BENZOIC ACTD. 207 



of permanganate solution (2 grammes KMnO^ and 4 grammes 

 KOH in 100 c.c.) are added to the solution, which is then heated 

 on the water-bath for 15 minutes ; if the colour is discharged 

 further additions of permanganate are added till the colour is 

 permanent. The excess is removed after acidifying with sul- 

 phuric acid by oxalic acid, and the solution can then be distilled 

 or extracted with a solvent to separate benzoic acid, if present. 



Benzoic acid gives the reactions below ; if salicylic acid is 

 present a little bromine water is added, and a turbidity or pre- 

 cipitate will be produced ; bromine water should be added till 

 all the salicylic acid is precipitated, and the precipitate removed 

 by filtration. The excess of bromine should be boiled off, and 

 the following tests applied : 



(a) Add a few pieces of magnesium and hydrochloric acid 

 till gas begins to be evolved ; benzoates are reduced to benzal- 

 dehyde, which has a characteristic smell. 



(6) Evaporate a little of the solution with soda-lime, and 

 ignite in a current of inert gas (nitrogen formed by passing 

 air through alkaline pyrogallol serves) ; benzoates are reduced 

 to benzene (characteristic smell), which may be collected in a 

 mixture of nitric and sulphuric acids, which form nitro -benzene 

 (another characteristic smell) ; this may be converted into 

 aniline, diazotised and condensed with /?-naphthol (red colour). 



(c) Evaporate a little of the solution to dryness, add 2 c.c. 

 of aniline and 0-02 gramme rosaniline hydrochloride, and boil 

 for twenty minutes ; a blue colour is produced if benzoates are 

 present. 



(d) Evaporate a little of the solution to dryness, add a little 

 gallic acid and 1 c.c. sulphuric acid ; if benzoates are present 

 anthragallol is produced, which, on dilution and making alkaline 

 gives a red colour passing to brown. 



(e) Jorissen tests for benzoic acid by oxidising it to salicylic 

 acid by hydrogen peroxide. To the solution to be tested (about 

 25 c.c.) 2 drops of 1 per cent, ferric chloride solution are added, 

 and if salicylic acid be absent 2 drops of a dilute solution of 

 hydrogen peroxide (one volume strength). On standing the 

 violet colour due to salicylic acid gradually appears. 



/9-naphthol is best detected by taking advantage of its easy 

 condensation with tetrazonium salts in faintly acid solution 

 to form dark red compounds. 



The author and Miller test as follows : To 1 gramme of 

 benzidine add 4 c.c. strong HC1 and about 60 or 70 c.c. of water ; 

 keep this solution cool, and add little by little 1 gramme sodium 

 nitrite dissolved in about 25 c.c. of water, cooling and shaking 

 well between each addition. When all the nitrite has been 

 added nearly neutralise, using phenol-phthalein as indicator. 



