Measure. Analyze. Learn.
In part one of the video, I showed you how to use
the Polarimeter to collect data, and how to
briefly analyze that data.
In this part of the video, I'm going to show you how to
do more accurate curve fitting of your curves,
using a Gaussian fit, and a cosine squared fit.
First, to do the Gaussian fit, we select the region of
one of the curves that's Gaussian like.
Then, from the "Analyze" menu,
we're going to select "Curve Fit�"
Select the blank first.
And now, scroll down to the bottom of the list of equations,
and select the "Gaussian" equation.
Then, tap on "Try Fit"
Notice in the window here, a black curve fitting line
has shown up over your waveform.
You want to make sure that that black curve matches your data.
If it doesn't, restart the curve fit.
Since it matches quite nicely in this instance,
we're going to look at the coefficient.
The coefficient of interest here is the "B" coefficient.
This value gives us the angle at
maximum illumination for the blank.
I would now repeat this process for the sucrose sample,
and take the difference of the two B coefficients.
And, that will give me the optical rotation.
So, to do a cosine squared fit, simply select
the "Cosine Squared" fit equation from the menu.
Now, make sure you deselect any region in
the graph that has been selected,
because, the cosine squared fit takes into account
all the data you've collected.
And that's why it's the most accurate interpretation of your data.
Select "Try Fit".
You see the curve fitting black line,
lies perfectly on graph screen.
The coefficient that we care about with the cosine squared fit
is the opposite sign of the "C" value.
The C value here reads negative 145.3.
Therefor, the angle at maximum illumination is positive 145.3.
This is for the blank.
I would now repeat this process for the sucrose sample.
Get my C value.
The difference between the two C values is
therefor the optical rotation of the sucrose sample.
Now that you have the optical rotation value,
you can determine the specific rotation for a sample,
or determine the concentration of a known sample
using Biot's Law.
For more Experiment ideas using the Polarimeter,
see our website for several free experiments.
Or, the 'Organic Chemistry with Vernier' book has
some great options as well.