Specific Rotation Calculation: Polarimeter Demystified

Step 1 of 2

(a)

Mass of sample = 1.00gm

Volume of ethanol = 20.0mL

Volume of sample = 5mL

Width of polarimeter tube  = 20.0cm = 2dm

Counterclockwise rotation \(=-1.25^{\circ}\)

Specific rotation  can be calculated as follows:-

\([\boldsymbol{\alpha}]_{\mathbf{l}}^{\mathbf{T}}=\boldsymbol{\alpha} / \mathbf{l} \mathbf{c}\)

Where:

\([\alpha]_{1}^{\mathrm{T}}=\) specific rotation in degrees. (The correct units are deg \(\mathrm{cm}^2\mathrm{\ g}^{-1}\), but are usually just given as degrees).  l is the wavelength of light used for the observation (usually 589 nm, the D line of a sodium lamp unless otherwise specified.  This wavelength is responsible for the orange-yellow color of the common sodium vapor street light). T is the temperature in \({ }^{\circ} \mathrm{C}\).

\(\alpha=\) observed rotation in degrees.

\(l=\) cell path length in decimeters. (1 decimeter = 1 dm = 10 cm. A standard polarimeter tube is 1.00 dm in length.)

\(C=\) concentration in \(\mathrm{g}\mathrm{\ ml}^{-1}\) for a pure liquid compound (i.e., the liquid's density), or \(g\ 100\mathrm{ml}^{-1}\) for a solution.

Now we have to calculate the concentration of liquid (density) = mass/volume

Substituting the values of mass and volume the

density \(=1.00\mathrm{g}/20.0\mathrm{\ mL}=0.0500\mathrm{\ g}/\mathrm{mL}\)

Thus the specific rotation:-

\(\begin{array}{c}  {[\alpha] \mathrm{D}^{25}=\frac{-1.25^{\circ} }{(0.0500)(2.00)}=-12.5^{\circ}} \end{array}\) 

Hence the specific rotation of the samples \(=-12.5^{\circ}\)