Titration is a procedure used in chemistry in order to determine the molarity of an acid or a base. A chemical reaction is set up between a known volume of a solution of unknown concentration and a known volume of a solution with a known concentration. The relative acidity (basicity) of an aqueous solution can be determined using the relative acid (base) equivalents. An acid equivalent is equal to one mole of H+ or H3O+ ions. Similarly, a base equivalent is equal to one mole of OH- ions. Keep in mind, some acids and bases are polyprotic, meaning each mole of the acid or base is capable of releasing more than one acid or base equivalent. When the solution of known concentration and the solution of unknown concentration are reacted to the point where the number of acid equivalents equals the number of base equivalents (or vice versa), the equivalence point is reached. The equivalence point of a strong acid or a strong base will occur at pH 7. For weak acids and bases, the equivalence point need not occur at pH 7. There will be several equivalence points for polyprotic acids and bases.
How to Estimate the Equivalence Point
There are two common methods of estimating the equivalence point:
- Use a pH Meter
For this method, a graph is made plotting the pH of the solution as a function of the volume of added titrant.
- Use an Indicator
This method relies on observing a color change in the solution. Indicators are weak organic acids or bases that are different colors in their dissociated and undissociated states. Because they are used in low concentrations, indicators do not appreciably alter the equivalence point of a titration. The point at which the indicator changes color is called the end point. For a properly performed titration, the volume difference between the end point and the equivalence point is small. Sometimes the volume difference (error) is ignored; in other cases a correction factor may be applied. The volume added to achieve the end point may be calculated using this formula:
VANA = VBNB
where V is volume, N is normality, A is acid, and B is base.