1 O Chem Melting Point ALL with Graphics (CC)

1 O Chem Melting Point ALL with Graphics (CC)


[ Music ]>>Welcome. Now the purpose of this video
is to demonstrate how to take a melting point by
the capillary tube method. And so we’re going to talk about how to prepare
the sample, load the tube, run the apparatus, but first I want to talk a little bit about
melting point and why we bother to do this. Melting point is the way — is one of the things
that we use to characterize solid materials, and as I mentioned before, most of the products
that we make in this class will be solids. So this will be one of the ways
that we will characterize a solid. You know, another way would be you look
at its color, maybe determine its density, figure out its solubility, but melting point
is one of the physical properties of a solid. It’s not possible to predict
the melting point of a material, although you could make some assumptions,
I guess, based on analogies with compounds of similar structure and molar masses,
but you can’t really predict it, so what we use the melting point for is
to characterize compounds that we’ve made, and we can also use it to more or less
determine the purity of a compound that we made. So what we’ll do in this class,
everything we’ve made has been made before, so we’re going to use the melting
point to see how pure the material is. Now pure materials have melting point ranges,
and by the way, when we measure melting point, it’s a range, not a specific temperature. So sometimes the melting point will be recorded as a specific temperature, but
generally that’s erroneous. Good data tables will present the
melting point as a range of temperatures, and the way that we determine the melting
point, it will definitely be a range. We put the material in a heater that ramps the
temperature and we watch how the material begins to liquefy and then finally goes all to liquid. Well, that’s not an instantaneous process. It takes a little time, and time is temperature
on our apparatus, so we are measuring a range. Now pure materials, pure organic materials, generally have melting point ranges
of 1 to 2 degrees Centigrade. And if the material is impure, the tendency
is that the melting point will be lower than the literature value, and
the range will often be broader. It will start to melt earlier
and take a longer time to melt. So we can use the melting point to determine
how well you purified your material. So we’ll be looking at that. Initially in this first experiment,
we’re going to use the melting point to just determine the identity of some unknown
compounds from a short list of compounds. So at this point, you should be looking at a
diagram, a table, that shows the seven compounds that we’ll use in this experiment. And you can see their melting points are kind
of spread out from 120s up into the 180s. So there — most of them, most of them are
sufficiently separated from each other that, if you do a good job, it
won’t be hard to determine. Two of them do have close melting points
and to differentiate between these two, we’ll have to do a different test,
which I’ll talk about when we get there. But we’re going to use the melting point
to determine purity of our compounds. So, at this point, I think I’ll
set up to show you how we’re going to perform the capillary melting point. [ Music ] Viewers of this series may recall a
recrystallization of benzoic acid. And here is our product of benzoic
acid from a previous recrystallization. Benzoic acid is one of the materials
that’s on your unknown list, too, so you may get this as an unknown. When it comes out of the bottle, frankly,
it’s not going to look as good as this. It’s not going to be nice crystals. But, you know, these are beautiful
crystals, but we’re not going to be able to physically load those little needles into
one of these tiny capillary melting point tubes. I just don’t have that kind of dexterity. So what we have to do, is you have to take these
nice looking crystals and we’re going to have to pulverize them, grind them up
until they’re very fine powder, and then the powder we can load. So I’m putting some of the
crystals on a watch glass, and I don’t have to take very much
because, look, we’re only loading a teeny, tiny tube here, so — and I’m
only going to put a little bit in the tube, so I don’t really need much. This is going to be more than enough
for us to load the capillary tube. But what I have to do now is take this glass
rod and spend a minute grinding these things into a very fine powder,
because in the needle form, like they came out of their
crystallization, they’re not going to load into the tube, so I need a fine powder. So I’m using a glass stirring rod
to grind these crystals very fine. All right, so I’m — I probably have enough. It’s probably fine enough. I’m going to push it into a little pile
there, and now we’re ready to load the tube. One of the secrets is, when
you’re doing a melting point with a capillary tube method,
don’t use too much sample. I’ll show you what’s too much in a minute,
but we’re only going to need a little bit. So close end, open end. We’re going to load the open end. I tap it into that pile of
crystals, maybe you can see. There’s a little bit in the end. I can tap it on the bench. All right. Well, I tapped — I’ve tapped
a little into the tube. We want to have it — we want to have this
sample nicely compacted, and there’s a device that we use, and to a non-chemist,
this looks like an ordinary glass tube. But to a chemist, this is the G-force sample
compactor, utilizing the force of gravity to compact the sample, and it’s operated thusly. The tube is placed in the vertical
position, capillary tube, closed end down, loaded in the top, dropped — that thing’s
bouncy, and I don’t know if you can see that. I take it out. That’s compacted, and I’ll do it —
I’ll do it one or two more times. Bounce, bounce, bounce, bounce. One more. And so this is a nice way to compact
the sample without maybe risking having that capillary break in your hand
while you’re working on the bench. Now take a look, if you can, at the
amount of sample that I put in the tube. It’s not much. It’s supposed to be somewhere between a
16th of an inch and an eighth of an inch. Let me show you this one. I loaded this one previously. You see the difference? Way too much. Way too much. Here’s why. When the material starts to melt,
generally it starts to melt from the bottom. And it’s going to take a longer time to melt
all of this material than it is to melt this. Time is temperature. So the effect is, the melting point range that
you measure for this sample is going to be — it’s going to melt over a broader range, so
it’ll be — you’ll probably have a longer time, higher temperature, higher final
temperature, and a much broader range. The correct amount is this. About a 16th of an inch in the tube. That’s the correct amount. All right. Now here’s our melting point apparatus,
and it’s a gadget called a Mel-Temp. That’s the manufacturer. And it comes with this canopy, a heat shield,
over here, because these parts in here get hot, but I’ve removed the canopy today just
to demonstrate where to load the sample. Because with the canopy on, and
you’ve never done this before, this is hard to tell where
the sample is going to go. And the main reason it’s hard to tell is
because here’s this big, open tube up here. Now, the Mel-Temps are made
to operate with a thermometer, a mercury or alcohol thermometer stuck in here. And so that’s a common way to run it. Irvine Valley College is space age, high
tech, and we run off of a Type K thermocouple, which is inserted into this heating block. And so we don’t use this thermometer holder,
and this is not where the sample goes. Don’t put it in there. This is going to be very hard for
you to see, probably, on the camera, but there are three little
slots here, right in there, and this thermocouple takes
up one of the three slots. You’ll see when you look through the
magnifying glass, there will be three slots, and the thermocouple will be in the left slot. That leaves two open slots. So I’ll load the sample tube
into one of those other slots. So that’s what it looks like without the canopy. I don’t want to run this
melting point without the canopy, because I just don’t need a
video of me burning myself. So I’m going to put the canopy back on,
and then we’ll take the melting point. All right, the canopy is back on. I shouldn’t burn myself, and now
I’m going to load the sample. So I can look in the top and kind of get an idea where these three slots are,
and I put the tube in. And when I push the on button, a light
will go on in here, and then I can look into the magnifying glass and
I should see my capillary tube. And I can move it up and down to make sure,
oh, yes, I’m looking at the capillary tube. And right to the left of the capillary
tube, would be the thermocouple. So you should see the thermocouple
wire in there. Occasionally the thermocouple will come
out of that slot and, boy, if that happens, then your temperatures are just haywire. And so it’s always kind of a good idea that when
you look in there to look at the capillary tube, make sure that you can see the
thermocouple sitting next to it, and then we know everything’s good. So, at this point, with the green button
turned on, the light went on in here, but we’re not applying any heat
yet, so there’s a dial here. Generally, what you want to do
when you ramp this temperature — I’m going to set this to about 4 to begin with. The dial goes up to 10, and
we’re going to put it at about 4. I have an idea, you know, benzoic
acid, we know where it melts, so I have an idea of about
where it’s going to melt. And I think 4 is going to be a high
enough setting that we’ll get the — oh, let me turn the thermocouple on here,
and we can watch the temperature go up. Because one of the other things I want to tell
you is it takes some time to heat this block, so it’s, you know, an electric
current’s going through there, and the block is slowly getting hot, and so we’ll watch the temperature
climb on the thermocouple. And we’re at about, almost 30 degrees now,
but the melting point of benzoic acid is up in the 120s, so we have a ways to go. If you’re the impatient sort and you want
to crank this up higher, okay, but beware. Because just as it takes a while for this to
heat up, when you start to ramp the temperature, when you turn the knob down, it’s
going to continue to ramp quickly and then it will slowly decrease
the heating rate. There’s a lot of heat in the block that’s
going to continue to keep this hot, so turning this dial is not going to make an
instantaneous temperature change, all right? So be aware of that. If you’re ramping at a very high rate, then
you may continue to ramp at a very high rate, even though you’ve turned the knob down. What we’d like is when you
go through the melting point, when you’ve past the melting point, you’d like
this temperature to be increasing about — at a rate of about 2 degrees a minute. That’s probably a pretty good
rate, two degrees a minute. So at this point, it looks like we’re going
a lot faster than two degrees a minute. I think as we get up above 100 degrees,
this heating rate is going to slow down. So I’m just leaving it on 4 for the time
being, and we’ll watch this thing go up. So at this point, there’s not a lot to do
except you’ll look in here and make sure — see what’s happening to your sample. What you are likely to see
is nothing for a long time, and then you might start to
see some things happening. The most difficult thing about doing a melting
point when you’re brand new is determining when is it actually starting to melt? Here’s what we want to do. We want to take the temperature from the
time you see the first drop of liquid, the first drop of liquid, to
the point where it’s all liquid. And that will be, we hope,
a one or two degree range. The problem is, the sample undergoes
some other physical changes often in the tube, and beginners get confused. Oh, is it really melting? Because it’s changing. It’s doing something. Well, what often happens is the sample will
shrink away from the walls of the tube, so it’ll just kind of move away from the tube, and you’ll see that movement
and you’ll, “Oh, it’s melting!” No, it’s not melting yet. And then the other thing that
happens is called it’s sweating. A lot of times some of these
materials will sweat. And what you’ll see there is the
bottom of the tube where the sample is, the wall will look like it’s getting fog on it. It’ll start to fog up and
you’ll think, oh, it’s melting. But it’s not melting yet. Wait until you see the first drop of liquid. So we’re going to put the camera
here, and you’ll get to look in here and watch this sample melt. And I think you’ll be able to see the sample
may shrink, and then you’ll see the first drop of liquid, and we’ll take the range,
and I don’t know what it’ll be. But we’ll see what we get. All right, well, we’re over 100 degrees, almost
at 110, and I’m seeing the temperature go at more than two degrees a minute. I’m going to back this down closer to 3 now. The temperature was up at
4, I’m backing it down. I want to get into the 120s at a slower rate. So I’m going to start before we get
to the 120s, because if I turn it down at 120, it’s too little, too late. So I’m going to try to back it down to
around, I’ve got it to about 3.2 right now. And look, the setting on your
instrument, your mileage may vary. Right? Yours may not be the same as mine. But watch the thermocouple. See how rapidly it’s increasing, and
if it looks like you’re going to go through the melting point
too fast, then back off. Now what if you don’t know
what the melting point is? It’s common practice if you don’t
know what the melting point is, put a tube in there and crank it up pretty good. And it will go through the
melting point and it will go fast, but you’ll have a ballpark idea of where it is. It’s not going to be data
you’re going to want to keep, but you’ll go, oh, around the 150 it melted. Now, use a second tube, a new tube, and
run it again and go through the 150s at a much slower rate, and that’ll be a keeper. I want to say a word about
using tubes over again. The answer is don’t do that. Don’t ever do that. The reason is, when the material heats up, it
might oxidize or otherwise thermally decompose. Impurities in a sample lower the melting point. And this is one of the reasons I mentioned,
you know, why we do the melting point. If you have a low range and a broad range, that
indicates that the sample is not that pure. So we want a high, narrow range. If you melt this and it oxidizes, now you’ve got
the oxidation product in there as an impurity, and the chances are very good that
the second time you run this tube, you’re going to get a different answer
than what you got the first time. And it’ll be an answer you don’t like. So never run the same tube twice. Always a new sample. All right, it looks like —
let’s look at this temperature. It looks like we’re definitely
— we definitely slowed it down. Now it’s creeping. So I think — I think what we’ll
do is we’ll come back and look at this when it gets into the 120s. We’ll pick it up again when
we get into the 120s. All right, well the temperature
has hit 120 degrees. We’re ramping fair — at a decent rate,
I think, to get this melting point, and I know the benzoic acid melts around 122,
so at this point, I’m going to put my eye to this thing and we’re going to
put the electronic eye to this thing so you can follow the melting
from this point on. As you get close to what you
know to be the melting point, you want to keep your eye on this. So we’re going to bring the camera in now
and you’ll be able to see the sample melt. All right, well, we’re looking
through the eyeglass, eye piece now, of the melting point apparatus,
and here’s the benzoic acid tube in the melting point apparatus and look closely. You’re going to see some movement here, and we’re going to see this sample
start to what we call sweat. We’re also going to see it start to
shrink away from the side of the tube. So — The sweating actually started around 111.5
degrees, and this benzoic acid doesn’t melt until it gets to at least 122,
according to the literature, so we’re going to turn this camera off
for a minute and let the temperature creep up and then we’ll start it again. All right, well now we’re back —
now we’re back at the benzoic acid, and we are approaching the
literature melting point. We’re in the low 120s, and we
should see some movement here. You probably can tell that the sample has
shrunk away from the side of the tube, and now it looks like it’s starting to melt. The temperature now, right now, is about 124
degrees, and clearly the sample has moved away from the side of the tube and we’ve got — we’re starting to get liquid
at the bottom of the tube. So melting has begun at around 124. That’s the temperature that the literature
says should be the final temperature for the melting range of benzoic acid. On our instrument today,
we’re just starting at 124. So we’ll see how we end up. Now you can see that we’re sort of at the
— what I would call an iceberg stage. It looks like an iceberg
floating in the ocean there. The temperature now is about 125. At this point, if you look at the
liquid on the top, you have a meniscus and the temperature now is
126 degrees, and in Europe, they use this meniscus point
as the melting point. Here we’ll take it all the way
to the point where the liquid — until the sample has gone completely liquid. All right. Well, it’s all melted now and — so I hope you
had a good look at the idea that it’s a process. The initial drop, that’s where we start to
take the temperature, and the final temperature of the range is where the
sample all goes to liquid. By the way, at this point, I can turn this off. We’re done. Turn that off. Please remember to turn it off. If you leave this thing turned
on and the dial on, then this heating block just continues to heat. So please don’t do that. Don’t burn out the instrument. Also when we’re done, you can
turn off the thermocouple also. We’ve characterized our unknown,
or in this case, benzoic acid, the material that we had prepared
in our recrystallization experiment. We’ve characterized it by
its melting point and — so this will be very typical of what you’ll do
in the lab many times as we make solid products. So I think the points I want to
emphasize again, not too much sample in the tube, just about a 16th of an inch. That’s all you want in the tube. It needs to be finely pulverized. It needs to be finely compacted. Air spaces between particles in the tube
are going to cause a broad melting point. Air is a thermal insulator, so these particles
need to be physically touching each other with minimal air in between in order
to get a narrow melting point range. Pure materials are going to have a narrow range. Any impurity in there is going to
lower the range and broaden the range. When you’re done, capillary tubes go
into the waste capillary tube jar, which are kept in the room
next to the Mel-Temp apparatus. So there we are, and I’ll be checking you
out, just help you when you get to yours. [ Music ]

15 thoughts on “1 O Chem Melting Point ALL with Graphics (CC)

  1. Thank you so much. I had a hard time visualizing the melting point technique when I read those steps in the O-Chem Lab Survival Manual. This video should be required viewing for new students since O-Chem labs cannot be done simply by reading about it. 

  2. Finally I understand my lab lesson ,great video,but i have a question…Is there any good explanation of why do we pack the sample?

  3. This was so helpful. I have been reading about the experiment and i have being so confused but just watching this has given me more insight.

  4. I am very happy that Dr. Davison took the time to make this video. Now I have an idea of what melting point is all about. Thank you again

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