Acid-Base Titration

The fundamental principle of acid-base titration is the neutralization reaction, typically represented as $H^+(aq) + OH^-(aq) \rightarrow H_2O(l)$.

At the equivalence point, the stoichiometry of the reaction dictates that the moles of acid ($n_a$) and moles of base ($n_b$) are in their balanced ratio. For a monoprotic reaction, this is simply $M_a V_a = M_b V_b$.

For polyprotic acids (like $H_2SO_4$), the stoichiometry must account for multiple protons: $n_H \cdot M_a V_a = n_{OH} \cdot M_b V_b$, where $n$ represents the number of dissociable ions per molecule.

The pH at the equivalence point is determined by the properties of the resulting salt. If a weak acid is titrated with a strong base, the resulting conjugate base undergoes hydrolysis, making the equivalence point pH greater than 7.

Equivalence Point vs. End Point: The equivalence point is a mathematical certainty based on moles, while the end point is an experimental approximation based on human or instrumental perception.

Buffer Region: In weak acid/strong base titrations, a buffer region exists before the equivalence point where the pH changes slowly; this does not occur in strong acid/strong base titrations.

Identify the Half-Equivalence Point: In titrations involving a weak species, the pH at exactly half the volume required for equivalence equals the $pK_a$ of the weak acid ($pH = pK_a$). This is a common shortcut for finding dissociation constants.

Check Stoichiometry: Always verify if the acid or base is polyprotic. Forgetting a factor of 2 for $H_2SO_4$ or $Ba(OH)_2$ is one of the most frequent causes of calculation errors.

Reasonableness Check: If you are titrating a weak acid with a strong base, your calculated pH at the equivalence point MUST be basic (> 7). If your result is 7 or lower, re-evaluate your hydrolysis calculation.

Significant Figures: Volume readings from a buret are typically recorded to two decimal places (e.g., $25.00$ mL). Ensure your final concentration reflects the precision of these measurements.

The 'pH 7' Myth: Many students assume the equivalence point is always at pH 7. This is only true for strong acid-strong base reactions; the presence of conjugate bases or acids from weak reactants will shift the pH.

Indicator Lag: Using too much indicator can actually shift the end point because indicators are themselves weak acids or bases that consume a small amount of titrant.

Over-titration: Adding titrant too quickly near the end point often results in 'overshooting,' where the solution color becomes too intense, leading to an overestimation of the analyte concentration.