What are pH and pOH
pH (potential of hydrogen) measures the acidity or basicity of an aqueous solution. It is the negative logarithm of the H⁺ ion concentration:
pH = −log[H⁺] pOH = −log[OH⁻] pH + pOH = 14 (at 25 °C)
The pH scale
- pH < 7 → acidic solution (more H⁺)
- pH = 7 → neutral solution (pure water at 25 °C)
- pH > 7 → basic/alkaline solution (more OH⁻)
The scale is logarithmic: each unit represents a factor of 10 in concentration. A pH of 4 is 10× more acidic than pH 5 and 100× more acidic than pH 6.
Autoionization constant (Kw)
Water undergoes autoionization: H₂O ⇌ H⁺ + OH⁻. At 25 °C, the product of the concentrations is constant:
Kw = [H⁺] · [OH⁻] = 1 × 10⁻¹⁴
That is why, in pure water: [H⁺] = [OH⁻] = 10⁻⁷ mol/L, giving pH = pOH = 7.
Example: pH of gastric juice
Gastric juice has [H⁺] ≈ 0.032 mol/L (3.2 × 10⁻²):
pH = −log(3.2 × 10⁻²) pH = −(log 3.2 + log 10⁻²) pH = −(0.505 − 2) pH = 1.5
Frequently asked questions
Why is pH 7 neutral?
Because at 25 °C pure water has exactly [H⁺] = [OH⁻] = 10⁻⁷ mol/L. Since pH = −log(10⁻⁷) = 7, that is the balance point between acidity and basicity.
How can a single pH unit make such a big difference?
Because the scale is logarithmic. pH 3 has 10× more H⁺ than pH 4, and 100× more than pH 5. That is why the stomach (pH 1.5) is so much more acidic than lemon (pH 2.4).
Can pH be negative or greater than 14?
Yes, in very concentrated solutions. 12 mol/L sulfuric acid has pH ≈ −1, and very concentrated NaOH can have pH > 15. But in high school we work in the 0–14 range.
What changes with temperature?
Kw increases with heat. At 100 °C, Kw ≈ 10⁻¹², so pure water has pH ≈ 6 — while still being neutral. The pH+pOH relation changes too.
How do you measure pH in practice?
With universal indicator paper (precision ~0.5), a pH meter (precision 0.01) or acid-base indicators such as phenolphthalein (turns pink at pH > 8.3) and bromothymol blue.
Why is blood pH tightly controlled?
Blood pH must stay between 7.35 and 7.45. Changes of only 0.1 already cause serious acidosis or alkalosis. The body uses the bicarbonate buffer system (HCO₃⁻/H₂CO₃) to maintain that balance.