®
Polarimeter
PS-2235
2
013-14434A
1
See the PASCO web site at www.pasco.com for more information
about PASCO data collection software
*NOTE: SPARKvue also supports a Chromebook device it is con-
nected through USB.
Charge the Battery
The Polarimeter battery is partially charged at the factory. The
battery can be charged using the USB cable connected to a USB
port or USB charger. Leave the Polarimeter off while charging.
As the battery is charging, the Battery Status LED (light-emitting
diode) will blink red. When the battery is fully charged, the
Battery Status LED shines green. The charger circuit inside the
unit turns itself off when the unit is fully charged, so it can’t be
overcharged.
Battery Life
On a full charge, the Polarimeter will typically run for over eight
hours during data collection before it needs to be charged.
Introduction
The Polarimeter is specifically designed for measuring the angle
of rotation of plane-polarized light that is caused by optically
active substances. The Bluetooth and USB connectivity enable its
use with tablets and computers.
Theory of Operation
At one end of the chamber is a fixed polarizer disk in front of the
LED light source. At the other end of the chamber is a rotatable
polarizer disk - called the analyzer - and the photo-detector. The
analyzer can be turned using the thumb-wheel. An optical
encoder records the position of the analyzer (rotatable polarizer),
and the photo-detector measures the intensity of light transmitted
through the substance in the Sample Cell.
Plane-polarized light from the end of the chamber with the fixed
polarizer enters the Sample Cell and passes through the optically
active substance. The substance rotates the plane polarized light
clockwise or counter-clockwise by a specific amount. As the
thumb-wheel of the analyzer is turned, the encoder on the
analyzer measures the angle, and the photo-detector records light
intensity.
The data collection software produces a graph of light intensity
compared to angle, and can be used to analyze the data to
determine various properties of the optically active substance.
The Sample Cell is precisely designed so that the distance inside
the cell between the two quartz windows is 100 millimeters
(mm). Therefore, the path length of the substance inside the
Sample Cell is 100 mm (1 decimeter).
Polarimetry
Since 1812, it has been know that certain molecules are optically
active and can rotate plane-polarized light. For example, table
sugar is an optically active substance. Table sugar has a
tetrahedral carbon atom which has four different groups attached
to it. This carbon is known as a
chiral
center, a
stereogenic
center, or an
asymmetric
carbon. Chiral centers can be organic or
inorganic. The geometry of the chiral center allows the formation
of mirror image molecular forms under the right conditions. One
of the molecular forms is able to rotate plane-polarized light to
the right (dextrorotary), and the other mirror image molecular
form is able to rotate plane-polarized light to the left (levorotary)
by the same angle.
Experimentation is the only way to determine which form of
molecule is which. The procedure consists of preparing a solution
of known concentration and placing it between two polarizing
filters. A polarimeter measures the amount and direction of
rotation of the plane-polarized light.
Specific angle.
, is a fundamental property of chiral
substances. The expression of the specific angle to which the
substance causes plane-polarized light to rotate at a particular
temperature, wavelength,
path length, and concentration, is as
follows.
where
is the measured rotation in degrees (clockwise or
counterclockwise), T is the temperature (degrees Celsius),
is
the wavelength of the light source (nanometers),
l
is the path
non-polarized
light
Polarizer
Sample Cell
Analyzer
plane-polarized
light
sample
rotated-polarized
light
T
T
lc
----
=
