Polarimeters

The polarimeter is used for determining the specific rotations of optically active substances and also for determining concentrations of solutions of optically active substances of known specific rotation. 

The polarimeter consists of two Nicol prisms N and Nx (Fig. 43) set at a distance from one another and on a common axis. Nx is the polariser and N the analyser. The polarimeter tube T, containing a definite 

Light vibrating in only one plane passes through the polarimeter tube to the analyser N, which can be rotated about the main axis. The eyepiece at E consists of a system of lenses for focussing. 

The flame produced from common salt or a sodium lamp is adjusted to give a maximum of light, the tube T being removed for the time being. 

I = length in decimetres of liquid in polarimeter tube. d = density of liquid at f. 

---

A half-shadow polarimeter (Lippich type) is illustrated diagram-maticaUy in Fig. Ill, 169, 1. Here two polarized rays are produced by means of the main Nicol prism P and a smaU Nicol prism P the latter... 

For further details and a description of the triple-shadow polarimeter, see text books of practical physical chemistry, for example, Daniels, Mathews and Williams, Experimental Physical Chemistry, 4th Edition, 1949, p. 34 (McGraw-Hill) Findlay and Kitchener, Practical Physical Chemisiryi 1964, p. 180 (Longmans, Green and Co. Ltd.). 

The experimental facts that led van t Hoff and Le Bel to propose that molecules having the same constitution could differ m the arrangement of their atoms m space concerned the physical property of optical activity Optical activity is the ability of a chiral sub stance to rotate the plane of plane polarized light and is measured using an instrument called a polarimeter (Figure 7 5)... 

The observed rotation a of an optically pure substance depends on how many mol ecules the light beam encounters A filled polarimeter tube twice the length of another produces twice the observed rotation as does a solution twice as concentrated To account for the effects of path length and concentration chemists have defined the term specific rotation, given the symbol [a] Specific rotation is calculated from the observed rotation according to the expression... 

Cholesterol when isolated from natural sources is obtained as a single enantiomer The observed rotation a of a 0 3 g sample of cholesterol in 15 ml of chloroform solution contained in a 10 cm polarimeter tube is -0 78° Cal culate the specific rotation of cholesterol... 

Polarimeter (Section 7 4) An instrument used to measure op tical activity... 

Although the usual absorption and scattering spectroscopies caimot distinguish enantiomers, certain techniques are sensitive to optical activity in chiral molecules. These include optical rotatory dispersion (ORD), the rotation by the sample of the plane of linearly polari2ed light, used in simple polarimeters and circular dichroism (CD), the differential absorption of circularly polari2ed light. 

Polarization is the most common method for the determination of sugar in sugar-containing commodities as well as many foodstuffs. Polarimetry is apphed in sugar analysis based on the fact that the optical rotation of pure sucrose solutions is a linear function of the sucrose concentration of the solution. Saccharimeters are polarimeters in which the scales have been modified to read directiy in percent sucrose based on the normal sugar solution reading 100%. 

Determination of Rotatory Power. —The 1 otatory p o < 1 of ethyl tartiate, which is an optically active substance, is determined by means of a polarimetcr. One of these insti U-ments known as Laurent s polarimeter is shown in l- igs. 71 and 72. 

When the solution is just cold the crystals, previously le-moved, are sown evenly over the bottom of the dish at distances of I—2 cms. apart and left for two days. The crystals will have now grown to a size which will enable the facets to be readily recognised. Each crystal is dried and carefully examined with a pocket lens in order to determine the position of the hemi-hedral facets, and placed in separate heaps. These facets lie to the right or left hand of the central prism face, as shown in Fig. 74. The crystals should be weighed, dissolved, and the solution diluted and examined in the polarimeter. The specific rotation may then be calculated. See Appe7idix., p. 264. 

The polarimeter is an instrument with which the essential oil chemist cannot possibly dispense. The hypothesis, first seriously enunciated by Le Bel and van t Hoff, that substances which contained an asymmetric carbon atom i.e. a carbon atom directly united to four different atoms or radicles) were capable of rotating the plane of polarisation of a beam of polarised light, has now become a fundamental theory of organic chemistry-. The majority of essential oils contain one or more components containing such a carbon atom, and so possess the power of effecting this rotation. In general, the extent to which a given oil can produce this effect is fairly constant, so that it can be used, within limits, as a criterion of the purity or otherwise of the oil. 

Figure 9.5 Schematic representation of a polarimeter. Plane-polarized light passes through a solution of optically active molecules, which rotate the plane of polarization.

A polarimeter. The sample lube contains an optically active compound. The analyzing filter has been tuned clockwise to restore the light field and measure the rotation caused by the sample. 

Polar molecule A molecule in which there is a separation of charge and hence positive and negative poles, 183-185 dipole force, 237 orientation, 183 Polarimeter, 600 Polarity, 184-185 Pollutants, 6 Polyamide, 615-616 Polyatomic ion, 36,39 Polyatomic molecules, 654 Polyester A large molecule made up of ester units, 614-615 Polyethylene, 611-612... 

These were actually fused silica polarimeter end plates. 

Photoelastic measurement is a very useful method for identifying stress in transparent plastics. Quantitative stress measurement is possible with a polarimeter equipped with a calibrated compensator. It makes stresses visible (Fig. 5-2). The optical property of the index of refraction will change with the level of stress (or strain). When the photoelastic... 

A polarimeter is used to determine the optical activity of a substance by measuring the angle through which plane-polarized light is rotated by a sample. 

POL The value determined by direct or single polarization of the normal weight of a sugar product or process stream in a polarimeter (saccharimeter). The normal weight (26 g in a 100-mL aqueous solution) of pure sucrose reads 100% or 100° pol in a polarimeter calibrated for sucrose measurement with a standard quartz plate. 

In situ polarimetry could be performed using an eight axis SXR polarimeter [87] with multilayer optics mounted immediately behind the experimental chamber. Full determinations of the four Stokes parameters are time consuming. 

The SU5 beamline [89] at the Super-ACO synchrotron (LURE, Paris ) employed an electromagnetic undulator to produce fully variable polarization in the VUV region [83, 90, 91]. This beamline was equipped with a gas filter for the suppression of unwanted higher order radiation [92] and had a VUV polarimeter [93] permanently installed just before the experimental chamber that could be rapidly lowered into the beam for polarization determinations. Full polarization analyses had been performed in commissioning, with 53 values ranging from 0.9 to 0.96 for rep and from 0.9 to 0.99 for Icp [93]. The remainder was determined... 

---