THE POLARIMETER 95 



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. 



Ir . , . . f of substance in i c.c. of ) 



If a layer i dm. long containing i gram { ]iquid produces a rotation ):> 



Then t, i lao 



' > * !> if u "* )> tnlaD 



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

 .: a = m x I x OD 

 a 



1 x aa 



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 : 



Initial reading of polariscope, without any solution = o 30' 

 Final with glucose = 3 45' 



Difference (a) = 3 15' or 3'25* 



Length of tube containing the solution (/) = 2 dms. 



Specific rotation of glucose ( 2 ) = 52'5 



From which m = 55 = -0309 



2 x 52-5 



.. the strength of the solution is 3-09 per cent. 

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



