Video Transcript
Phenolphthalein is an indicator
often used in titrations. In basic solutions, it has a pink
color. What does it change into in acidic
solutions?
First of all, a titration is a
quantitative analytical method used to determine the concentration of an analyte
using a known concentration of a second solution. The diagram shows the apparatus of
a generic titration experiment. In this question, we are concerned
with an acid–base titration. The contents of the buret depend on
the contents of the conical flask. For example, if the buret contains
a basic solution, then the conical flask contains an acidic solution.
During the acid–base titration
experiment, the solution in the buret is slowly added to the solution in the conical
flask. During this process, the acid
reacts with the base in a neutralization reaction to produce a salt and water. To determine when all of the acid
has reacted with the base, an indicator, which undergoes a color change over a
specific pH range, can be used. In this experiment, the indicator
phenolphthalein is used. In an acidic solution, which has a
pH less than seven, phenolphthalein is colorless. At a pH of around 8.2,
phenolphthalein turns a faint pink color. If the pH value continues to rise
and the solution becomes even more basic, phenolphthalein will turn darker pink.
In many acid–base titration
experiments, chemists want to add just enough base to an acidic solution containing
phenolphthalein to get a faint pink color. The point at which the indicator
has just changed color and doesn’t change back is called the end point. This color change indicates that
the equivalence point was just passed, which is the point at which the acid and base
have completely neutralized one another. The volume reading can then be made
by using the markings on the buret to accurately determine how much base was used to
neutralize the acidic solution.
In conclusion, the indicator
phenolphthalein is colorless in an acidic solution.
