Polarimeter
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A polarimeter [1] is a scientific instrument used to measure the angle of rotation caused by passing polarized light through an optically active substance.[2]
Some chemical substances are optically active, and polarized (unidirectional) light will rotate either to the left (counter-clockwise) or right (clockwise) when passed through these substances. The amount by which the light is rotated is known as the angle of rotation.
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[edit] Polarimeter History
Polarization by reflection was discovered in 1808 by Etienne Malus (1775–1812).[3]
[edit] Construction
The polarimeter is made up of two Nicol prisms (the polarizer and analyzer). The polarizer is fixed and the analyzer can be rotated. The prisms may be compared to as slits S1 and S2. The light waves may be considered to correspond to waves in the string. The polarizer S1 allows only those light waves which move in a single plane. This causes the light to become plane polarized. When the analyzer is also placed in a similar position it allows the light waves coming from the polarizer to pass through it. When it is rotated through the right angle no waves can pass through the right angle and the field appears to be dark. If now a glass tube containing an optically active solution is placed between the polarizer and analyzer the light now rotates through the plane of polarization through a certain angle, the analyzer will have to be rotated in same angle.
[edit] Operation
Polarimeters measure this by passing monochromatic light through the first of two polarizing plates, creating a polarized beam. This first plate is known as the polarizer.[4] This beam is then rotated as it passes through the sample. The sample is usually prepared as a tube where the optically active substance is dissolved in an optically inactive chemical such as distilled water, ethanol, methanol. Some polarimeters can be fitted with tubes that allow for sample to flow through continuously.
After passing through the sample, a second polarizer, known as the analyzer, rotates either via manual rotation or automatic detection of the angle. When the analyzer is rotated to the proper angle, the maximum amount of light will pass through and shine onto a detector.
[edit] Types of polarimeter
[edit] Laurent's half-shade polarimeter
[edit] Biquartz polarimeter
[edit] Lippich polarimeter
[edit] Manual
The earliest polarimeters, which date back to the 1830s, required the user to physically rotate the analyzer, and the detector was the user’s eye judging when the most light shone through. The angle was marked on a scale that encircles the analyzer. This basic design is still used in the simplest polarimeters.
[edit] Semi-automatic
Today there are also semi-automatic polarimeters, which require visual detection but use push-buttons to rotate the analyzer and offer digital displays.
[edit] Fully automatic
The most modern polarimeters are fully automatic, and simply require the user to press a button and wait for a digital readout.
[edit] Sources of error
The angle of rotation of an optically active substance can be affected by:
- Concentration of the sample
- Wavelength of light passing through the sample (generally, angle of rotation and wavelength tend to be inversely proportional)
- Temperature of the sample (generally the two are directly proportional)
- Length of the sample cell (input by the user into most automatic polarimeters to ensure better accuracy)
Most modern polarimeters have methods of compensating for or controlling these.
[edit] Calibration
Polarimeters can be calibrated – or at least verified – by measuring a quartz plate, which is constructed to always read at a certain angle of rotation (usually +34°, but +17° and +8.5° are also popular depending on the sample). Quartz plates are preferred by many users because solid samples are much less affected by variations in temperature, and do not need to be mixed on-demand like sucrose solutions.[5]
[edit] Applications
Because many optically active chemicals such as sucrose, are stereoisomers, a polarimeter can be used to identify which isomer is present in a sample – if it rotates polarized light to the left, it is a levo-isomer, and to the right, a dextro-isomer. It can also be used to measure optical activity of racemic mixtures.
[edit] Chemical industry
Many chemicals exhibit a specific rotation as a unique property (like refractive index in many cases) which can be used to distinguish it. Polarimeters can identify unknown samples based on this if other variables such as concentration and length of sample cell length are controlled or at least known. This is used in the chemical industry.
By the same token, if the specific rotation of a sample is already known, then the concentration and/or purity of a solution containing it can be calculated.
Most automatic polarimeters make this calculation automatically, given input on variables from the user.
[edit] Food, beverage and pharmaceutical industries
Concentration and purity measurements are especially important to determine product or ingredient quality in the food & beverage and pharmaceutical industries. Samples that display specific rotations that can be calculated for purity with a polarimeter include:
- Steroids
- Diuretics
- Antibiotics
- Narcotics
- Vitamins
- Analgesics
- Amino Acids
- Essential Oils
- Polymers
- Starches
- Sugars [6]
Polarimeters are used in the sugar industry for determining quality of both juice from sugar cane and the refined sucrose. Often, the sugar refineries use a modified polarimeter with a flow cell called a saccharimeter. These instruments use the International Sugar Scale (as defined by the International Commission for Uniform Methods of Sugar Analysis (ICUMSA).
[edit] References
[edit] See also
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