48 PRACTICAL ORGANIC AND BIO-CHEMISTRY 



8. Behaviour towards Reagents. 



(a) Reaction of aqueous solution to litmus. 



A marked acid reaction indicates an acid or a phenol ; if there is an 

 odour, it may be a volatile fatty acid or a phenol. 



A neutral reaction indicates a salt of an acid or a base ; an al- 

 cohol, aldehyde, ketone (note smell), carbohydrate. An ester 

 in alcoholic or ethereal solution has also a neutral reaction. 



An alkaline reaction indicates a base, or an acid dissolved in 

 excess of alkali. 



(b) Sodium carbonate: acids insoluble in water, e.g. uric acid, also 



cystine, tyrosine dissolve ; bases insoluble in water do not 

 dissolve or if in solution are precipitated. 



(c) Sodium hydroxide: ammonium salts are decomposed with 



evolution of ammonia and bases are liberated from their salts. 

 On boiling, amides are decomposed, and esters are hydrolysed. 



(d) Hydrochloric acid: acids insoluble in water do not dissolve or 



if in solution are precipitated ; bases insoluble in water dis- 

 solve, e.g. tyrosine, cystine, aniline. 



(e) Sulphuric acid. 

 (/) Nitric acid. 



(g) Bromine water. 

 \K) Permanganate. 



(i) Effect of heating with soda lime. 

 The exactly neutral solution may be tested with 

 (/) Schiffs reagent for aldehydes. 



(k) Ammoniacal silver nitrate for aldehydes, reducing carbo- 

 hydrates, etc. 



(/) Fehlings solution for aldehydes, reducing carbohydrates, etc. 

 (m) Ferric chloride for phenols (violet or green colour), 

 for acetoacetic acid (claret colour), 

 for formates, acetates (reddish-brown colour, 



precipitate on boiling), 

 for lactates, oxalates (yellow colour). 

 (n) Calcium chloride for oxalates, urates, etc. (insoluble calcium 



salts are precipitated). 



(o) Sodium nitroprusside and sodium hydroxide : 

 Acetone gives a red colour, changing to purple with acetic acid. 

 Creatinine ,, a red ,, ,, yellow ,, ,, 



Indole ,, a blue-violet ,, ,, blue ,, ,, 



Confirmatory tests must be made after an indication of the nature 

 of the substance has been obtained, according to the reactions given 

 under the various groups of compounds. 



