112 BACTERIOLOGICAL AND ENZYME CHEMISTRY 



the amount of copper oxide reducing sugar present in any 

 solution. 



Briefly, tlie process consists In adding successive small 

 measured quantities of tlie sugar solution to a given volume 

 of Fehling solution tiU complete reduction takes place, the 

 end of the reaction being determined by means of a special 

 indicator. 



The Fehling solution is prepared as follows : — 



(a) Copper Solution. — RecrystaUised copper sulphate (69'2 

 grams) is dissolved in water and the solution made up to 

 one litre at 60° F. with distilled water. 



(b) Alkaline Tartrate Solution. — Rochelle salt, i.e., sodium 

 potassium tartrate (346 grams) and caustic soda (130 grams) 

 are dissolved in about 600 c.c. of distilled water, the solution 

 cooled and made up to one litre at 60° F. with distilled 

 water. 



The two solutions are to be kept separate, and equal 

 volumes mixed for each day's work, from which mixture the 

 volumes specified in the analytical method are measured out 

 at 60° F. 



Preparation of the Indicator. — One gram of ferrous 

 ammonium sulphate and the same quantity of ammonium 

 thiocyanate are dissolved in 10 c.c. of water at a moderate 

 temperature, say 120° F., and immediately cooled ; 5 c.c. of 

 concentrated hydrochloric acid are then added. The solution 

 so obtained has invariably a brownish-red colour, due to the 

 presence of ferric salt, which latter must be reduced. For 

 this purpose zinc dust is the most satisfactory reagent to 

 employ, and a mere trace is sufficient to decolourise the 

 solution if pure reagents have been employed. 



When kept for some hours, the indicator develops the 

 red coloration by atmospheric oxidation. It may, however, 

 be decolourised by the addition of a further quantity of zinc 

 dust, but its delicacy is decreased after it has been decolourised 

 several times. For practical purposes the indicator may be 



