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