ACID-BASE TITRATION CURVES

ACID-BASE TITRATION CURVES

A titration curves is a graph of pH against the volume of solution added for a titration involving strong and weak acids and bases.

Titrations are common analytical procedures that chemists perform, with the usual goal of determining the concentration of one of the reactants.

ACID-BASE TITRATION CURVES

As you know, the equivalence point is the point in a titration when the acid and base that are present completely react with each other.

If chemists know the volumes of both solutions at the equivalence point, and the concentration of one of them, they can calculate the unknown concentration.

ACID-BASE TITRATION CURVES

In any titration the initial solution is either the acid or the base being analyzed, so the pH at the beginning of the pH curve (at volume = 0 L) is equal to that of the solution in the flask.

This information is a good starting point for determining the nature of the solutions that are reacting.

ACID-BASE TITRATION CURVES

If the initial pH is very low, say pH = 1, then the solution being titrated is probably a strong acid.

If the pH is somewhat higher than this, but still below pH = 7, then the solution being titrated is probably a weak acid.

In the event that a base is being analyzed, the initial pH may be about 10 or 11 (weak base) or much higher, pH = 13 for a strong base.

ACID-BASE TITRATION CURVES

The initial pH identifies the solution being analyzed as a strong or weak acid or base.

ACID-BASE TITRATION CURVES

During a titration, the pH of the solution in the reaction flask does not change in a regular manner as more solution is added from the burette.

The middle of the steep rise that occurs in a titration curve is the equivalence point.

The endpoint of a titration occurs when the indicator changes colour, which happens over a range of about 2 pH units.

ACID-BASE TITRATION CURVES

The pH changes colour rapidly near the equivalence point.

Therefore, the change in colour usually takes place in a fraction of a millilitre, with the addition of a single drop of solution.

When an indicator is chosen for a titration, the end point pH (which is essentially the equivalence point pH) must be within the pH range over which the indicator changes colour.

TITRATION CURVE FOR A STRONG ACID WITH A STRONG BASE

TITRATION CURVE FOR A WEAK ACID WITH A STRONG BASE

If the acid being used for the titration is a weak acid, the shape of the titration curve differs significantly.

Consider the titration of acetic acid (CH3COOH) with sodium hydroxide (NaOH).

Acetic acid is a weak acid. The product of its reaction with NaOH is a

weak base, the acetate ion (CH3COO-).

TITRATION CURVE FOR A WEAK ACID WITH A STRONG BASE

At the equivalence point of this titration, the products in the reaction flask will include this weak base; hence, the pH will be greater than 7, due to the reaction of the acetate ion with water:CH3COO-

(aq) + H2O(l) CH3COOH(aq) + OH-(aq)

This is called a salt hydrolysis reaction.

TITRATION CURVE FOR A WEAK ACID WITH A STRONG BASE

TITRATION CURVE FOR A STRONG ACID WITH A WEAK BASE

How does the shape of the titration curve change if a strong acid and a weak base are used?

Consider the titration of HCl with ammonia (NH3).

The product of the reaction of ammonia with HCl is a weak acid, the ammonium ion (NH4

+). At the equivalence point of this titration, the

products will include this weak acid, hence the pH will be less than 7, due to the reaction of the ammonium ion with water:NH4

+(aq) + H2O(l) NH3(aq) + H3O+

(aq)

TITRATION CURVE FOR A STRONG ACID WITH A WEAK BASE

TITRATION CURVE FOR A WEAK ACID WITH A WEAK BASE

In the final case of a weak acid-weak base titration, the pH increases steadily and does not change rapidly at the equivalence point.

Determining the equivalence point using an indicator is therefore difficult.

In this case, the titration must be followed using a pH meter, and the titration curve used to determine the equivalence point.

TITRATION CURVE FOR A WEAK ACID WITH A WEAK BASE

TITRATION CURVES