Determination of Vitamin C by Redox Titration (Iodometric Titration of Ascorbic Acid) Lab


I’m going to show you how we’re going
to transfer the ascorbic acid into the Erlenmeyer flask. The ascorbic acid we need 25.00 mL. So, what we’re going to use is one of these,
which is called a volumetric pipette. We’re going to transfer it by means of this,
which is a pipette bulb. Back in the olden days people used to suck
these things up with their mouths, it’s not advised: you never know what you might
be sucking up, it’s not a very good idea to use your mouth. Use a pipette bulb such as one of these. The pipette bulb itself has 3 buttons on it:
A, B, and C. A button, you can compress that, and you can compress the bulb to create the
vacuum that we needed to suck up the liquid. The B button here, will allow me to press
that and it will suck the liquid up. The C buttons allows me to release pressure
in case I go a little bit too far and need to adjust the level. You’ll notice there’s a line at the top
here of the volumetric pipette, that line represents 25.00 mL and I’ll show you in
a sec how the meniscus looks on the liquid and this needs to be at the bottom of the
meniscus. The bottom of the
“U” shape. Alright, so let’s take a look at how we
can transfer this. I have a beaker here; the beaker must be clean
and dry. This is what I’m going to transfer my ascorbic
acid solution into. If it’s wet at all, that’s going to affect
the molarity of it. I’m just putting a bit in here. Now what I’m going to do here is grab my
volumetric pipette here and I’m going to attach the bulb to the top. I’ve already pressed the A button and compressed
the bulb, so it’s ready to roll. So what I’m going to do is I’m going to
stick it in here, I showed you the B button earlier, but I’m going to compress that. And what you’ll notice is that the liquid
will begin to climb here up the volumetric pipette. Remember I want to try and get it on the line. I’ll bring it up close in a second and show
you the meniscus. Now presently it’s above the line I want
it to be. You see how it’s a “U” shape there,
you want the U, the bottom of it to be on this line here. So, I’m going to use the C button to release
some of the ascorbic acid here. And I’m going to try and get it onto that
line. Okay I’ve done that, so now I’m going
to bring it over here and then I can use the C button to release into my Erlenmeyer Flak. Now the Erlenmeyer flask does need to be clean,
but it does not necessarily need to be dry; it can be a little bit wet. Now the reason for that is because we’re
transferring a known amount of the ascorbic acid, we’re only interested in the moles
for the titration. Now we can also simply take the bulb off here
and it will obviously go into the flask like this. And the way that these are designed is that
there is always going to be a little bit left in the bottom, so just sort of roll it around
on the side here and you’ll always be left with a little bit in the bottom of the volumetric
pipette. But that’s okay so we’ve transferred the
amount we need to the Erlenmeyer flask. Filling the burette with iodine solution I’m now going to show you how to clean out
the burette and fill it with iodine solution. If you’re short, it is probably better to
take the burette off of the stand, just make sure the stopcock is closed before you add
anything to the burette. A little bit of water through the funnel here,
take the funnel out. Just sort of sloth here around the top with
the water; do this a couple of times just to rinse it out. I’m just going to do it once here. After we’ve rinsed it out with the water,
I like to rinse it out with whatever I’m going to put in it. And one thing about rinsing is that its always
better to do a few rinses with a small amount than one large rinse with a big amount. So what I’m going to do is put a small amount
of iodine into the burette. I’m doing it down here again, making sure
the stopcock is closed. If you’re tall enough its probably better
to leave it in the burette stand while you’re doing the filling. Not everybody is tall enough to do that. So, it runs down the sides and that pretty
much cleans it out. And then you can release it. So, do that a couple of times, just a small
amount of iodine into the beaker. I’m going to put this back into the burette
stand. I know you can’t see me adding iodine to
the top here because that’s out of the shot of the camera. But we’re going to be interested in is what’s
happening with the burette mainly. So what I’m going to do is add the iodine
to the burette, I’m adding it through the top here. Of course, the stopcock is closed. Now the thing with the burette is that I don’t
have to fill it all the way to the top, I’m actually going to fill this to around the
2 mark. I’ll tell you why in a second. Now the other thing you’ll notice, it may
be hard to see on the camera, but right here we have an air bubble that actually counts
as volume. We gotta get rid of that. An easy way to do that is to just do that
with the stopcock a couple of times and you’ll notice its now a completely full part down
here and we’re good. What I’m going to do is I’m just going
to take a little bit more out here, and I’m going to bring the burette down; I’m going
to try and arrange it in from of the camera so that you can see the graduations and talk
about how to measure from a burette. So, what I want to show you now is how we
can measure something on the meniscus here. As you see I haven’t filled it all the way
to the top and there’s no real reason to. When we take a burette reading we record an
initial reading and then a final reading once we have delivered the iodine. The difference of course is going to be the
amount of iodine delivered by the burette. The way that the burette is set up is you
can see this is a 3 up here and this is a 4 down here. Now you can see the liquid is forming something
of a meniscus. A meniscus is the “U” shape here for the
liquid. When we’re measuring the meniscus, we measure
the bottom of the meniscus and in this instance the bottom of the meniscus is between the
sixth and seventh graduation between the 3 and the 4. So, the 3 is up here, the four is down here
the sixth and seventh is down here and the measurement I would use for that would either
be 3.65 or 3.70. In all cases, these burette measurements are
always done to 2 decimal places. The reason for that is because between the
3 and the 4 there are 10 graduations which means that each one of those is worth 0.1. If we measure half of that graduation it becomes
0.05 which means every measurement now is going to have to be done to 2 decimal places. It’ll either be point something zero or
pint something five. In this case we can do 3.65 or 3.70. Always though measure at the bottom of the
meniscus. Titration of Ascorbic Acid with the Iodine
Solution I’m now going to show you how to titrate
the ascorbic acid with the iodine solution. The first thing we need to do is we’ve got
our 25.00 mL of ascorbic acid. I’m going to add 1 mL or 20 drops of 0.2
M Acetic acid, that’s about one of these pipettes full. I’m now going to also add 10 drops of starch
indicator. If I do not at the indicator, it will not
turn purple. I’ve set this up so the lid of the flask
is at the point of the burette. Just a round about there. I’ve started this now at about 5.4 mL and
I’m now going to add the iodine from the burette and wait for it to turn a light shade
of purple. At this point you can start to see a slight
blue occurring as we add the iodine to the ascorbic acid just at the very point of entry. You’ll notice as we swirl it the blue goes
away. As we get closer to the endpoint it will persist
a bit longer. But it will go away when swirled. Now I know I’m very very close to the endpoint;
very close, so I’m going to slow right down now and add very small amount of this solution
just one drop here. One I can use because the molarity doesn’t
matter anymore all it does is add moles, I can push in a drop with the wash bottle like
that. Yes this is getting really really close now. One needs to exercise some patience. Don’t over-add because what will happen
is you’ll overshoot. It’s getting really close now, rinse in. In fact, I think that is the endpoint right
there. As you can see there, a slight shade of purple:
that’s what you’re looking for! And that’s when I’d read the burette,
how much was there and I’d read to 2 decimal places, because that’s what the burette
is capable of measuring to. I’m going to show you now what happens if
we continue with the titration and we get an overshoot, we get that blue color and it
can turn an even darker purple. Titration of Juice sample with Iodine Solution So now I’m going to show you how to do the
titration of the juice and what we’re going to do is we’re going to add 1 mL of the
0.2 M Acetic Acid, about 20 drops one of these pipettes full. And then 10 drops of our starch indicator
to give a nice purple color. As we noticed before with the other titration
of iodine we begin to see a nice blue color down in here as we add it, but as we swirl
it the blue color goes away. We’re getting fairly close to the endpoint
now so I’m going to slow down a bit. Just adding a couple drops at a time. Slowly slowly. So again, we’re looking for that very light
blue color in the entire solution to persist. I feel like we’re getting pretty close now,
ah yes there we go very close. And that is probably about it, I’m going
to add a little bit of DI water here to the top since that is going to be part of the
measurement anyway. So that is the perfect color there, just a
very very slight tinge of purple amongst the background of the juice. It can be a little hard to see, it may be
harder to see than one you might get with just the colorless solution. Now what I’m going to show you what happens
if we overshoot but this is the color we want and remember that I would measure the burette
and I would take that measurement to 2 decimal places. Now what I’m going to do is just add a bit
more to show you what it looks like if we overshoot. There we go, see it becomes a light blue color
and I’m going to keep to become that dark purple. But what we’re aiming for is that light
blue tinge at the beginning there.

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5 thoughts on “Determination of Vitamin C by Redox Titration (Iodometric Titration of Ascorbic Acid) Lab

  1. When the color is changed, what does it say about the vitamin C content of the juice used? (Is it the more drops of iodine solution used, the less vitamin C content there is?

  2. This helped me de-stress before I did this very lab! Thank you for the nice detailed video and explanation! Now I know exactly what to look for!

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