Ionization of Water and pH

Ionization of Water

• Water can ionize slightly by reacting with another water molecule.

  • One molecule acts as a base (accepts H⁺) and the other as an acid (donates H⁺).
  • Produces hydronium ion (H₃O⁺) and hydroxide ion (OH⁻).
  • Double-headed arrow indicates that this reaction is not extensive.

• The ion product constant for water (Kₗ or K_W) is 1 × 10⁻¹⁴.

  • K_W = [H⁺][OH⁻]
  • In neutral solutions, [H⁺] = [OH⁻] = 1 × 10⁻⁷ M.

pH Scale

• pH is the negative logarithm of H⁺ concentration: pH = -log[H⁺].
• Scale ranges from 0 (very acidic) to 14 (very basic). Neutral pH = 7.
• Biological fluids pH usually ranges from 6.5 to 8.
  • Buffers maintain pH stability in this range.

Impact on Biological Systems

• Enzymatic activity is highly sensitive to pH.
  • Example: Pepsin is most active at pH ~2, while Trypsin is most active around pH 6.2 to 6.3.
• Strong acids/bases completely dissociate in water, weak acids/bases only partially dissociate.

Acid & Conjugate Base Pairs

• Weak acids like HA in water reach equilibrium, donating an H⁺ to form A⁻.

  • HA + H₂O ⇌ A⁻ + H₃O⁺
  • Simplified: HA ⇌ A⁻ + H⁺

• Ka (acid dissociation constant) equation: Ka = [H⁺][A⁻] / [HA]

  • Larger Ka = stronger acid; smaller Ka = weaker acid.

• pKa = -log(Ka)

  • Smaller pKa = stronger acid; larger pKa = weaker acid.

Titration and pKa

• During titration, adding base to acid removes acidic H⁺, increasing pH.
  • Midpoint of titration: equal amounts of acid and its conjugate base, pH = pKa.
  • Example: Formic acid (HCOOH) titration shows clear equivalent points marking complete ionization steps.

Polyprotic Acids

• Example: Phosphate ions with three ionizable hydrogens.
  • Shows multiple pKa values as successive hydrogens are removed upon base addition.

Buffers

• Buffers resist pH changes by having a weak acid and its conjugate base in solution.
  • Effective within ~1 pH unit of the buffer's pKa.
• Henderson-Hasselbalch Equation: pH = pKa + log([A⁻]/[HA])
  • Describes relationship between pH, pKa, and component concentrations.
  • Only works with weak acids and bases, not strong ones.

Buffer Effectiveness

• Effective buffers contain both the weak acid and its conjugate base in near-equal concentrations.
  • Example: Acetic acid (pKa ≈ 4.76), effectiveness range ~pH 3.76 to 5.76.

Biological Relevance: Amino Acids

• Example: Glycine's ionizable groups (amine and carboxyl) show variable protonation based on pH.
  • Carboxylic acid group deprotonates first (around pH 2.5).
  • Amine group deprotonates later (around pH 9.5).

Zwitterions

• At certain pH (isoelectric point, pI), amino acids exist as zwitterions (net charge = 0).
  • pI is found by averaging the pKa values of the groups ionized before and after the zwitterion state.
  • Example for glycine: (pKa1 + pKa2) / 2 = pI