In the case of ferric chlorid a one-half per cent solution should be employed. A 2 per 

 cent solution of ferric alum may be used.o In either case, and especially with ferric chlorid, 

 an excess of reagent should be avoided, although an excess of 0.5 cc of 2 per cent ferric 

 alum solution may be added to 50 cc of the solution of salicylic acid without impairing the 

 results. 



A correction should finally be made for the salicylic acid removed from the ether by the 

 water with which it is washed. This amounts to about 1.5 per cent of the total salicylic 

 acid present. If the utmost possible accuracy is desired, the method may be repeated on 

 the wash waters, thus practically eliminating the error due to the removal of salicylic acid 

 by the wash water. 



Where the nature of the substance is such that extraction by organic solvents is not prac- 

 ticable, as in the case of the presence of a large amount of fat, the salicylic acid may first 

 be separated by distillation, as described, under its qualitative estimation. In such cases it 

 is necessary to collect at least 600 cc of the distillate and continue the distillation until the 

 last 200 cc of the distillate gives no color on the addition of a drop of ferric solution. The 

 distilling apparatus should in all cases be tested with known amounts of salicylic acid in 

 order to determine the amount of distillate necessary to carry over a definite weight of 

 salicylic acid. 



It is sometimes practicable to determine the salicylic acid directly in the distillate by the 

 colorimetric method with ferric chlorid given above. If mineral acid used in the distillation 

 be carried over mechanically, however, the accuracy of the method is greatly impaired. Sali- 

 cylic acid may be recovered from the distillate, after making alkaline and evaporating 

 if desired, by extraction with ether and estimating colorimetrically as directed above. 



2. BENZOIC ACID. 



(a) QUALITATIVE DETECTION. 



Separate benzoic acid as directed for salicylic acid. If benzoic acid be present in consid- 

 erable quantity, it will crystallize from the evaporated ether in shining leaflets with charac- 

 teristic odor on heating. Dissolve the residue in hot water, divide into two portions, and 

 test by the following methods: 



(1) First method* 



Make the residue alkaline with ammonium hydroxid, expel the excess of ammonia by 

 evaporation, take up the residue with water, and add a few drops of a neutral 0.5 per cent 

 solution of ferric chlorid. The presence of benzoic acid will be indicated by the formation 

 of a brownish-colored precipitate of ferric benzoate. 



(2) Second method* 



Evaporate to dryness and treat the residue with 2 or 3 cc of strong sulphuric acid, c Heat 

 till white fumes appear; organic matter is charred and benzoic acid is converted into sulpho- 

 benzoic acid. A few crystals of potassium nitrate are then added. This causes the forma- 

 tion of metadinitrobenzoic acid. When cool, the acid is diluted with water and ammonia 

 added in excess, followed by a drop or two of ammonium sulphid. The nitrocompound 

 becomes converted into ammonium metadiamidobenzoic acid, which possesses a red color. 

 This reaction takes place immediately, and is seen at the surface of the liquid without sti-ring. 



(b) QUANTITATIVE ESTIMATION. 



Evaporate the ether extract obtained as directed under salicylic acid to dryness, thor- 

 oughly dry in sulphuric acid desiccator (preferably in vacuum) and sublime under a watch 



a This solution should be boiled until a precipitate appears, allowed to settle, and filtered. The acid- 

 ity of the solution is slightly increased in this manner, but so precipitated it keeps clear for a consider- 

 able time, and the turbidity caused by its dilution with water is much less and does not appear for a 

 much longer time than if the unboiled solution be employed. This turbidity is especially objectionable 

 in the quantitative estimation of salicylic acid, as it interferes with the exact matching of the color. 



b Moller, Bui. soc. chim. Par., 1890 (3), 3: 414. 



c If this be the only method employed, the sulphuric acid may be added directly to the residue left on 

 the evaporation of the ether. 



