We learned all about acids and bases in the general chemistry series, and we also learned
about the concept of an acid-base titration. This is a neutralization reaction where we use an acid
or base of known concentration to determine the concentration of the other reagent. This
is because we can find the stoichiometric amount of acid or base titrant that must
be used to precisely neutralize the analyte, and then use the volume of titrant added to calculate
the concentration of the analyte. For example, one mole of hydrochloric acid reacts with one
mole of sodium hydroxide to produce one mole of sodium chloride and one mole of water. Knowing
the number of moles we add of one substance tells us something about the number of moles
of the other substance initially present. The point at which the number of moles of acid and
base precisely neutralize one another is called the equivalence point. To get to this point,
you have to slowly add your acid or base little by little until the reagents are exhausted,
thus leaving only water and salt. We have already gone over titration calculations
in lecture-based tutorials, so that will not be the focus here, although we will discuss
these calculations in a general way. Rather than actually performing stoichiometric calculations,
we are going to focus on the details of physically performing the titration, which should help
make any relevant calculations easier to do. For example, we have already learned that to get
to the equivalence point of a titration between a strong acid and base like HCl and NaOH, we can
measure the pH until we reach a pH of 7, since precisely equimolar amounts of strong acid and
strong base will leave only salt and water, which gives us neutral pH. But rather than measuring pH,
what we normally use in chemistry labs when performing titrations is an indicator. An
indicator is a substance that exhibits a visible color change once the titration reaches its
equivalence point. For many acid-base titrations, a common indicator will be phenolphthalein. This
is because it turns bright pink as the solution is changing from acidic to basic, as upon having just
neutralized all the acid, any additional base will react with phenolphthalein instead, and it
is the deprotonated form that glows pink, so as soon as we see this color persist in solution,
we’ll know that we’re at the equivalence point. So to summarize, we start with an acid, we can
add a bit of the indicator, and then titrate with a base. Again, the solution will stay clear until
it suddenly turns pink at the equivalence point, notifying us that the titration is complete,
and allowing us to do our calculations. Today we will be using titration to
find the concentration of a hydrochloric acid solution by using 0.1 molar sodium hydroxide as
the titrant. To start, we’ll need a ring stand fixed with a buret clamp, an Erlenmeyer flask,
a buret, a beaker, glass funnel, our acid and our base, some indicator, and a graduated cylinder.
First let’s assemble our titration setup. We’ll add our clean burette to the ring stand
and clamp. This is where we’ll add our base, which again is referred to as the titrant in
this case. Our acid will go in an Erlenmeyer flask below the burette; this is our analyte.
Before adding base to the burette, let’s make sure it works correctly. We can add water on top
using a funnel. The burette can be tall so you might need to lower it or step up on something.
Sometimes the stopcock leaks, and you don’t want your base to leak out as that will affect
your calculation, so that’s why we start with water. After you see that there are no leaks,
turn the stopcock parallel to the burette to open it, making sure it is not clogged. After
you ensure that water flows well, remove all the water and close the stopcock.
Next, we’re going to load up our base, making sure that we are wearing safety goggles and gloves,
as we are using corrosive chemicals. Let’s fill the burette all the way to the top using a funnel.
In your lab notebook, note the starting volume reading on the burette. Remember to read from
the bottom of the meniscus and record it to the appropriate number of significant figures.
Now let’s put our acid in the flask and place this directly below the burette. In this experiment,
we are using 20 milliliters of HCl solution. But again, we don’t know
the concentration. Let’s also add a few drops of the indicator. We’re now ready for the titration.
We will start with what we call a rough titration, or quick and dirty titration. This is a quick
run to get an idea of where our equivalence point will be. This rough titration saves a
lot of time but it is not very accurate, so we won’t use this data in our results. To do this,
let’s open the burette and let the base go down quickly as you swirl the flask, which could
also be stirred with a magnetic stir bar if you desire. You’ll stop every now and then to check
if there is a color change that’s persistent. When you see this persistent change, you will
know that you have reached the equivalence point. Record the end volume reading on the burette and
subtract that from the initial reading to get the volume added. This volume is an estimate
of how much it will take to reach the equivalence point. But because we poured very quickly, we have
most likely added a bit more than was needed. The next titration will be more accurate. Since
we have an idea of how much base we need, we can quickly add about three quarters of the
volume we recorded from the rough titration. We know that we won’t get to the equivalence
point with this amount. But after that, we have to be more careful. A safe approach is to add
a half a milliliter at a time, but you can go as slowly as one drop at a time. After you add half
a milliliter, you’ll swirl and see if there is a color change. When the base drops into the acid,
you may see some fleeting color change right where the drop enters, but it doesn’t stay like
that when you mix the solution, which means the equivalence point has not been reached. We need a
color change that remains even after mixing. Keep adding, mixing, and checking. It is important to
be careful when we’re close, as we don’t want to miss the equivalence point by adding too much. As
you can see, this pink color is not changing anymore, so we are at the equivalence point.
Record the final volume reading on the burette and get the difference from the first reading. As we
already know, the volume of base added, multiplied by its concentration, over the volume of acid,
will give us the concentration of our acid. In this type of experiment, we generally
perform three titrations for our concentration determination to be more accurate. The average
of the three numbers will give us a solid estimate for the concentration of the acid analyte.
Titration certainly gets much more complicated than this, even simply by utilizing a weak acid
or base as either the titrant or analyte, which will change the pH of the equivalence point and
therefore the indicator that is used. There are also several other types of titrations,
such as redox titrations. But for now, we have covered the most basic titration setup,
so that we have a firm grasp of how to do this type of analytical chemistry.