Applications of Refractivity

APPLICATIONS OF REFRACTIVITY

The various applications of refractivity are enumerated below :

(a) It is feasible to determine the molar refractivities of different substances experimentally and subsequently comparing their values with theoretical ones,

(b) Based on the fact that molar refractivity is an additive property, it may be utilized to determine the refractivities of homogeneous mixtures (as solutions).

Thus, the molar refraction of a solution having two components (viz., the solute and the solvent) is given by the expression :

R_{1, 2} = N₁R₁ + N₂R₂        ...(i)

where, N₁ = Mole fraction of the solute,

N₂ = Mole fraction of the solvent,

R₁ = Molar refractivity of the solute, and

R₂ = Molar refractivity of the solvent.

Evidently, from Eq. (i), it is quite possible to determine the molar refractivity of an unknown solute R₁ provided we know the mole fraction N₁ and N₂ and the refractivities of the solute R₂ and the homogeneous solution R_{1, 2}.

Besides, the concentration of the solute in the solution may be determined by employing the follow-ing expression, provided the refractivities of the solute, the solvent and the solution are known :

        ..........................(ii)

(c) Determination of Critical Micelle Concentration (CMC)

In general, substances that form micelles in water offer two distinct regions in their molecules : first, the hydrophobic entity (caused due to the hydrocarbon chain), and secondly, the hydrophilic entity (caused due to the polar group). It has been observed that a number of monomers usually hold all the hydrocarbon chains together specifically in the centre of the micelle which are ultimately responsible for minimising the free energy of the system. Thus, the particular concentration at which the micelles are first observed is termed as the critical micelle concentration (CMC). Interestingly, the physical characteristics of the substances forming micelles afford sharp changes at the CMC. Therefore, a plot of refractive index (RI) Vs concentration (g/L) must depict a visible change in slope at the CMC.

Example :

Determination of critical micelle concentration (CMC) of Butyric Acid by refractometry :

Butryic acid (CH₃CH₂CH₂COOH) is found to form micelles in an aqueous medium in a concentra-tion range fairly suitable for measurements with the Abbe’ refractometer. Materials Required : Butyric acid solution (25%, w/v in DW) : 200 ml ; volumetric flasks (50 ml) : 6 ;

Procedure : 

Prepare precisely 2.5, 5.0, 7.5, 10.5, 15.0 and 20.0% solutions of butyric acid in water by measuring suitable volumes (from a stock solution of 25% w/v) with the help of a burette into six 50 ml volumetric flasks, and finally making up the volume with DW. Using Abbe’ refractometer measure the refractive indices of all the above six solutions besides the stock solution (25%) at 25°C. Measure also the refractive index of DW.

Results : 

Plot a graph having the various concentrations of butyric acid along the abscissa and the refractive indices along the ordinate, whereby two straight lines are obtained intersecting at the CMC as shown in Figure 18.5 below :