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Mr. Andersen demonstrates the azide-winkler method of dissolved oxygen analysis.
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Transcript Provided by YouTube:
00:00
Hi and welcome to the AP Biology Lab 12 – Dissolved Oxygen Lab. I’m going to
00:05
kind of do a hands on walk through of how to do dissolved oxygen. Today I’m going to
00:09
be using the Hawk dissolved oxygen test kit. It’s nice because you can put all of your
00:14
chemicals in it and you can actually take it out in the wild. I’m going to do this using
00:19
chemicals that I have right here. It’s kind of nasty so you want to make sure you’ve got
00:22
lab coat, got some goggles. And you’ve got some rubber gloves as well. And so my head
00:30
is about to disappear but my hands are going to kind of talk you through the whole process.
00:34
So to start you got to get some water. This is water that I grabbed from the creek that’s
00:38
right outside. It’s kind of cold because it’s springtime. And so this looks like it’s around
00:43
13 degrees Celsius. And so that’s like 55 degrees, something like that. So temperature
00:50
is one thing that can effect dissolved oxygen. Next I’m going to clear some of this stuff
00:54
out of the way. And I’m going to get my bottle. So. I’m going to take the lid off. And so
01:02
when you do this, you can do it, this is just water that I’ve taken right out of the stream.
01:09
What we’re measuring is the amount of dissolved oxygen. And so I’m going to gradually pour
01:13
this in. But if we have bubbles in it then that’s going to screw up our results. Because
01:20
what we’re really trying to measure is those bubbles. And so you can see that I’m pouring
01:24
it over the top. And usually you let it run for a few minutes so you get all of those
01:31
oxygen bubbles out of the way. Next thing I’m going to do is I’m going to cap it. Force
01:38
that extra water out of the way. So it looks like I didn’t trap any bubbles in there. Okay.
01:46
Next I’m going to add my chemicals. And so there are two packets that I’m adding. The
01:52
first one is dissolved oxygen packet number 1. And that is actually manganous sulfate.
01:58
So I’m going to use my clippers to open that up and pour some in. Alright. There we go.
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Now I’m going to put my dissolved oxygen 2 packet it. This is actually three chemicals.
02:14
We have sodium azide, lithium hydroxide and then potassium iodide. So let me add that.
02:24
And now I’m going to mix it up. Now if I start to see a brownish color and you can start
02:30
to see that as well, if I start to see a brownish color like this that means that there’s oxygen
02:36
in the water. And so I’m going to shake it vigorously. And the precipitate is starting
02:42
to form. And them I’m just going to let it set for a second. And when you let it set,
02:46
you want to make sure that the flocculant which is this solid material sinks to the
02:50
bottom. So I’m going to let it sit like that. I may have pause the video here for a second.
02:55
But it takes awhile for that flock to settle out. Okay. The amount of flock is settled
03:00
down about half way. And so now what I want to do is I want to shake it up again. So I’m
03:04
going to shake it up again really vigorously and try not to get this on the computer. But
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now I’m going to let it sit again. I don’t know if you can see that, but I’m going to
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let it sit until the flock goes down half way again. Before we go on to the third step.
03:20
Okay. The flock has settled out again. And so you can see that it looks like we have
03:24
some oxygen in here. The next thing I’m going to do is I’m going to take the top off. And
03:30
now I’m going to add the third chemical. This is called sulfamic acid. And what this should
03:36
do is it should when it mixes with that, it should get rid of that precipitate. And then
03:45
it should turn, so it should turn it to a yellow color. And so I’m going to shake it
03:49
again. And we should be able to get rid of that flocculant. Alright. So that yellow color
03:57
means that there is oxygen present. Now what we’re doing in this lab is simply a titration.
04:03
And if you’ve never done a titration in chemistry before, what we have to make sure we have
04:08
is a known quantity. And so the way this kit works is that they have a little vial. And
04:16
that vial allows you to pour some of it in. This is a known quantity. So I’m going to
04:20
pour known quantity in here. Like that. And now we want that to go into this other jar.
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And so what I usually do is just invert it. And I tip it like that. Okay. Now to actually
04:38
do the titration part, what we have is this, which is yellow. And now we’re simply going
04:43
to start adding drops of the fourth chemical in the experiment. This is called sodium thiosulfate.
04:51
So it comes in a little dropper like this. And so this is the titration portion. What
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I’m going to do is I’m going to take one drop of it and add it. And between each drop I’m
05:02
going to mix it up. This is four. And let me add five. You want to kind of be careful.
05:08
So that’s five. And you can see that it’s still yellow. But we’re starting to turn it
05:12
towards clear. That’s seven. Let me add an eighth drop. Sometimes you have to hold it
05:18
up to something that’s white. So that’s pretty close. So that’s nine. I think maybe one more
05:24
is going to do it. So let me add one more drop. So let me mix it up. And so now that’s
05:31
totally clear. And so 10 drops is how much we had to add to make that turn from that
05:37
yellow color that we had a second ago to that clear color that we have right now.
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This post was previously published on YouTube.
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Photo credit: Screenshot from video.