The polarimetry measures the rotation that a polarized light beam undergoes when it passes through an optically active substance that can be a crystal (for example tourmaline) or a sugar solution.
It is a simple technique, belonging to the optical methods of analysis and with numerous applications, especially in the chemical and agri-food industry to determine the concentration of sugary solutions..
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The physical foundation of this technique resides in the properties of light as an electromagnetic wave, consisting of an electric field and a magnetic field moving in mutually perpendicular directions..
Electromagnetic waves are transverse, which means that these fields, in turn, propagate in the direction perpendicular to them, according to figure 2.
However, since the field is made up of numerous wave trains that come from each atom, and each one is oscillating in different directions, natural light or that coming from an incandescent light bulb is not polarized..
In contrast, when the oscillations of the field occur in a preferential direction, the light is said to be polarized. This can be achieved by letting the light beam pass through certain substances capable of blocking unwanted components and allowing only one to pass through it..
If, in addition, the light wave consists of a single wavelength, we have a beam linearly polarized monochrome.
Materials that act as filters to accomplish this are called polarizers or analyzers. And there are substances that respond to polarized light, rotating the plane of polarization. They are known as optically active substances, for example sugars.
In general, polarimeters can be: manual, automatic and semi-automatic and digital.
Manual polarimeters are used in teaching labs and small laboratories, while automatic ones are preferred when a large number of measurements are required, as they minimize the time spent on measurement..
The automatic and digital models come with a photoelectric detector, a sensor that emits a response to the change of light and greatly increases the precision of the measurements. There are also those that offer reading on a digital screen, being very easy to operate.
To illustrate the general operation of a polarimeter, a manual optical type is described below..
A basic polarimeter makes use of two Nicol prisms or Polaroid sheets, in the middle of which the optically active substance to be analyzed is located..
William Nicol (1768-1851) was a Scottish physicist who devoted much of his career to instrumentation. Using a crystal of calcite or Iceland spar, a mineral capable of splitting an incident light beam, Nicol created in 1828 a prism with which polarized light could be obtained. It was widely used in the construction of polarimeters.
The main parts of a polarimeter are:
- The light source. Generally a sodium, tungsten or mercury vapor lamp, whose wavelength is known.
- Polarizers. The old models used Nicol prisms, while the more modern ones usually use Polaroid sheets, made of long-chain hydrocarbon molecules with iodine atoms..
- A sample holder. Where the substance to be analyzed is placed, whose length is variable, but known exactly.
- An eyepiece and indicators fitted with vernier scales. For the observer to accurately measure the rotational power of the sample. Automatic models have photoelectric sensors.
- Additionally, temperature and wavelength indicators. Since the rotational power of many substances depends on these parameters.
In the procedure described there is a small inconvenience when the observer adjusts the minimum light, since the human eye is not capable of detecting very small variations in luminosity..
To overcome this problem, the Laurent polarimeter adds a half-wavelength retarding half-sheet, made of birefringent material..
In this way, the observer has two or three adjacent regions of different luminosity, called fields, in the viewer. This makes it easier for the eye to distinguish between brightness levels..
You have the most accurate measurement when the analyzer is rotated so that all fields are equally dim..
Biot's law relates the rotating power α of an optically active substance, measured in sexagesimal degrees, with the concentration c of said substance -when it is a solution- and the geometry of the optical system.
For this reason, emphasis was placed in the description of the polarimeter, that the values of the wavelength of the light and that of the sample holder had to be known..
The constant of proportionality is denoted [α] and is called specific rotary power of the solution. It depends on the wavelength λ of the incident light and the temperature T of the sample. The values of [α] are usually tabulated at 20 ºC for sodium light, specifically, whose wavelength is 589.3 nm.
Depending on the type of compound to be analyzed, Biot's law takes different forms:
- Optically active solids: α = [α] .ℓ
- Pure liquids: α = [α]. ℓ.ρ
- Solutions with solutes that have optical activity: α = [α]. ℓ.c
- Samples with various optically active components: ∑αi
With the following additional magnitudes and their units:
- Specimen holder length: ℓ (in mm for solids and dm for liquids)
- Density of liquids: ρ (in g / ml)
- Concentration: c (in g / ml or molarity)
Polarimeters are very useful laboratory instruments in various areas and each type of polarimeter has advantages according to its use..
A great advantage of the technique itself is that it is a non-destructive test, appropriate when analyzing expensive, valuable samples or that for some reason cannot be duplicated. However, polarimetry is not applicable to any substance, only to those that have optical activity or substance chiral, as they are also known.
It is also necessary to consider that the presence of impurities introduces errors in the results..
The angle of rotation produced by the analyzed substance is in accordance with its characteristics: the type of molecule, the concentration of the solution and even the solvent used. To obtain all these data, it is necessary to know exactly the wavelength of the light used, the temperature and the length of the sample holder container..
The precision with which you want to analyze the sample is decisive when choosing an appropriate equipment. And its cost too.
- They are usually cheaper, although there are low-cost digital versions as well. As for this there is a lot of offer.
- They are suitable for use in teaching laboratories and as training, because they help the operator to become familiar with the theoretical and practical aspects of the technique.
- They are almost always low maintenance.
- They are strong and durable.
- Reading the measurement is a bit more laborious, especially if the substance to be analyzed is of low rotational power, therefore the operator is usually specialized personnel.
- They are easy to handle and read, they do not require specialized personnel for their operation.
- The digital polarimeter can export the data to printer or storage device.
- Automatic polarimeters require less measurement time (around 1 second).
- They have options to measure by intervals.
- The photoelectric detector allows to analyze substances with low rotational power.
- Efficiently control temperature, the parameter that most influences the measurement.
- Some models are expensive.
- Require maintenance.
Polarimetry has a large number of applications, as mentioned at the beginning. The areas are diverse and the compounds to be analyzed can be organic and inorganic as well. These are some of them:
- In pharmaceutical quality control, helping to determine that the substances used in the manufacture of medicines have the appropriate concentration and purity.
- For the quality control of the food industry, analyzing the purity of sugar, as well as its content in beverages and sweets. Polarimeters used in this way are also called saccharimeters and they use a particular scale, different from the one used in other applications: the ºZ scale.
- Also in food technology it is used to find the starch content of a sample.
- In astrophysics, polarimetry is used to analyze the polarization of light in stars and to study the magnetic fields present in astronomical environments and their role in stellar dynamics..
- Polarimetry is useful in detecting diseases of the eye.
- In satellite remote sensing devices for the observation of ships in the high seas, areas of pollution in the middle of the ocean or on land, thanks to the taking of images with high contrast.
- The chemical industry uses polarimetry to distinguish between optical isomers. These substances have identical chemical properties, since their molecules have the same composition and structure, but one is a mirror image of the other..
Optical isomers differ in the way they polarize light (enantiomers): one isomer does it to the left (left-handed) and the other to the right (right-handed), always from the observer's point of view..
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