SOLUBILITIES 79 



2. Sparingly soluble in cold water but more so in hot, and 

 precipitated from solution by alcohol, e.g. gums, mucilages, 

 starch, and pectins. 



3. Insoluble in water but soluble in dilute caustic alkali, 

 and precipitated from solution by the addition of acid or 

 alcohol, e.g. hemicelluloses. 



4. Insoluble in water, alkali, and organic solvents, but 

 soluble in cuprammonia, e.g. cellulose. 



GENERAL TEST FOR CARBOHYDRATES AND THEIR 



DERIVATIVES. 



In attempting to characterize an unknown organic sub- 

 stance, there is one test which should always be employed 

 at the outset, and that is Molisch's reaction. This test is 

 extremely delicate, and may be applied to a substance in 

 aqueous solution or, if the substance is insoluble in water, to a 

 little of the liquid obtained by boiling the solid with dilute 

 sulphuric acid. By this treatment the substance, if it contains 

 a carbohydrate, will be hydrolysed and then will yield suffi- 

 cient monosaccharide to give the test which is carried out as 

 follows : — 



A few drops of 15 per cent alcoholic solution of a-naphthol 

 are added to about a third of a test tube full of the solution to 

 be tested and concentrated sulphuric acid is carefully poured 

 down the side of the tube. At the junction of the two 

 liquids a green ring is produced * and over this a red zone ; 

 on gently agitating the colour changes to purple. 



Alternatively, 1-2 drops of the solution are mixed with 

 about 4 drops of a 4 per cent alcoholic solution of a-naphthol ; 

 about I c.c. of concentrated sulphuric acid is then added and 

 the whole is gently agitated. A purple colour indicates the 

 presence of carbohydrate. 



The reaction depends upon the production of furfural, by 

 the action of the sulphuric acid on the carbohydrate, and its 

 condensation with the a-naphthol. 



This reaction is given by all true carbohydrates and all 



* No attention should be paid to the production of a green colour, 

 which is given by the action of sulphuric acid on alcoholic a-naphthol, 

 even in the absence of carbohydrate. 



