Acid Base Titration

Acid-base reactions are most often used to analyze the amount of acid or base in a solution by titration.  An example of an acid-base titration where a hydrochloric acid solution with an unknown concentration is titrated with a sodium hydroxide standard solution . In this titration, a known volume of the hydrochloric acid solution would be "titrated" by slowly adding small amounts of the NaOH standard solution.   The titrant, sodium hydroxide in this case, reacts with and consumes  the acid through a neutralization reaction (shown in Equation 1).  The exact volume of base needed to react completely with the acid is measured.  This is called the equivalence point of the titration - the point where the molar amount of base added and the molar amount of acid in the unknown are equal.

HCl (aq)  +   NaOH (aq)   →   NaCl (aq)  +  H2O (l)

Knowing the exact concentration of and volume of the titrant gives the number of moles of sodium hydroxide.  The latter, in turn, is related by stoichiometry to the number of moles of hydrochloric acid initially present in the unknown.

Endpoint and Equivalence Points in Titrations

The terms endpoint and equivalence point often get used to mean the same thing but these two terms can refer to slightly different points in a titration. The endpoint of a titration is the point at which an color change, often from an indicator added to the reaction signals that reaction is complete. Indicators are usually added to acid-base titrations to detect the equivalence point, but they are often not exact. The equivalence point is the point in the reaction where the amount (or moles) of titrant neutralizes the amount (or moles) of analyte.

For example, in the case of the neutralization reaction shown in Equation 1, the pH of the solution would be acidic (<7) before the equivalence point and basic (>7) after the equivalence point.  The pH at the equivalence point should be exactly 7, corresponding to the neutral products (sodium chloride and water).  An indicator like phenophthalein that changes color just above a pH of 7 is a suitable indicator for the titration of a strong acid with a strong base and would tell us the endpoint of this reaction and comes close to determining the equivalence point.

Animated Molecular View of a Weak Acid-Strong Base Titration - University of Guelph, (Ontario, Canada) Chemistry Department