page_title: Neutralization (chemistry)
section_title: Weak acids and strong bases
hierarchical_section_title: Neutralization (chemistry) / Weak acids and strong bases
caption_attribution_description: English: weak acid/strong base titration curves
context_page_description: In chemistry, neutralization or neutralisation is a chemical reaction in which acid and a base react quantitatively with each other. In a reaction in water, neutralization results in there being no excess of hydrogen or hydroxide ions present in the solution. The pH of the neutralized solution depends on the acid strength of the reactants.
context_section_description: A weak acid is one that does not dissociate fully when it is dissolved in water. Instead an equilibrium mixture is formed.
  AH + H₂O ⇌ H₃O⁺ + A⁻
  Acetic acid is an example of a weak acid. The pH of the neutralized solution is not close to 7, as with a strong acid, but depends on the acid dissociation constant (pKₐ) of the acid. The pH at the end-point or equivalence point in a titration may be easily calculated. At the end-point the acid is completely neutralized so the analytical hydrogen ion concentration, TH, is zero and the concentration of the conjugate base, A⁻, is effectively equal to the analytical concentration of the acid; writing AH for the acid, [A⁻] = TA. Defining the acid dissociation constant, pKₐ, as
  [A⁻][H⁺] = Kₐ[HA];      pKₐ = −log₁₀Kₐ
  and the self-dissociation constant for water, Kw, as
  Kw = [H⁺][OH⁻];      pKw = −log₁₀Kw
  the equation for mass-balance in hydrogen ions is easy to write down. 
  TH = [H⁺] + Kₐ[A⁻][H⁺] − 
  The term  is equal to the concentration of hydroxide ions. At neutralization, TH is zero.
  [H⁺] + Kₐ[A][H⁺] −  = 0
  [H⁺]² + KₐTA[H⁺]² − Kw = 0
  [H⁺]² = 
  log [H⁺] =  log Kw −  log (1 + KₐTA)
  pH =  pKw −  log (1 + )
  In most circumstances the term 1 +  is much larger than 1, and is equal to  to a good approximation.
  pH ≈  (pKw + pKₐ − log TA )
  This equation explains the following facts:
  The pH at the end-point depends mainly on the strength of the acid, pKₐ.
  The pH at the end-point also depends on the concentration of the acid, TA.
  The pH rises more steeply at the end-point as the acid concentration increases.
  When a weak acid is titrated with a strong base the end-point occurs at pH greater than 7. Therefore, the most suitable indicator to use is one, like phenolphthalein, that changes color at high pH.
caption: Titration curves for addition of a strong base to a weak acid with pKa of 4.85. The curves are labelled with the concentration of the acid.
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