200 COMPOSITION OF THE DIGESTIVE JUICES. [CH. VIII. 



down the funnel with a little water and remove it. To the flask 

 add 20 cc. of the standard silver nitrate, measuring this by means 

 of a pipette. Then add 5 cc. of the iron alum, and make the volume 

 up to the mark with distilled water. Mix thoroughly and filter 

 through a dry paper into a clean, dry, 100 cc. measuring cylinder. 

 Collect at least 90 cc. of the fluid and note its exact volume. Trans- 

 fer it to a beaker or flask, wash out the cylinder with a little water 

 and titrate with the thiocyanate from a burette until a faint but 

 permanent pink colour is obtained. 



Calculation. Suppose that 92 cc. of the filtrate were taken, 

 and that this required 11*4 cc. of thiocyanate. 



100 



Then 100 cc. of the filtrate would require 11-4 x =12-4 cc. 



92 



So the chlorides have precipitated 20 - 12-4 = 7-6 cc. of the 



standard AgNO 3 . 



So total chlorides in 10 cc. = 7-6 x 0-00365 gram. HC1. 

 So total chlorides in 100 cc. = 0-277 gram. HC1. 



245. Mineral Chlorides. Repeat the above exercise in every 

 particular, except that the sodium carbonate is not added. The 

 calculation is made in the same way as in the above case. 



Active HC1 is the difference between these two results. It 

 should be compared with the result of Ex. 243. 



246. Free Hydrochloric acid. To 10 cc. of the filtered fluid 

 in a small beaker add i cc. of o-i N. sodium hydroxide from a 

 burette. Stir well and place a drop of the mixture, together with a 

 drop of freshly prepared Gunzberg's reagent (see p. 390) , into a white 

 evaporating basin. Heat on a boiling water bath. If free HC1 is 

 still present the film will develop a brilliant carmine tinge as it dries. 

 In that case add another cc. of the NaOH, and repeat the evaporation 

 with another drop of the reagent. Proceed in this way, adding i cc. 

 of soda at a time until a negative test is obtained. Then add 0-5 

 or less of o-i N. HC1 from a burette and repeat the test. If it is 

 now positive add o-i or 0-2 cc. of soda and repeat again. It is 

 necessary to determine the amount of soda and HC1 that must be 



