Module 6 : Reaction Kinetics and Dynamics
Lecture 27 : Experimental methods in chemical kinetics
  27.2

Polarimetry

This method is useful when reactants/products are optically active. Optically active compounds contain asymmetric centers (e.g., a carbon atom to which four distinct groups / atoms are attached). Such molecules rotate the plane of polarization of polarized light. In a plane polarized light the electric and magnetic fields of radiation oscillate in a given plane (unlike the usual light from a torch or a light bulb wherein the radiation is not polarized, i.e., different photons coming out have different planes of polarization). A polarized light can be separated from an unpolarized beam of light by using appropriate prisms (polarizers) which permit only polarized light to pass through. The method is illustrated for the following reaction
 

C12 H22O11 + H2O C6H12 O6 + C6H12O6

 (27.1)
sucrose       water            dextrose    levulose

Sucrose rotates the plane of polarized light to a particular angle(clockwise rotation, dextrorotatory). Let us denote this by 0. As the hydrolysis starts, the plane of polarization starts tilting away from 0 because the products have a different degree of polarization.

 
As the reaction reaches a final equilibrium, the polarization (degree to which the polarimeter has to be rotated to allow the light to pass through) reaches a value . At each stage of the reaction, the plane of polarization may be denoted by t as illustrated in the following figure.
 
 
Figure 27.1 Different planes of polarization at times a) t = 0, b) t 0, c) t
 
The angle ( - t ) may be taken as proportional to the concentration of sucrose. The rate of the reaction may be written as
 
d[sucrose] /dt = d/dt = - k (27.2)
 
by plotting vs time t, the rate constant for the hydrolysis (or inversion in this case) of sucrose can be determined.
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
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