120 PRACTICAL ORGANIC AND BIO-CHEMISTRY 



(1) Gerhard? s Ferric Chloride Test. Dilute ferric chloride solution, 

 added drop by drop, to about 10 c.c. of the solution gives a claret- 

 red colour. 



The delicacy of this reaction is very nearly i in 100,000. 



In applying this test to urine a precipitate of ferric phosphate is 

 formed. Ferric chloride is added so long as a precipitate is produced 

 and the ferric phosphate filtered off. On adding ferric chloride to the 

 filtrate the claret colour appears if aceto-acetic acid be present. 



The delicacy of the reaction with urine is less than given above 

 on account of the presence of other substances which give a red 

 colour with the iron salt 



There is also the disadvantage in testing urine that aromatic 

 compounds salicylates, antipyrine which are excreted after their 

 administration, also give a violet colour with ferric chloride. 



To avoid confusion the urine is shaken with benzene or chloroform 

 which removes salicylic acid. The urine is then acidified with sul- 

 phuric acid and shaken with ether which extracts the aceto-acetic 

 acid. The ethereal solution on shaking with dilute ferric chloride 

 solution will give the claret colour. The colour disappears on stand- 

 ing for 12-24 hours. Thiocyanic acid is also extracted by ether, 

 but the colour of ferric thiocyanate is permanent. 



The statement that the colour if produced by aceto-acetic acid 

 disappears on boiling is, according to Kennaway, not exactly true. 

 The colour in either case becomes paler and redder, and in the case of 

 aceto-acetic acid a reddish flocculent precipitate appears on boiling. 



(2) LegaFs Sodium Nitroprusside Test. On adding 3 drops of 

 a freshly prepared 5 per cent, solution of sodium nitroprusside to 

 about 10 c.c. of the solution and rendering alkaline with a few drops 

 of caustic soda, a deep red colour is formed. The colour changes 

 to magenta on acidifying with acetic acid. 



When applied to urine it should be remembered that creatinine 

 gives a similar colour reaction (p. 171 ). 



(3) Rothera's Sodium Nitroprusside Test. 10 c.c. of the solution 

 are saturated with ammonium sulphate by adding 5 gm. of the 

 crystals ; 3 drops of 5 per cent, sodium nitroprusside and 2 c.c. 

 of strong ammonia are then added. A fine permanganate colour is 

 produced. 



i part of aceto-acetic acid in 100,000 gives the reaction in 2 

 minutes, I part in 400,000 in 5 minutes. 



This reaction was described as characteristic for acetone but was 

 shown by Hurtley to be far more delicate for aceto-acetic acid. 



