144 THE CARBOHYDRATES 



The specific rotation of a substance is defined as the angular 

 rotation produced by a column of liquid i dm. in length, 

 which contains i gram of the active substance in each cubic 

 centimetre. This quantity is expressed by the symbol a^^, 

 the numeral indicating the temperature at which the measure- 

 ments were made, and the letter d standing for the yellow 

 line of the sodium flame which is used as the source of 

 illumination. The use of this quantity a^ for determining the 

 number of grams of active substance in a given solution will 

 be rendered apparent from the following considerations. 



Supposing we have a solution containing an unknown 



number of grams, m, of active substance per c.c, and we fill a 



tube of length / dm.* with this solution and observe its angular 



rotation to be a. 



^, , , , ... rof substance in I c.c. of "\ 



If a layer i dm. long containing i gram|jjq^j^p^^^^^^^^^^^^^.^^ j o^, 



Then „ / ,, „ „ i „ „ ,, la 



D 



/ „ ,. ,. m „ „ „ mla 



And this would be the observed angle of rotation (a). 

 .-. a = w X / X ttjj 

 a 



D 



/ X aj,- 



The angle of rotation is determined as follows : — 



1. Find the zero reading when no solution is between the 

 polarizer and analyser. For this purpose the mean of at least 

 three readings, differing by only two or three minutes, should 

 be taken. 



2. Fill the tube with the liquid, taking care to avoid the 

 introduction of air-bubbles. 



3. Insert the tube and determine the new reading at which 

 the illumination of both halves of the field is equal. The mean 

 of three readings should again be taken. 



The difference between the initial and the final readings is 

 the angle of rotation. 



The following experiment performed on a solution known 

 to contain glucose may be quoted in illustration of the 

 method : — 



* The length of the tube must be expressed in decimetres. 



