308 METHODS OF ANALYSIS [Chap. 



25 DISTILLATION AND TITRATION. 



Dissolve the soap, obtained as directed under 24, by adding 135 cc. of recently 

 boiled water (132 cc. if potassium hydroxid was used in the saponification) and warm 

 on the water bath, with occasional shaking, until the solution is clear. Cool to 

 60°-70°C., add 5 cc. of the dilute sulphuric acid (8 cc. if potassium hydroxid was 

 used in the saponification), stopper loosely and heat on the water bath until the 

 fatty acids form a clear, transparent layer. Cool to room temperature, add a few 

 pieces of the pumice stone and connect with a glass condenser by means of a bulb 

 tube. Heat slowly with a free flame until ebuUition begins and distil, regulating 

 the {lame so as to collect 110 cc. of distillate in as nearly 30 minutes as possible. 

 Mix this distillate, filter through a dry filter, and titrate 100 cc. with the standard 

 barium or sodium hydroxid solution, using phenolphthalein as an indicator. The 

 red color should remain unchanged for 2-3 minutes. 



Multiply the number of cc. of N/10 alkali used by 1.1, divide by the weight 

 of fat taken and multiply by 5 to obtain the Reichert-Meissl number. Correct the 

 result by the figure obtained in a blank determination. 



Leffman and Beam Method. — Official. 



26 REAGENTS. 



Glycerol-soda solution. — Add 20 cc. of the sodium hydroxid solution, prepared as 

 directed under 23 (a), to 180 cc. of pure, concentrated glycerol. 

 The other reagents used are described under 23. 



27 DETERMINATION. 



Add 20 cc. of the glycerol-soda solution to about 5 grams of the fat in a flask, 

 weighed as directed under 24, and heat over a free flame, or on an asbestos plate, 

 until complete saponification takes place, as shown by the mixture becoming 

 perfectly clear. If foaming occurs, shake the flask gently. 



Add 135 cc. of recently boiled water, drop by drop at first, to prevent foaming, 

 and 5 cc. of the dilute sulphuric acid, distil without previous melting of the fatty 

 acids, using an apparatus similar to that illustrated in 28, Fig. 13, regulating the 

 flame so as to collect 110 cc. of distillate in as nearly 30 minutes as possible. Filter, 

 titrate the volatile acids and calculate the Reichert-Meissl number, as directed 

 under 25. Conduct a blank determination and correct the result accordingly. 



INSOLUBLE VOLATILE ACIDS (POLENSKE NUMBER). 



28 Polenske Method^. — Tentative. 



Proceed as directed under 27 up to the point at which 110 cc. of distillate have 

 been collected, except that only 20 minutes are allowed for the distillation, em- 

 ploying an apparatus of the exact dimensions illustrated in Fig. 13. Substitute a 

 25 cc. cylinder for the receiving fla^sk to collect any drops that may fall after the 

 flame has been removed. Immerse the flask containing the distillate almost com- 

 pletely in water at 15°C. for 15 minutes, filter the UOcc. of distillate and determine 

 the approximate Reichert-Meissl number, if desired, as in 27, avoiding too violent 

 shaking of the distillate and consequent emulsification of the insoluble acids pre- 

 vious to filtration. Remove the remainder of the soluble acids from the insoluble 

 acids upon the filter paper by washing with 3 successive portions of 15 cc. of water, 

 previously passed through the condenser, the 25 cc. cylinder and the 110 cc. receiv- 

 ing flask. Then dissolve the insoluble acids by passing 3 successive 15 cc. por- 



