28 PHYSIOLOGICAL CHEMISTRY. 



boil five to ten minutes. Now allow the liquid to cool, remove 5 cc. with 

 a pipette, dilute this to 25 cc. with water and add a few drops of iodine 

 solution; a blue violet color results, showing that starch or a starch-like 

 substance is still present. The remainder of the acid liquid in the flask 

 is next boiled steadily for one hour, a little water being added from time 

 to time to replace that lost by evaporation. At the end of an hour remove 

 5 cc., dilute and test with iodine solution as before. The characteristic 

 starch reaction is now absent, while the liquid has become thin and trans- 

 parent. 



Neutralize the free sulphuric acid by addition of a slight excess of chalk 

 or fine marble dust, heat gently to complete the reaction. Then filter and 

 evaporate the filtrate nearly to dryness on a water-bath. Allow to cool 

 and notice that the residue has a sweet taste. It is, in fact, glucose and 

 the experiment illustrates the method of manufacturing glucose on the 

 large scale. Test it by dissolving a little in water and adding a few drops 

 of Fehling's solution, described below. On boiling, a yellowish precipitate 

 appears which becomes bright yellow and finally red. 



Under certain conditions it is possible to obtain a pure 

 crystalline product from the syrup made as just illustrated. 

 This is known as crystallized grape sugar or pure anhydrous 

 dextrose. The common commercial product, sold as glucose 

 syrup, often contains much unconverted dextrin from the 

 incomplete hydrolysis of the starch. At the present time large 

 quantities of glucose, both solid and liquid, are made and used 

 in the fermentation industries, by bakers and confectioners 

 and in the household. Glucose is sweet, but not as sweet as 

 cane sugar, and, because of the fact that it readily undergoes 

 fermentation, it can not replace cane sugar for certain pur- 

 poses, such as the preparation of the syrups of the pharma- 

 copoeia or the canning or preserving of fruits. 



The typical aldose reactions are well shown with a solution 

 of glucose. For the first of these we require a reagent, re- 

 ferred to above as Fehling's solution, which is made as 

 follows : 



Fehling's Solution. Ex. Dissolve 69.28 gm. of pure crystallized copper 

 sulphate in distilled water and dilute to make a liter of solution. In a 

 second portion of distilled water dissolve 100 gm. of pure solid sodium 

 hydroxide and 350 gm. of pure recrystallized Rochelle salt by aid of heat. 

 Cool and dilute to make one liter. These solutions when mixed yield the 

 Fehling's solution proper. It is best to mix equal volumes, quite accurately- 

 measured, just before the reagent is needed for use. 



