DETERMINATION OF HYDROCHLORIC ACID. 239 



absolute alcohol and once with ether, let the excess of ether 

 evaporate, place the filter together with the barium sulphate 

 in a weighed platinum crucible, heat, at first gently and then 

 more strongly, with the crucible half open, until all the car- 

 bon is burned, and weigh when cold (see page 206). One 

 molecule of barium sulphate, BaS0 4 , corresponds to two 

 molecules of hydrochloric acid, HC1; 233.46 parts by weight 

 correspond to 72.92 parts of hydrochloric acid, HC1. The 

 entire determination should be made in duplicate as a check. 

 Instead of precipitating the barium as sulphate and 

 weighing we may also proceed as follows : Add to the aqueous 

 solution of barium chloride obtained ammonia and ammonium 

 carbonate, filter off the precipitated barium carbonate, wash 

 thoroughly, and dissolve in dilute hydrochloric acid. For 

 this purpose it is best to wash the barium carbonate into a 

 beaker, dissolve it in dilute hydrochloric acid, filter the solu- 

 tion through the filter which was used to collect the barium 

 carbonate, and wash. The solution is evaporated to com- 

 plete dryness on the water-bath to remove any hydrochloric 

 acid still present, a few cubic centimeters of water are added, 

 and the solution is again evaporated to dryness. The residue 

 is dissolved in water and titrated with a dilute solution of 

 silver nitrate of known strength, after the addition of suffi- 

 cient potassium chromate solution. 1 A solution of silver 

 nitrate containing 2.9054 g. of AgN0 3 to the liter, 1 cc. of 

 which equals 0.001 g. NaCl, should be used. If this silver 

 solution is used the percentage amount of HC1 in the gastric 



nX 3.646 

 juice may be calculated from the formula x = ~ , in 



which n indicates the number of cubic centimeters of the sil- 

 ver solution used for 10 cc. of the gastric juice. 



1 The amount of the potassium chromate solution added must not be 

 too small, as the potassium chromate reacts with the barium chloride to 

 form insoluble barium chromate and potassium chloride. 



