A simple technique for determining the unknown concentration of
a solution is to react it with another solution of known concentration until
all of the first solution has completed reacting.
However, in order for this to work, you must overcome one problem - How
do you know when you have added enough of one solution to the other such that
the reaction is complete?
For neutralization reactions (reaction between acids and bases) this is
relatively easy because there are a number of substances which change color
depending on the pH of the solution in which they are dissolved. Phenolphthalein
is the substance we will use because it is colorless in acidic solution and
bright pink in basic solution. Below is the colorless version:
Phenolphthalein (colorless form)
The phenolphthalein molecule changes slightly depending on the pH of the
solution it is in.
Colorless Phenolphthalein
pH < 7
Pink Phenolphthalein
pH > 7
If we add a few drops of phenolphthalein to an acidic solution, it should
indicate (turn pink) the end of the reaction when all of the acid has been
neutralized and extra base is added.
Consider the following reaction:
H2SO4(aq)
+ 2 NaOH(aq)
--->2 H2O(l)
+ Na2SO4(aq)
When we first add base, it reacts with some of the acid, forming water and
sodium sulfate. But if not enough base is added the overall solution will
still be acidic. Eventually, enough base has been added so that the only substance
left in the beaker are water and sodium sulfate, giving you a neutral solution
(pH 7). Once another drop of base is added the solution becomes basic. If
phenolphthalein is present, a pink color will appear.
Titration calculations are the same as any other type of stoichiometry problem.
Using the equation above calculate the concentration of the unknown base if
35.0 mL of NaOH reacted with 53.0 mL of 0.25 M sulfuric acid?