Acid-Base Titration

Acid-Base Titration HnA + n OH- n A- + n H2O Equivalence Point : The point at which the reaction is just complete End Point : The place where experimentally the reaction is observed to be complete

• At Equivalence Point : A solution initially containing [A-] moles of HA, now contains an equivalent amount of A-. • The amount of base needed to reach the equivalence point is independent of the strength of the acid. THE WHOLE BASIS OF TITRATION RELYS ON THIS FACT!

• Although, the location of the equivalence point does not change, the shape of the curve does change.

Observing the equivalence point • There are as many equivalence points as there are replaceable hydrogens in an acid. • Some equivalence points MAY occur without a noticeable rise in pH! • Two requirements are necessary for a jump in pH to occur. The successive Ka values must differ by several orders of magntude 2. The pH of the equivalence point must not be very high or very low.

Observing the equivalence point – I knew there was a use to studying calculus!

Detecting the equivalence point • pH Measurements are the most direct way of measuring the equivalence point • Less exacting methods include using an indicator dye, which is it self an acid or a base. The protonated and deprotonated forms have different colors, and the pKa should be close to the pH expected at the equivalence point. • The observed color changes of an indicator changes slowly over 1.5 to 2 pH units. • Indicators are therefore only useful in titrations of acid and bases that show definite breaks in the titration curve.

Common acid-base indicators

Phenolphthalein H2In In2− In(OH)3− < 0 0−8.2 8.2−12.0 >12.0 colorless pink to fuchsia colorless