Video Transcript
In this video, we will learn how to
describe acid–base titration methods and their use in determining acid and base
concentrations. In this video, we’re specifically
going to be looking at acid–base titrations. So, what’s the purpose of an
acid–base titration?
When doing an acid–base titration,
our main purpose is to work out the concentration of an acid or base, which is
currently at an unknown value. We do this by using an acid or base
that we do know the concentration of. Of course, if our unknown solution
is an acid, we want our known solution to be a base and vice versa. So, let’s have a look at how to set
this up.
Here is an example setup of the
equipment you’ll need to carry out an acid–base titration. At the bottom, you’ll need a
conical flask. This is sometimes called an
Erlenmeyer flask. The shape of this flask makes it
ideal for swirling liquids. This is something you’ll need to do
during the titration. By using a conical flask to swirl
your liquids, you’re far less likely to splash anything out of the flask than if you
were using something with a wide neck like a beaker. You’ll also need a buret. Burets are good for accurate liquid
measurements in instances where you don’t know at first quite how much liquid you’re
going to need. Of course, if you know exactly how
much liquid you’ll need, you would use a pipet.
You’ll also need a retort stand,
sometimes called a clamp stand. This has a heavy metal base and a
metal pole that extends upwards. It’s onto this pole that you can
attach clamps with which to secure your buret. Of course, you don’t want to make
this too tight and risk damaging the glassware. When setting up your retort stand,
you need to be careful about which orientation you have it in. You want to make sure that
everything that you’re clamping is overhanging the large part of the base. If you have it the wrong way round,
you risk everything toppling over. This can be both dangerous and
expensive. You may also decide to place a
white tile directly beneath your conical flask. This can be helpful if you’re
looking for subtle color changes.
Now that we have our basic setup,
we need to work out how to run this titration. Let’s begin by looking at our two
main solutions. Our two solutions are referred to
as the titrant and the titrand. You may also hear the titrant
referred to as the standard solution. This is the solution that we do
know the concentration for, and this is the solution that we must place in our
buret. On the other hand, our titrand
might be referred to as the analyte. This is our solution which has an
unknown concentration, and it’s this solution which should go into your conical
flask. Bear in mind that even though we
aren’t certain of the concentration of our titrand, we must use an exact known
volume. This will be important for the
calculations that you perform after you’ve done the experiment. We won’t go into the calculations
in this video.
So, how could you remember which
goes where? You could remember that the unknown
concentration is a known volume. And since you have to know the
volume at the start, that must go into the conical flask. There’s no point in using a buret
to add an exactly known volume. Alternatively, you could remember
that the titrant ending in T goes in the top and the titrand ending in D goes down
at the bottom. Once we’ve added both of our
solutions to the correct glassware, it’s time to add a few drops of indicator. We’ll look at how to select the
right indicator later on. We’re now ready to run our
titration.
It’s a good idea if your first run
is very rough. So rather than adding your titrant
dropwise right from the beginning, add it a few centimeters cubed at a time until
you notice the indicator change. Once the indicator has changed
color and swirling it doesn’t make it turn back, you’ve got a rough idea of how much
titrant you need. Let’s say that in this example, it
took about 40 milliliters before the indicator changed. Now, we’ll need to clean out our
conical flask and refill everything. Our subsequent runs will now need
to be much more careful. We can start our more accurate run
by adding a bit less than in step one. As an example, let’s say we add
about 35 milliliters.
Next, we need to add the titrant a
couple of drops at a time, making sure that we give our conical flask a good
swirl. We continue adding a drop or two of
our titrant at a time and swirling and then adding a few more drops and swirling
until our indicator reaches the end point. This means that the indicator is
just turning whichever color it’s supposed to. Of course, you’ll need to repeat
this experiment several times to make sure that you’re getting the right result. You want your results to be both
accurate and precise.
Now that we have a rough idea of
how to carry out an acid–base titration, let’s take a closer look at our
titrant. Remember that the titrant is also
called the standard solution, and this is the solution for which we know the exact
concentration. So how do we make up a standard
solution? Often, it depends what type of
solution you are preparing. It could be that you need to add an
exact known mass of a solid to a volumetric flask and make it up exactly with your
liquid. For example, you might need an
exact mass of sodium hydroxide pellets and make it up to the line on your flask with
water. Alternatively, it could be that you
need to mix two liquids together. This might be the case, for
example, if you need to prepare a hydrochloric acid solution. It could be that the hydrochloric
acid you already have is too concentrated, so you need to mix it with some water to
dilute it.
Either way, the key points are that
you have to know exactly how much of each substance you’re putting into your
standard solution. To measure exact masses, of course,
you’ll want to use a balance. To measure volume, you’ll want a
pipet or volumetric flask. However you make up your standard
solution, make sure that you make plenty of it. You’ll need to run your titration
for maybe five times, so you’ll need to have enough standard solution for all of
these runs. You probably want a little bit
extra as well, just in case.
Preparing the titrand is much more
straightforward. You simply add an aliquot of your
titrand to the conical flask. Aliquot is simply the scientific
way of saying a small sample. Remember that since your titrand
has an unknown concentration, we must use a known volume. You could use something like a
measuring cylinder, but a volumetric pipet is much more accurate. You could also use a graduated
pipet.
Now that we have our two main
solutions ready, we should have a look at indicators. So, why do we need an indicator for
our acid–base titration? In this example, let’s imagine that
our titrant that is the solution in the buret is a base and our titrand or analyte
in the conical flask is an acid. Let’s say that the starting pH of
our acid is four. As we add drops of base and swirl,
our pH will start to increase. We’re aiming to add just enough
base that will neutralize our acid, leaving us at pH seven. As we get close to neutral, our
indicator may start to just change color. But as we swirl, the color changes
back. As we exactly neutralize the acid
with our base, we would expect the color to remain permanently even when we
swirl. And we just want a hint of color
change. This moment is our end point. If we were to carry on adding our
titrant, the pH will continue to rise, and the color will become much darker. This means we’ve gone too far. If this happens, we’ll have to
start again.
Indicators help us to recognize
this end point. The indicator in this example is
phenolphthalein. Phenolphthalein is colorless in
acids but turns pink in bases. So when using phenolphthalein as an
indicator, you can recognize the end point because you have just a tiny hint of pink
which doesn’t go away with swirling. Of course, if you’re doing this the
other way round and your titrant is an acid and your titrand is a base, what you’re
really looking for is for that lovely pink color to have just disappeared
completely. Another commonly used indicator is
methyl orange. Methyl orange is red in an acid and
yellow in a base. The end point for this indicator is
when your solution is a hint of orange. The way that indicators work is
that they themselves are actually weak acids or bases, and this is what helps them
to change color.
There are, of course, many, many
other indicators, but these are the two most common. It should also be noted that not
all indicators change color at pH seven, and you could also use a pH meter instead
of using an indicator. We don’t tend to use litmus paper
or universal indicator for acid–base titrations. Using litmus paper is just too
fiddly and not accurate enough. And universal indicator changes
colors over a wide range of pHs, which is not very helpful for titrations.
Now that we have a solid
understanding of titrations, let’s have a look at some of the things that might go
wrong. The first potential source of
problems in titration experiments is the equipment. Incorrect use of the equipment can
cause your results to be incorrect. An inaccurate buret reading is a
common problem, and there are lots of reasons that you might get an inaccurate
reading. Imagine you need to read the value
from this buret. Where do we take the reading
from? Which of these is correct? The answer is the bottom one. You should always take your reading
from the bottom of the meniscus.
Other tips for always ensuring an
accurate buret reading are get rid of any bubbles, make sure that you always know
your starting value before you run your titration — usually, this is at zero, but
not always — never start using your buret if the liquid is above zero mLs, and if
you filled your buret with a funnel, always remove the funnel before beginning your
experiment. This is because the end of the
funnel can display some of the liquid and thereby change your readings or the funnel
could contain extra drops of liquid which will also alter your readings.
But what else could go wrong with
the equipment? If you don’t use an exact volume of
titrand in your conical flask, that can also affect your results, as can incomplete
mixing. If you end up of droplets of
titrand stuck on the neck of the conical flask, this has been accounted for in your
buret reading but has not mixed with your titrand. This means that your results will
not be accurate. You can avoid these droplets by
being careful how you use your buret and by ensuring that you give your titrand a
good swirl.
You also want to avoid droplets
stuck to the end of the buret. If this happens, gently touch it
onto the side of the conical flask and swirl it in. Muddling up the solutions is also a
possibility with titrations. To avoid this, make sure everything
is carefully labeled. This will hopefully stop you
putting the wrong solution in the wrong piece of glassware.
The indicator can also cause
issues. For example, you might have chosen
an indicator where it’s really difficult to detect the color change. An inappropriate indicator or
simply not being careful with your experiment could mean that you miss your end
point. There are also safety hazards
associated with titrations. Doing these experiments means using
acids and bases, many of which are harmful or corrosive, so care must be taken. You should always make sure that
your equipment is secure and safe before you start your experiment. This will prevent your buret and
all of your equipment from tipping over.
You should also always fill your
buret safely. This means never filling your buret
above eye level. To do this, you could place your
buret on a stool or on the floor. This prevents you from splashing
potentially harmful chemicals onto your head, face, or into your eyes. You may also choose to use a funnel
to fill your buret to avoid spillages. This is perfectly fine, but
remember to remove the funnel before starting the experiment. Now, let’s have a look at some
questions.
A student wants to use titration to
determine how much acid is needed to neutralize a known volume of base. They set up the experiment as
shown. At the start of the experiment,
which solution should go into the Erlenmeyer flask and which should be used to fill
the buret?
This question is asking about an
acid–base titration. This type of experiment is used to
determine an unknown concentration. The solution which fills the buret
is called the titrant. You can remember that the t at the
end of titrant could stand for top, meaning it goes in the top. The titrant has a known
concentration, and it’s sometimes referred to as the standard solution. Our titrand goes into the
Erlenmeyer flask, sometimes called a conical flask. This is the solution with an
unknown concentration, sometimes called the analyte.
The question tells us that we have
a known volume of base. Our titrand with the unknown
concentration must have a known exact volume. So, in this case, the base is our
titrand and should go into the Erlenmeyer flask. This means that the acid should go
into the buret.
So, let’s summarize. We know that the titrant with known
concentration should go in our buret, while the titrand with an unknown
concentration goes in our flask. And there’s a range of indicators
we can use to find our end point. And the keys to a successful
titration are a careful setup, dropwise addition, and plenty of swirling.
