(3) Endpoint. The endpoint is defined as the point where equivalent
amounts of two reactants have been brought together, the point of neutralization.
Indicator. An indicator is a dye that changes color at the endpoint.
INDICATORS AND GENERAL RULES OF USE
a. Shown below are some of the indicators that may be used in acid-base
titrations. These indicators vary in sensitivity and pH range and are chosen according
to the equivalence point or stoichiometric endpoint of the reactants used in the titration.
Methyl violet 6B
1.5 - 3.2
3.1 - 4.4
3.4 - 4.8
3.8 - 5.4
4.2 - 6.2
6.0 - 7.6
6.8 - 8.4
8.2 - 10.0
b. The following general rules apply in choosing the proper indicator for titrations
unless a special indicator is specified: (1) when titrating a strong acid with a strong
base, use phenolphthalein, (2) When titrating a weak acid with a strong base, use
phenolphthalein, (3) when titrating a weak base with a strong acid use methyl red, (4)
never titrate a weak base with a weak acid because indicators will not give a sharp color
change (when the endpoint is reached) in this combination. When using
phenolphthalein and other colorless indicators, arrange the titration for the observation
of the appearance of color and NOT the disappearance of color.
PERFORMING AN ACID-BASE TITRATION
The general method that is normally used in performing acid-base titrations is
outlined as follows:
a. Place a specific volume of unknown into an Erlenmeyer flask and record the
b. Place two to three drops of indicator into the solution to be titrated. The
amount of indicator to be used may be stated in a specific procedure.