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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.
- 1 History
- 2 Construction
- 3 Operation
- 4 Types of polarimeter
- 5 Sources of error
- 6 Calibration
- 7 Applications
- 8 References
- 9 See also
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.
Polarimeters measure this by passing monochromatic light through the first of two polarising plates, creating a polarized beam. This first plate is known as the polarizer. This beam is then rotated as it passes through the sample. 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.
Types of polarimeter
Laurent's half-shade polarimeter
The earliest polarimeters, which date back to the 1830s, required the user to physically rotate one polarizing element (the analyzer) whilst viewing through another static element (the detector). The detector was positioned at the opposite end of a tube containing the optically active sample, and the user used his/her eye to judge the "alignment" when least light was observed. The angle of rotation was then read from a simple protractor fixed to the moving polariser to within a degree or so.
Although most manual polarimeters produced today still adopt this basic principle, the many developments applied to the original opto-mechnical design over the years have significantly improved measurement performance. The introduction of a half-wave plate increased "distinction sensitivity", whilst a precision glass scale with vernier drum facilitated the final reading to within ca. ±0.05º. Additionally, most modern day manual polarimeters also incorporate a long-life yellow LED in place of the more traditional and costly sodium arc lamp.
Today there are also semi-automatic polarimeters, which require visual detection but use push-buttons to rotate the analyzer and offer digital displays.
The most modern polarimeters are fully automatic, and simply require the user to press a button and wait for a digital readout. Fast automatic digital polarimeters reduce measuring time to just one second, regardless of the rotation angle of the sample. In addition, they permit continuous measurement, for example for kinetic investigations or in HPLC. Special techniques like a temperature controlled sample tube reduce measuring faults and ease operation. Results can directly be transferred to computers or networks for automatic processing.
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 errors.
Polarimeters can be calibrated – or at least verified – by measuring a quartz plate, which is constructed to always read at a certain angle of optical 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.
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 the ratio of enantiomers in solutions.
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.
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:
- Amino Acids
- Essential Oils
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).
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