Hello. Welcome to part three of our Carbohydrate
ID Prelab. We are going to go over the Barfoed’s test. In this test you are going to take one
mL of your sample, and you are going to take three mL of the Barfoed’s solution. How do
we precisely get one mL or three mL? Well, we will be using a disposable plastic pipette.
These are not the most precise instruments in the world, but we can still get an approximately
good estimate of one mL using them. You may not be able to tell. Here, I’ll put this right
there. Can you focus on that? I can’t tell. Right here there is a notch. That notch is
our one mL notch. So how do I fill this pipette specifically to that one mL notch. I take
my solution that I am working with, and I over fill. Squeeze the bulb, bring it up.
Now, the notch is right here. Here and below, that is one mL. Everything above is extra,
so how do I get it down to one mL? I simply eject until the line of the fluid is at that
one mL mark. Right there. Now that is precisely one mL, in this pipette. Now, if let go and
relax, Oh look. The liquid has moved up to here, but it hasn’t changed its amount, it
is still one mL. Now I can walk around, I can do things, I can hold it. I’m not going
to lose my one mL. Then I can say, “Hey, I’m going to move this one mL. I want it to go
back here.” I let it all out. There we go. I have moved exactly one mL. You can use this
marking, fill this section, relax, carry it around, and you’ve got one mL. If you need
three mL, you do that three times. You’re good. That is how we move a precise volume.
What do we do next? In the Barfoed’s test, we have to put our test tubes in a hot water
bath for precisely two minutes. How do we do that? We will use test tube tongs, because
we don’t put our hands in boiling water, and if glass has been in boiling water for several
minutes it is going to be freaking hot and we’re not going to use our hands to carry
it. We simply pinch to open, relax to close. Once it is closed, like this, you can relax.
You don’t even have to hold it. It will just hold your test tube for you. That is it. Many
of you may have used these already. I don’t know. We’ve got them in the drawers back there.
If you can’t find one, let me know and I’ll make sure you get one. So, Barfoed’s, one
mL of sample, three mL of the Barfoed’s solution, boil for two minutes, precisely two minutes.
Don’t over boil. That’s bad for this test, because this test indicates monosaccharides.
Monosaccharides will react in two minutes or less. Other carbohydrates will react in
more than two minutes. So if you go longer than two minutes, you could get false positives.
This is supposed to tell us what the monosaccharides are. What does that look like? Well, the Barfoed’s
solution itself is a blue, so it is going to turn your liqud blue no matter what happens.
This is a negative test. There are no monosaccharides in here. However, this guy, this brick red
precipitate, down there at the bottom, you can see. Look, it looks like crushed up brick
dust, That is your positive indicator of monosaccharides. So there is definitely a monosaccharde in
here. Now, I don’t know what is in here, I just know what is in here is a monosaccharide.
I don’t know what’s in here, I just know that what’s in here is not a monosaccharide. This
sample, did not come from our monosaccharides. There it is. Barfoed’s test. I hope this helps.
Struggle well. One last note. I should have said it earlier. This stuff is a solid. Which
means, we do not pour it down the sink. The other stuff, we can pour down the sink. But
solids, precipitates, we have to pour in the garbage can. Just, that’s important because
these sinks get clogged with solids all the time, big headache, big problem, don’t do
it. Trash. Not sink. Great. How do I stop this thing? Seriously, where is the control
panel? Oh, there it is. Ok. Got it.