THE ACTION OF AMYLASE ON STARCH 113 



too delicate, and it is recommended to prepare it the day 

 before it is required for use, as it gives the best results after 

 the second decoloration. 



The titration is carried out as follows : 



The Fehling solution must first be standardised by 

 taking, say, 1 gram of pure dextrose, and dissolving in 200 c.c. 

 of distilled water. 



Five c.c. of the Fehling solution are accurately measured 

 into a 150 c.c. boiling flask, and raised to boiling over a 

 small naked Bunsen flame. The sugar solution obtained as 

 above is added from a burette in small quantities at first 

 of about 5 c.c., the mixture being kept rotated and boiled after 

 each addition until reduction of the copper is complete, 

 which is ascertained by rapidly withdrawing a drop of the 

 liquid by a glass rod, and bringing it at once into contact 

 with a drop of the indicator on a porcelain or opal slab. 



The reduction is complete as soon as no red coloration, 

 due to the formation of ferric thiocyanate, is produced. 



Having once standardised the Fehling solution, the 

 amount of reducing sugar present in any given solution can 

 be simply determined, care being taken in the case of a 

 solution containing an active enzyme to stop the action of 

 the latter by addition of caustic soda (say 10 c.c. caustic 

 soda to 100 c.c. of the solution), so that alteration in the 

 composition of the solution may not take place in the course 

 of the titration operations. 



It is possible in this way to follow the course of change, 

 say when starch solution is acted upon by amylase, by with- 

 drawing portions of the solution from time to time, stopping 

 the reaction with caustic soda and titrating as above. 



The presence of maltose in the products of the action of 

 amylase on starch can be demonstrated by the preparation 

 of an osazone, by the reaction described on p. 87. 



In order to prepare an osazone, to O'Ol gram of the sugar 

 in about half a test-tube full of water, O'l gram of phenyl 



