USE OF POLARIMETER 31 



solution of caustic soda, distinctive colorations or 

 precipitates are produced. Cane sugar gives a violet 

 solution which retains its colour even on boiling, glucose 

 and fructose produce a blue coloration and a precipitate. 



The sugars also differ from each other with regard to their 

 action on heating with caustic-soda solution, the readiness with 

 which they are fermentable by yeast, and especially by their different 

 effects upon polarised light when a beam of the latter is passed 

 through an aqueous solution of the sugar (see 17). 



17. Quantitative Estimation of Sugars by Polarimeter, 



All the sugars give, on dissolving in water, solutions that are 

 optically active ; that is to say, a beam of plane polarised light on 

 being passed through such a solution has its plane of polarisation 

 rotated either to the right or to the left. 



The extent of such rotation depends upon several factors : — 

 (i.) The nature of the sugar ; (ii.) the strength of the sugar solution ; 

 (iii.) the length of the column of solution through which the light 

 is passed ; (iv.) the temperature of the solution ; (v.) the wave 

 length (colour) of the polarised light employed. 



^ ;o .^.i; d^==^ ;„^ g p ( 



€ 



Fig. 7. — Diagrammatic representation of a polarimeter. 



These factors are taken into account in calculating the specific 

 rotation of a substance, thus : — 



The specific rotation being defined as the angle of rotation 

 produced by a liquid which in the volume of i c.c. contains i gram 

 of the active substance, the length of the column through which the 

 light passes being i decimetre, we have 



100. rt! 



w = 



l.c 



where c is the number of grams of active substance in 100 c.c. 

 of the solution, / is the length of the column of liquid in deci- 



