73 



To further guard against error due to the solubility of the precipitate, the alcoholic 

 solution from which the precipitation was made was saturated with freshly precipi- 

 tated ki.-ir copper I.en/oate. A stock solution of the alcohol so saturated was made 

 up and u>ed throughout the work. It was found that the results obtained by precipi- 

 tatioii in neutral solution were too high and that a slight acidity in the solution was 

 necenarj . Thi- was accomplished by adding a little acetic acid to the copper reagent. 

 Tin- .-oluiion- employed and the method of determination were as follows: 



lr acetate reagent. A mixture of 250 cc of 95 per cent alcohol and 750 cc of 

 water is .-at urated with copper benzoate and 2 cc of glacial acetic acid added, fol- 

 lowed \>y a concentrated solution of sodium benzoate in 20 per cent alcohol to form a 

 .-mall amount of permanent precipitate. This solution is filtered just before using. 



Alcohol anfiiti'iiii. \ mixture of 1 part of 95 per cent alcohol and 3 parts of water is 

 saturated with basic copper benzoate, prepared by precipitating copper acetate solu- 

 tion with sodium I.en/oate. filtering, and washing. An excess of the basic copper 

 l>en/.oat e is added to the alcohol in order that additional alcohol may be added from 

 time to time. 



/>> tn-ii> 'million. The residue obtained by the evaporation of the chloroform extract 

 is dissolved in a small amount of the alcohol saturated with copper acetate mentioned 

 above and ua-hed into a .-mall beaker with the same solution, 25 cc of the copper 

 acetate solution is added, the mixture stirred, allowed to stand for one hour, filtered 

 on a g(xx-h, and the precipitate wa.-hed with the alcohol solution saturated with basic 

 copper l.en/.'iaie The .-liiti<,n i.- then dried and weighed as usual. If preferred, 

 the re.-idiie ma\ le washed into a 100 cc flask with the alcohol solution described, 

 the copper a< ei.iie reagent added, and the mixture brought up to mark with the 

 alcohol -at united ropt.er benzoate. The whole is then mixed and, after standing one 

 hour, tilier.d, and the copper determined volumetrically by any of the standard 

 methods. 



The following table shows the result* obtained, working on a known amount of ben- 

 /oic arid in aqueous solution. As before stated above, this precipitate consists of 

 copper ! . pre-ented l.y the formula <\ } lI-< '<)() < 'u Oil. 



Benzoic acid wrr-//n*//i an aqueous solution by Method III. 



An attempt was made to apply this method to the determination of benzoic acid 

 in ketchup. A number of samples were treated as follows: 



Three hundred grams were made up to 1,500 cc with saturated salt solution, allowed 

 to stand overnight, and filtered; 500 cc of the filtrate were acidified with sulphuric 

 arid and extracted with 3 portions of chloroform of 100, 50, and 50 cc. The extract 

 . \aporated in a current of air, dried over sulphuric acid, the residue weighed, 

 taken up in neutral alcohol, and titrated with tenth-normal sodium hydrate. Ihe 

 solution was then evaporated to dryness and benzoic acid determined gravimetnc- 

 ally. as desrril>ed above, by use of the copper precipitation method. 



