservation.
Common Mistakes
- Reversing the scale: Low pH = high acidity = high [H⁺]. pH 2 is more acidic than pH 5, not the other way around.
- Linear thinking on a log scale: pH 4 is not "twice as acidic" as pH 8 — it has 10,000× (10⁴) more H⁺. Always convert to concentrations for quantitative comparisons.
- Temperature dependence: Neutral pH at 37°C (body temperature) is 6.8, not 7.0, because Kw changes with temperature. pH 7 blood is actually slightly alkaline relative to physiological neutral.
Frequently Asked Questions
Blood pH must stay between 7.35–7.45. Below 7.35 is acidosis; above 7.45 is alkalosis — both life-threatening if severe. The body uses bicarbonate buffering (H₂CO₃ ⇌ H⁺ + HCO₃⁻), respiratory control (faster breathing removes CO₂, raising pH), and renal excretion to maintain this narrow range. A pH drop to 7.0 causes coma; 6.8 is fatal.
A buffer resists pH changes by containing a weak acid and its conjugate base in equilibrium. The Henderson-Hasselbalch equation pH = pKa + log([A⁻]/[HA]) shows buffering is strongest when pH ≈ pKa (1:1 acid/base ratio). Bicarbonate buffer (pKa = 6.1) is effective at blood pH because the lung continuously removes CO₂, effectively pulling the equilibrium to regenerate HCO₃⁻.
Normal rain is slightly acidic (pH ~5.6) due to dissolved CO₂. Acid rain (pH 4–4.5) results from SO₂ and NOₓ emissions forming H₂SO₄ and HNO₃. In soils and lakes, the 10–100× increase in [H⁺] leaches aluminium ions (toxic to fish) from soil particles, dissolves calcium and magnesium nutrients, and inhibits nitrifying bacteria essential for nitrogen cycling — collectively damaging entire food webs.