PAUL ROBERTS: Hi, I'm Paul Roberts, and I'm a chemistry teacher here at Andover College.
Today, I'm going to talk you through how to do core practical titration of acid and alkali.
Now, this is a real classic chemistry experiment. And if the students pay particular attention
to the measurements along the way, they should get some accurate, reproducible results.
Now before we start the experiment, we have some safety considerations. First of all,
we have our sodium hydroxide. Sodium hydroxide has a concentration of 0.1 moles per decimeter
cubed. This concentration, it is an irritant and it is also corrosive. If any spillages
occur on the skin, you need to wash your hands straight away and be very careful not to get
this in the eyes. Any contact with the eyes will require immediate medical attention.
Likewise, we have a concentration of hydrochloric acid with a similar ballpark number to the
sodium hydroxide concentration. It's going to be approximately 0.1 per decimeter cubed.
Again, it's a corrosive and an irritant at its concentration, so eye protection and lab
coats need to be worn throughout the practical.
So to start the titration, let's consider the hydrochloric acid. Now, the hydrochloric
acid is of an unknown concentration. So now, I'm going to charge the burette with hydrochloric
acid. Now, it's very important that I check that my burette tap is closed. It's closed
in the horizontal position. And a very important safety aspect is that burettes must always
be filled below eye height. Filling a burette-- reaching above your head to fill a burette
is dangerous because even with safety glasses on, there's a risk that you can pour the acid
over your head, and that is not a good idea.
So in my situation, I'm going to place the burette on the floor and fill using a small
funnel. You want a small funnel so it's difficult to accidentally overfill the burette. And
ideally, the funnel wants to have a narrow bore so it doesn't accidentally get jammed
in the mouth of the burette.
Now, I'm going to fill the burette with hydrochloric acid. I'm using the stock bottle here, but
it might be an idea for students to decant this first into a 100 ml beaker and then add
from there. So I'm adding it in there. My tap is closed. And I'm just keeping an eye
on the level. It will fill a lot faster than you expect. I'm going to fill to just below
full, and then move the burette back to the table. I'm going to rinse the burette. I'll
adjust the height a little bit. And I'm going to drain the acid out of the burette.
Now, this serves two purposes. First of all, any debris that may be in the burette will
be rinsed out. And also, it should clear any air bubbles that are in the jets of the burette.
This may take a little bit of time, but it's a very good practice to do to get good, reproducible
results.
Now, I'm not going to empty the burette completely. If I do that, there's a chance that I'll create
an air bubble in the jet, so I'm going to wait until the burette is round about 50,
and then I'll move it back down onto the floor and I'll refill. So I'm turning the burette
off. And this is now my waste beaker. This beaker, like all chemicals in this reaction,
could be disposed of down the sink with plenty of water.
So now I've rinsed my burette, I can refill it. And again in my situation, I'm placing
this on the floor. I'll take my hydrochloric acid, and I'm going to refill. Now, it's not
essential to hit exactly zero on the burette when refilling. In fact, you can waste a lot
of time to hit 0. So I'm going to stop so I can clearly see that my acid level is within
the scale of the burette. I've also, this time, removed the funnel. It's important to
remove the funnel from the burette as any drips that might take place during the titration
will affect the actual total value.
OK, so now, my burette is full of acid and clean acid. I'm going to move back onto my
table. So now the burette is full, we can move away to the hydroxide side of the titration.
This requires a pipette volume of 25 centimeters cubed.
Now, I'm going to insert the pipette filler onto the pipette. And again, there's a very
safe way to do this and there's an unsafe way. When inserting the pipette into the pipette
filler, it's very important not to push the pipette at any distance into the filler. If
you were to push the pipette into the filler in this arrangement, there's a chance this
glass here could break and the broken glass goes into your hand. This will most certainly
result in a trip to A&E.
Alternatively, insert the pipette into the pipette filler first by loosening the ferrule
at the end of the filler and gently rotating the pipette filler onto the pipette, not the
other way around. Now if this were to break at this moment, there would be no injury.
OK. Just inserting it on. Smooth pressure. Now in some cases, teachers may want to do
this for their students. Now, I can be sure that my pipette filler is firmly attached
to the pipette, turning it upside down, and gently pulling on the filler and there should
be a positive pressure. Never the other way around because if it falls out, you'll have
broken glassware.
Now, I'm going to fill the pipette with my sodium hydroxide. Now in the same way that
we rinse the burette, I'm also going to rinse the pipette. I'm rotating the wheel of my
pipette filler, filling the pipette, and then push the button and just let the pipette empty.
And this is essentially just rinsing any dust, or debris, or any other chemical residues
that may be in the pipette. Now, I can accurately measure out 25 centimeters cubed of the hydroxide
into a conical flask.
Now before I start this, I want my conical flask to be as close to my hydroxide as possible.
This is going to minimize any drips on the table. I'm making sure the wheel is pushed
all the way down, and then I'm going to draw up.
And this is the part that the students will probably find the most challenging. Now, what
I do at this point is I purposely go past the meniscus endpoint here. This is my endpoint,
25 centimeters cubed, that I'm looking for. I purposely go past. I lift the pipette out
of the solution, and I then gently roll down on the wheel until I get to my meniscus pointing
at 25 centimeters cubed. And then I can transfer across into my conical flask.
Now, the students, at this point, have a temptation to push down on the plunger and empty out
the pipette. Now unfortunately, doing that actually releases too much hydroxide. The
pipettes are designed to have a very small volume of solution or very small volume of
liquid in the bottom. If you want to make sure that there are no drops coming out the
bottom, you can just touch it to the base. But all the time, I'm making sure my thumb
is on the button, so letting gravity drain the pipette. OK, so that's now my 25 centimeters
cubed of sodium hydroxide accurately measured.
We can now start the titration. Before we start, however, we need to make sure we can
see a color change of titration. So I'm putting a white tile on the base of the burette. Now,
this titration has an indicator of methyl orange. And I'm going to add 5 drops of methyl
orange to my sodium hydroxide solution. If you add a few more drops, it's fine. You just
don't want to go adding squirts and squirtfulls. OK, so this is the starting color of the titration.
It's a kind of a mild orange color.
And I'm now going to adjust my burette so it's just the jet of the burette is just inside
the conical flask. I don't want the burette to be too low. If it's too low, then it's
going to be difficult to stir. If it's too high, then some acid might not actually find
its way into the conical flask and that will give bad results. Now, I've already taken
my initial reading. At this point, it's worth checking that your initial reading hasn't
changed. I've checked this and my initial reading is still 0.8 centimeters cubed.
So as we're going to be looking for a color change during this titration and the color
is going to be starting as a orange color and the methyl orange, and it's going to be
turning to a light pink color. Now as a tip, what you can do is prepare another flask of
25 centimeters cubed of sodium hydroxide, add the same amount of methyl orange indicator
to it, and use this as a reference start point. That way, you'll be able to easily see that
your titration has changed color. If we were doing the same titration using phenolphthalein
as an indicator, we'd expect a color change to go from pink to colorless. And in this
case, we wouldn't actually need a reference point.
So I can now start running acid into the conical flask. And I'm turning the tap to the vertical
position. And you want a nice, steady flow. Now, this is a technique that's been developed
over many years. I want to try and swirl my conical flask whilst having the tap open.
The tap is open, acid is running in, and I'm looking for a color change. At the moment,
you can see there's been no color change from the initial.
Notice I am not looking at the graduations in the burette. I'm just focusing on the color
change. Checking my reference, I'm now just starting to see a slight pink change of color,
but when I stopped the burette, that pink color disappears. So I'm now getting close
to my end point. I'm turning my tap very slowly. And I've just got it going on a dripwise and
I'm looking for a change of color.
OK, now notice I still have the same color as the starting color, but I'm now very close.
That pink color remained for quite a while. At this point, I want to accurately release
one or two drops at the most, using both hands on the burette tap. Let in two drops. The
solution has gone pink. I can swirl. That color pink slowly, slowly faded away. I'm
getting now very close to the end point.
I'm going to do the same again. Add two drops-- two drops maximum. 1, 2, and then swirl. Notice
that that color has almost changed. It's very, very close to that pale pink end point. And
again, two drops. 1, 2. And you can see now, we've reached the end point there. And comparing
the starting color with the final color, we've got a very faint some people describe this
as a salmon pink type color. And so this is now the end point of the titration.
So I can now read the endpoint of my burette. I'm reading from the bottom of the meniscus.
I can see it's 22.8 centimeters cubed. So not only do I have an endpoint reading, I
also have a start point reading for my next titration.
At this point, you might consider refilling the burette, but only 22 centimeters cubed
of acid has been released and the total volume of the burette is 50 centimeters cubed. That
means there's another 28 centimeters cubed remaining in the burette, so there's enough
volume left behind to enable another titration without having to refill. This will speed
up the titration process.
And going back to my earlier comments about the concentration of your unknown acid, this
is why if you keep your concentration with the unknown acid to be slightly higher, the
0.1 moles per decimeter cubed, that means you'll need less acid to complete the neutralization,
and so you'll be able to get more than one titration of acid out of a single 50 centimeter
cubed burette.
So now we've reached the end point of the titration, I'd like to show you what would
happen if you delivered too much acid. As you can see here, instead of reaching an end
point of a salmon pink color, we'd end up with more of a rose pink color of solution.
And in this example, too much acid has been added. And unfortunately, it would be considered
a poor quality titration.
We won't just do one titration in this situation. We'd typically look to repeat the titration
to get concordant results. And by concordant results, that would be two titers having volumes
within 0.2 centimeters cubed of each other.