This is an introduction to titrations:
in titration we use water solutions to make
our analytical work faster, easier, and
more accurate than we could if we were
doing this with dry materials.
Why water solutions?
why do chemists prefer to conduct reactions in water solution?
Water is a universal solvent;
it dissolves almost everything.
Now a true solution is
homogeneous. That means every milliliter of the solution, whether it comes from
the top or the bottom or the middle, is going to have the same composition as
every other milliliter.
The number of millimoles in one milliliter therefore
millimoles of salt solute will be the same wherever it comes from.
Water is a non-conductor of electricity.
There is no noise that's going to obscure the
conductivity titrations, for instance.
Water is the product of most acid-base
reactions
and there are many interesting salts
that are not soluble
in water and so part of our titration
could be seeing how much is taken out of
the solution. This enables our
precipitation reactions to be
quantitative.
There are a number of types of titration:
the acid-base titration is
of course the most common. It requires for finding the equivalence point a weak
acid indicator because there is no other obvious way to find when the number of
moles of acid and base are the same.
There are oxidation-reduction titrations.
Usually one or the other reagent is going to have a color change.
For example, potassium permanganate, which is a very very vivid purple when it is titrated
into a solution. The permanganate disappears (as it reacts)
until the equivalence point and then you have a very very slight tinge of purple
or pink.
The precipitation titration is a third type.
Precipitation titrations are
difficult because the equivalence point is hard to spot. If you're creating a
precipitate, what you're looking for is the point at which no precipitate is
being formed and that may be a difficult thing to see
and finally a conductimetric
titration relies on the different
sizes of ions reacted and in excess to
give a change in the shape of the
curve at the equivalence point.
Remember the equivalence point is the
number of milliliters of added reagent
that is exactly the same number of
millimoles of electrons being given and taken.
The acid-base titration:
the indicator that is used in an acid-base
titration has one color at a pH below the equivalence point and close to the
equivalence point that color changes. So let's look at a problem:
titrating an unknown molarity of hydrochloric acid 32.20 milliliters with 0.0520 molar sodium hydroxide
reaches the color change of 25.04 mL of base.
What is the molarity of the acid? (you may turn off
this playback and solve the problem and then check your answer.) (pause)
This is strong acid in a strong base. What is the equivalence pH when you titrate strong
acid with strong base? Of course it's a seven. Now we know that at the
equivalence point the molarity of the acid times the
volume of the acid
is equal to the molarity of the base times volume of the base.
Dividing both sides by volume of acid
we find molarity of the acid.
Product of molarity and volume of the base divided by
the volume of the acid.
We plug in the numbers for the molarity of the base
and the number of milliliters
of base titrated and for the
volume of the acid
and the answer is 0.0404 molar
or moles per liter
or millimoles per milliliter. Notice we
have one, two, three significant digits
because in the molarity of the base we
have three significant digits.
If the pKa of the indicator is close to the
equivalence pH then the rapid change in
pH around the equivalence point will
make the indicator change colors just
like a stoplight so as we run up to the
equivalence point we have a very small
change in pH but again this is a very
strong acid and strong base and so when
you get close to the equivalence point
you get a rapid increase in pH that will
cause the color change in this area so
that when you add one drop of additional
base you're going to see that color
change.
Methyl orange is the one chosen.
Here it changes colors just short of the
equivalence pH of seven when you're
titrating strong acid in the strong base
so methyl orange would be a decent
indicator if you titrate base into acid
It would work if you titrated acid
into base. You can again talk about that
in your discussion group