Polarimeter

A polarimeter or polariscope is an instrument of photoelasticity to determine the mechanical stresses in transparent solids. For this purpose, the optical rotation of the material to be examined is determined by an array of polarization filters. The change of the angle of linearly polarized light when passing through a sample of the chemical substance is determined. This property is called optical activity, and is an intrinsic property of the chiral nonracemic compounds or mixtures thereof. The rotation value is frequency and temperature dependent. Is typically measured at 589 nm ( Sodium D - line ), but also at other wavelengths. Because of the temperature dependence of the rotary value 22 ° C or 25 ° C is generally the temperature stabilized at 20 ° C measured; where you can also find other information in the literature.

Principle

A polarimeter is usually composed of two nicol. The fixed polarizer polarizes the light from the light source linear. The rotating analyzer is used behind the position of the plane of polarization observed.

If you cross the two polarizing filters, so no light can pass through; the visual field to which the viewer sees remains dark. One brings the sample material between the two polarization filters, there is a function of the angle of rotation, a whitening, which enables the measurement of the rotation value.

There are many other factors the rotation value influence ( layer thickness of the sample, solvent, temperature, wavelength of light, etc.) shall be the specific rotation at 10 cm thickness, 100 ml of solvent and 1 g of optically active substance and returns the other relevant parameters with of.

A measuring instrument commonly used is the Halbschattenpolarimeter by F. Lippich. In this, part of the light incident in A, before slightly rotated by an auxiliary prism H (1 ° to 10 °). Thus we see two " fields ".

Polarizer P and analyzer A are nicols, where P is fixed and A rotatable. If A and P are parallel, the picture is bright; one rotated by 90 ° A darken the picture. If A and P crossed, one half is dark, while the other half is bright. If one turns now A 360 ° so will alternate areas of large and small overall brightness ( total of 4). The arrangement (Figure 1 and 2) described above is only one of many ways to build a polarimeter. In other arrangements of the polarizer P is rotated while the analyzer A is fixed, both parts can also be rotated at the same time. Instead of a slow rotation, a polarisierndes element can rotate rapidly, with a detector in place of the eye can then be a sinusoidal variation () the brightness register. From the phase shift with respect to the waveform without the sample rotation value is then determined.

Automatic polarimeter

The correct measurement of the optical rotation with a manual polarimeter is time consuming and requires a certain experience of the laboratory personnel ahead. Therefore, automatic, electronically controlled polarimeter can be used with digital display, especially in the industrial field and in laboratories with a large number of measurements. Automatic Polarimeter avoid faulty measurements by operator and reading errors. Their small measurement time of up to one second per measurement and a computer interface allow measurement series, as they are indispensable in critical industrial processes such as food processing, chemical and pharmaceutical industries for continuous quality assurance.

Indication of the measuring

The angle of rotation α is frequency and temperature dependent. Is typically measured at 589 nm ( Sodium D - line ), but also at other wavelengths. Because of the temperature dependence of the rotary value 22 ° C or 25 ° C is usually thermostated at 20 ° C measured; where you can also find other information in the literature.