AROMATIC COMPOUNDS 273 



measuring flask and at the same time i c.c. of the standard uric acid 

 solution (below) into another 100 c.c. flask. To each are added 2 c.c. of the 

 uric acid reagent and 20 c.c. of saturated sodium carbonate solution. They are 

 allowed to stand for 2 to 3 minutes, diluted to the mark, mixed, and the colours 

 compared in a Duboscq colorimeter, the uric acid tint being placed at 20 mm. 



Adrenaline gives exactly 3 times the colour that uric acid does : the 

 readings in the colorimeter are proportional. 



The uric acid standard contains i mgm. uric acid dissolved in i c.c. of 

 4 per cent, lithium carbonate solution and is prepared by dissolving 250 

 mgm. in 25-50 c.c. of water + 25 c.c. Li 2 CO 3 solution by shaking for an 

 hour and diluting to 250 c.c. in a measuring flask. The other standard 

 solution which is permanent may also be used (see p. 557). 



TETRASUBSTITUTION PRODUCTS OF BENZENE. 



Picric acid, which is so easily formed from phenol, is trinitrophenol. 



N0 



INO a 

 OH 

 It is a tetrasubstitution product of benzene. 



Theoretically a large number of tetrasubstitution produces of ben- 

 zene are capable of existence, but the number of natural compounds 

 which are included here is very small. 

 Gallic Acid. 



Gallic acid is present in gall nuts, tea and 

 other plants. 



C \ V / OH It is prepared by the hydrolysis of tannin. 



It crystallises in silky needles which melt at 220. It is not very 

 soluble in cold water, but readily in hot water. It dissolves in alkalies 

 and the solution turns brown in the air. It resembles pyrogallol in its 

 reactions with ferric chloride, Fehling's solution, etc. 



It does not precipitate gelatin and is not precipitated by lead acetate. 

 Tannic Acid or Digallic Acid. 



This acid, the anhydride of 



gallic acid, occurs in gall nuts, 



OH sumach and other kinds of bark. 

 OH It may be prepared by heating 



gallic acid with phosphorus .oxychloride. Its constitution is not 

 definitely known and it is sometimes referred to as tannin, but the 

 synthetical product obtained from gallic acid has not the same pro- 

 perties as natural tannin which contains digallic acid in its constitution. 

 The natural product is here referred to as tannin, the synthetical as 

 digallic acid. 



Two other digallic acids are known, 



