P001 - Polarimetry and Polarimeters - a simple explanation

Product catalog summary

Introduction to Polarimetry and Polarimeters
Polarimeters are optical instruments used to measure the rotation or 'twisting' of light. They are utilized in both industrial and academic laboratories for purposes ranging from quality control to research into complex chemical structures. Bellingham + Stanley has been manufacturing polarimeters for over a century.

Principle of Polarimetry
Polarimetry involves polarized light passing through an optically active substance, causing the plane of polarization to rotate. This rotation is measured in angular degrees (°A). The process involves a sample tube placed between two polarizing elements, with the rotation detected by comparing the plane's position before and after passing through the sample.

Optical Rotation and Specific Rotation
Optically active chemicals, such as sugars, cause light rotation. The rotation is proportional to the concentration of the substance and the path length of the solution. Specific rotation is a characteristic of a chemical measured under standard conditions, often using the sodium wavelength of 589 nm.

Calculation of Specific Rotation
The specific rotation is calculated using the formula: [α]tλ = α / (10 x l x c) or [α]tλ = α / (10 x l x d x p), where α is the corrected optical rotation, l is the tube length, d is the relative density, c is the concentration in grams per mL, and p is the concentration in grams per gram solution.

Wavelengths and Angular Range
The sodium wavelength of 589 nm is commonly used, with other sources like Mercury 546 nm and near-infrared 880 nm gaining interest. Polarimeters can detect the angular difference but may give ambiguous results due to the cyclic nature of angular measurements. Users must select the appropriate measuring range or experiment with concentrations to determine absolute rotation.

Use of Polarimeters
Polarimeters are used in various applications, from quality control to research and development. Users may range from those following standard procedures to those investigating optical properties for the first time.

Catalog excerpts

P001 - Polarimetry and Polarimeters - a simple explanation-1

Bellingham + Stanley Longfield Road, Tunbridge Wells, Kent, TN2 3EY, UK Tel +44 (0) 1892 500400 Fax +44 (0) 1892 543115 TECHNICAL BULLETIN - P001 Title: Polarimetry & Polarimeters – a simple explanation Polarimeters are optical instruments for measuring the rotation or ‘twisting’ of light. Industrial and academic laboratories use polarimeters for a variety of purposes ranging from simple quality control to fundamental research into complex chemical structures. Bellingham + Stanley has been designing and manufacturing high quality polarimeters for 100 years. Principle of polarimetry Polarised light is light that has passed through a ‘polariser’, which forces the randomised electromagnetic waveforms into one plane. When this plane-polarised light then passes through an optically active substance (e.g. solution of an optically active chemical) the PLANE of polarisation is rotated by an amount which is characteristic of the test substance. Polarimeters detect the position of the PLANE and compare it to its original position, the difference being the rotation, normally expressed in angular degrees (°A). Every effort has been made to ensure the accuracy of the contents of this Bulletin. However, Bellingham + Stanley assumes no responsibility for errors contained herein or their consequences.

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P001 - Polarimetry and Polarimeters - a simple explanation-2

Bellingham + Stanley Longfield Road, Tunbridge Wells, Kent, TN2 3EY, UK Tel +44 (0) 1892 500400 Fax +44 (0) 1892 543115 A sample tube containing the test liquid (solution) is placed between two polarising elements (polaroid strip or a calcite crystal). The first element, the polariser, polarises the light before it passes into the sample. The second element, the analyser, can be rotated to counteract any rotation by the sample and hence locates the resultant angular position of the light plane and hence the amount of rotation caused by the sample. In the sugar industry, the rotation is expressed...

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P001 - Polarimetry and Polarimeters - a simple explanation-3

Bellingham + Stanley Longfield Road, Tunbridge Wells, Kent, TN2 3EY, UK Tel +44 (0) 1892 500400 Fax +44 (0) 1892 543115 Calculation of Specific Rotation (optically active liquids/solutions) is the corrected optical rotation is the specific rotation at t C in the polarised light of wavelength . is the length of the polarimeter tube in metres is the relative density of the liquid or solution at 20 C is the concentration of the solute expressed in grams per mL of solution is the concentration of the solute expressed in gram per gram solution Wavelength The sodium wavelength of 589 nm is by far the...

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P001 - Polarimetry and Polarimeters - a simple explanation-4

Bellingham + Stanley Longfield Road, Tunbridge Wells, Kent, TN2 3EY, UK Tel +44 (0) 1892 500400 Fax +44 (0) 1892 543115 The instrument cannot decide itself how many times the plane has passed the 180° reference position along the sample path length. It is up to the user to either know the range (angular segment) in which the result will fall approximately or to be prepared to experiment to establish the absolute rotation. It is for this reason that, with automatic digital polarimeters such as the B+S ADP 220, the user must select the measuring angular range, knowing (approximately) where the...

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