Measure. Analyze. Learn.
A chemical polarimeter is a device for measuring
stereo chemistry in molecules.
And, today I'm going to give you a brief introduction
on how to use the polarimeter.
First thing to note, is that is has two cables coming out of it.
One is an analog, and one is a digital.
They will only fit into specific ports on your interface.
Today, I'm using the LabQuest Mini,
with a computer and Logger Pro.
But, you can also use it with LabQuest.
Now that I have everything plugged in,
I'm going to open Logger Pro.
You need to have Logger Pro 3.8.5, or newer,
for the polarimeter to work properly.
Make sure you note the axes on the graph.
The Y axis should be illumination,
and the X axis should be angle.
All of your data with the polarimeter
is taken in reference to a blank.
Today I'm working with sugar solutions, so may blank is water.
I fill one of the sample cells to 10 centimeters with water.
I don't always have to fill it all the way to 10 cm,
the least I can get away with is 2 cm or about 5 ml.
Place the sample into the polarimeter. Snap it into place.
And now, start data collection, in Logger Pro.
And, after I've done that, I begin to rotate the analyzer.
And, a cosine squared waveform should appear on the screen.
Data collection will continue for 15 seconds,
or I can stop it early if I like.
Once data collection is stopped,
I need to make sure and store that run,
so I can compare my optically active sample to the water.
Now, I'm ready for my optically active sample,
which is a 30 percent sucrose solution.
I've also filled this one to 10 centimeters.
Place it in the polarimeter.
Begin data collection and rotate the analyzer.
You can see right away, qualitatively, that the
cosine squared waveform has been shifted to the right.
Sucrose is, therefor, a right handed molecule.
If I had used a negative sugar, like fructose,
I would have gotten a shift to the left.
This data can be analyzed quantitatively,
several different ways in Logger Pro.
The quickest and simplest way to quantitatively analyze
these results is to use the statistics feature.
To do that, I select the first peak after angle 0,
and tap on the '[Statistics]' button.
I want to analyze both the blank
and the sample at the same time, select 'OK'.
And so, the value that I care about when I'm trying to figure out
the optical rotation is the angle at maximum illumination.
So, the absolute peak here of this wave.
Statistics gives you that value right here
with the maximum Y at X.
So this is the value of the angle at maximum illumination.
For the blank here, it's 145.5.
And, for the sample, it's 166.8.
To get the optical rotation of my sample,
I just take the difference of those two values.
And then, I can use that value for further analysis of my sample.
To do a more accurate value of the optical rotation,
you can do a Gaussian fit, or a cosine squared fit.
And, I'm going to show you how to do that in the next video.