DETECTION OF BENZOIC ACID. 1167 



violent action is over, and greenish vapors have ceased to come off. Then cover 

 with a watch glass and boil rapidly to complete dryriess, but avoid overheating 

 and baking. A drop of nitric acid adhering to the cover does no harm. Cool suf- 

 ficiently and add 7 grams of ammonium chlorid, 15 cc of strong ammonia and 25 

 cc of hot water. Boil the covered mixture one minute, and then, with a rubber- 

 tipped glass rod, see that all solid matter on the cover, sides and bottom is either 

 dissolved or disintegrated. Filter into a beaker and wash several times with the 

 hot ammonium chlorid solution. A blue-colored nitrate indicates copper. In that 

 case add 25 cc of strong pure hydrochloric acid, and about 40 grams of granulated 

 test lead. Stir the lead about in the beaker till the liquid has become perfectly 

 colorless, and then a little longer to make sure that the copper is all precipitated. 

 The solution, which should be still quite hot, is now ready for titration. In the ab- 

 sence of copper the lead is omitted and only the acid added. About one-third of 

 the solution is now set aside and the main portion is titrated rapidly with the ferro- 

 cyanid till the end point is passed, using the uranium indicator as in the standardi- 

 zation. The greater portion of the reserve portion is now added, and the titration 

 continued with more caution till the end point is again passed. Then add the re- 

 mainder of the reserved portion and finish the titration carefully, ordinarily by 

 additions of two drops of ferrocyanid at a time. Make corrections of the final 

 reading of the burette precisely as in the standardization. 



Gold, silver, lead, copper, iron, manganese, and the ordinary constituents of ores, 

 do not interfere with the above scheme. Cadmium behaves like zinc. 



DETECTION OF BENZOIC ACID. 1 



Extract the substance with ether, evaporate off the ether from the resulting ex- 

 tract, and treat the residue with two or three cc of strong sulphuric acid. Heat till 

 white fumes appear. Organic matter is charred and benzoic acid is converted into 

 sulphobenzoic acid. A few crystals of potassium nitrate are then added. This 

 causes the formation of metadinitrobenzoic acid. When cool the acid is poured into 

 water and ammonia added in excess, followed by a drop or two of ammonium sul- 

 phid. The nitro compound becomes converted into ammonium metadiamidobenzo- 

 ate, which possesses a peculiar reddish brown color. The benzoic acid must first be 

 separated in a state of approximate purity before this test can be applied. Half a 

 milligram of the acid can be detected, in the absence of interfering bodies. 



1 Mohler, Bull, eoc. chim., (3), 3, 414. 



