180 METHODS OF ANALYSIS. 



by means of a centrifuge or by adding 10 or 15 cc of low boiling point gasoline 

 or petroleum ether and shaking again. 



Separate the clear, aqueous portion obtained from the emulsion and add it to 

 the first aqueous portion separated. Then pour the ether into another sepa- 

 ratory funnel, care being taken that none of the aqueous portion is left with the 

 ether. Return the aqueous portion to the separatory funnel and again extract 

 with ether, following the same procedure as before. Repeat this operation twice 

 again, four separate extractions with ether being made in all. 



In case of special difficulty in breaking up the emulsion in any of the 

 extractions a small amount of ether may be allowed to remain with the aqueous 

 portion rather than the reverse, as it is removed in sucessive extractions. Wash 

 the combined ether extracts by shaking in a separatory funnel with one-tenth 

 their volume of water (using, however, not less than 20 cc of water at each 

 washing). Care must be taken at each washing to separate the aqueous portion 

 completely from the ether, but none of the ether should be allowed to run into 

 the wash water. 



Distil slowly the greater part of the ether, transfer the remainder to a 

 porcelain dish, and allow to evaporate spontaneously. Thoroughly dry in a 

 vacuum desiccator <* over sulphuric acid, extract the dry residue with ten por- 

 tions of 10 or 15 cc each of carbon bisulphid or low boiling point petroleum 

 ether, rubbing the contents of the dish with a glass rod or other suitable instru- 

 ment and transferring the successive portions of solvent to a second porcelain 

 dish. The extracted residue should finally be tested with a drop of ferric-alum 

 solution, and if any reaction for salicylic acid be given it should be taken up in 

 water, reextracted with ether, and the operation repeated. The gasoline extract 

 is finally allowed to evaporate spontaneously. 



Dissolve the residue in a small amount of hot water and dilute to a definite 

 volume. Dilute aliquots of the solution and match, in Nessler tubes or with a 

 colorimeter, the color obtained by adding a few drops of ferric-chlorid or ferric- 

 alum solution with that of a standard solution of salicylic acid containing about 

 1 mg of salicylic acid in 50 cc. A 0.5 per cent solution of ferric chlorid should 

 be used or a 2 per cent solution of ferric alum.& 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 oc of the 

 solution of salicylic acid without impairing the results. 



Salicylic acid may often be separated from fat extracted with the ether by 

 washing the ether solution with dilute ammonium hydroxid. Then evaporate 

 the aqueous liquid almost to dryness and test with ferric solution. 



In the case of foods which yield to the gasoline solution of the ether residue 

 a color that obscures the ferric-chlorid reaction (for example, tomatoes), the 

 ether solution may be evaporated, the residue dried in a desiccator or in a cur- 

 rent of dry air, sublimed, and collected on a watch glass cooled with ice. Then 

 dissolve the sublimate in hot water and test with ferric alum. 



In examining a substance whose ether extract does not give a color or precipitate with 

 ferric solution, the drying of the residue and its extraction with gasoline may be omitted. 

 The residue may then be transferred by means of warm water directly from the distilling 

 flask to the graduated flask, in which it is made up to a definite volume. Substances 

 interfering with the ferric reaction may often be removed by precipitation with ferric 

 chlorid or lime, as directed on p. 181. 



6 This solution should be boiled until a precipitate appears, allowed to settle, and fil- 

 tered. The acidity of the solution is slightly increased in this manner, but so precipi- 

 tated it keeps clear for a considerable 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 solu- 

 tion is employed. This turbidity is especially objectionable in the quantitative estimation 

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



