6.3 Titrations and pH Curves

Acid-base titrations are crucial for determining unknown concentrations of acids or bases. The process involves gradually adding a known solution to an unknown one, monitoring pH changes to pinpoint the equivalence point where acid and base moles are equal.

Titration curves plot pH against titrant volume, revealing key information about the reaction. The curve shape differs for strong vs weak acids/bases, with strong acid-strong base titrations showing sharp equivalence points and weak acid-strong base titrations displaying buffer regions and gradual pH changes.

Acid-Base Titrations and pH Curves

Process of acid-base titrations

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• Titration process gradually adds known concentration solution (titrant) to unknown concentration solution (analyte) using burette for controlled addition
• Endpoint detected with indicators (phenolphthalein) or pH meters measuring neutralization reaction between acid and base
• Stoichiometric relationship and conservation of mass/charge govern acid-base reactions
• Various titration types include acid-base, redox, and complexometric (EDTA)
• Setup uses burette for precise titrant addition, Erlenmeyer flask for analyte, magnetic stirrer for mixing

Interpretation of titration curves

• Curve plots titrant volume (x-axis) vs solution pH (y-axis)
• Equivalence point occurs when acid moles equal base moles, shown as steep vertical region
• Buffer regions appear as flat portions, resist pH change when weak acid/base and conjugate present
• Half-equivalence point marks buffer region midpoint where pH equals weak acid/base pKa
• Initial/final plateaus represent pure acid or base solution pH

pH calculations in titrations

• Before titration: use acid/base dissociation equations
• During titration, pre-equivalence: apply Henderson-Hasselbalch $pH = pKa + log([A^-]/[HA])$, account for dilution
• At equivalence: strong acid-strong base pH = 7, weak acid-strong base pH > 7, strong acid-weak base pH < 7
• Post-equivalence: calculate excess base, consider conjugate base/acid hydrolysis
• Polyprotic acids show multiple equivalence points, use stepwise dissociation constants

Strong vs weak acid titrations

• Strong acid-strong base: sharp equivalence at pH 7, no buffer region, symmetrical S-shaped curve, rapid pH change near equivalence
• Weak acid-strong base: equivalence at pH > 7, buffer region present, asymmetrical elongated S-curve, gradual pH change near equivalence
• Initial pH differs: strong acid very low, weak acid higher
• Indicators: strong acid-strong base uses wide range, weak acid-strong base needs specific indicators (bromothymol blue)