XXIII] FATS AND OILS 307 



22 INSOLUBLE ACIDS (HEHNER NUMBER).-OFFICIAL. 



Allow the flask, containing the cake of insoluble fatty acids from 21 and the 

 paper throu<fh which the soluble fatty acids have been filtered, to drain and dry 

 for 12 hours. Transfer the cake, together with as much of the fatty acids as can 

 be removed from the filter paper, to a weighed, wide-mouthed beaker flask. Then 

 place the funnel, containing the filter, in the neck of the flask and wash the paper 

 thoroughly with hot absolute alcohol. Remove the funnel, evaporate off the alco- 

 hol, dry for 2 hours at 100°C., cool in a desiccator and weigh. Again dry for 2 hours, 

 cool and weigh. If there is any considerable decrease in weight, re-heat for 2 hours 

 and weigh again. Calculate the percentage of insoluble fatty acids. 



SOLUBLE VOLATILE ACIDS (REICHERT-MEISSL NUMBER). 



(As these determinations are entirely empirical, the directions given must be 

 followed exactly. In reporting results the method used should always be stated.) 



Reicheri-Meissl Method. — Official. 



23 REAGENTS. 



(a) Sodium hydroxid solution (1 to 1). — The sodiimi hydroxid should be as free as 

 possible from carbonates. Protect the solution from contact with carbon dioxid. 

 Allow to settle and use only the clear liquid. 



(b) Potassium hydroxid solution. — Dissolve 100 grams of the purest potassium 

 hydroxid in oS cc. of hot water. Cool in a stoppered vessel, decant the clear solu- 

 tion and protect from contact with carbon dioxid. 



(C) 55% alcohol by volume. — Distilled over sodium hydroxid. 



(d) Dilute sulphuric acid. — Dilute 200 cc. of the strongest acid to 1 liter with 

 water. 



(e) Barium {or sodium) hydroxid solution. — Standardize an approximately N/10 

 solution. 



(f ) Indicator. — Dissolve 1 gram of phenolphthalein in 100 cc. of 95% alcohol. 



(^) Pumice stone. — Heat small pieces to a white heat, plunge in water, and keep 

 under water until used. 



24 SAPONIFICATION. 



Weigh 5.75 cc, about 5 grams, of the filtered sample, into a saponification flask 

 and proceed by 1 of the following 3 methods. 



(1) Under pressure with alcohol. — Place 10 cc. of the 95% alcohol in the flask 

 containing the fat (the flask must be made of strong, well-annealed glass, capable 

 of resisting the tension of alcoholic vapor at 100°C.) and add 2 cc. of the sodium 

 hydroxid solution. Insert a soft, cork stopper into the neck of the flask, tie down 

 and place the stoppered flask on a water or steam bath for at least an hour, 

 rotating the flask gently from time to time. Cool before opening. 



(2) Under pressure without the use of alcohol. — Place 2 cc. of the potassium hy- 

 droxid solution in the flask containing the fat (the flask being round-bottomed and 

 made of well-annealed glass to resist pressure), cork and heat as directed under 

 (1). This method avoids the formation of esters and the removal of the alcohol 

 after saponification. 



(3) With a reflux condenser and the use of alcohol. — Place 10 cc. of the 95% alcohol 

 in the flask containing the fat, add 2 cc. of the sodium hydroxid solution and heat on 

 a steam bath until the saponification is complete, using a reflux condenser. 



After the saponification, if alcohol was used, remove it by evaporation on a 

 steam bath. 



