196 PHYSIOLOGICAL CHEMISTRY: 



in which the alcohol facilitates the precipitation of the barium 

 chromate, while the acetate prevents in part the precipitation of 

 the calcium carbonate and in part the setting free of hydrochloric 

 acid. The potassium-bichromate solution should contain about 

 8.5 grms. potassium dichromate to the litre. Its titre must exactly 



JSJ 



correspond with an barium-chloride solution, and the procedure 



is the same as in the titration of the BaCl 2 solution obtained from 

 the contents of the stomach. A paper moistened with tetramethyl- 

 paraphenylendiamin is used as indicator; this is colored blue by a 

 bichromate in acetic-acid solution. In titrating we add chromate 

 solution as long as the barium chromate precipitated does not ap- 

 parently increase, then test with the indicator paper after each addi- 

 tion until it gives a decided blue coloration within one minute, and 

 stop adding chromate solution. As the titre of the chromate solution 



has been determined by an BaCl 2 solution, it is easy to calculate 



the quantity of HC1 in 10 cc. of the gastric juice corresponding to 

 the number of cc. of the chromate solution used. If the total 

 acidity is determined in a second portion of the gastric juice, then 

 the quantity of lactic acid or other organic acids represented as 

 HOI may be calculated. This method, which is the most exact 

 known at present, gives not only the amount of free hydrochloric 

 acid, but also the hydrochloric acid combined with the albumin 

 and peptone. 



In testing for volatile fatty acids the contents of the stomach 

 should not be directly distilled, as volatile fatty acids may be 

 formed by the decomposition of other bodies, such as albumin and 

 haemoglobin. The neutralized contents of the stomach are there- 

 fore precipitated with alcohol at ordinary temperature, filtered 

 quickly, pressed, and repeatedly extracted by alcohol. The 

 alcoholic extracts are made faintly alkaline by soda, and the alcohol 

 distilled. The residue is now acidified by sulphuric or phosphoric 

 acid and distilled. The distillate is neutralized by soda and evapo- 

 rated to dryness on the water-bath. The residue is extracted witji 

 absolute alcohol, filtered, the alcohol distilled off, and this residue 

 dissolved in a little water. This solution may be directly tested 

 for acetic acid with sulphuric acid and alcohol or with ferric chlo- 

 ride. Formic acid may be tested for by silver nitrate, which 

 quickly gives a black precipitate; and butyric acid is detected by 

 the odor after the addition of an acid. In regard to the methods 

 for more fully investigating the different volatile fatty acids, the 

 reader is referred to other text-books. 



