F W. Mar — Estimation of Barium as the Sulphate. 289 



and settles rapidly led me to investigate the conditions of so 

 rapid a precipitation. These quickly settling precipitates were 

 noticed, in the first instance, in the action of sulphuric acid 

 upon solutions containing a very large amount of potassium 

 chloride with hydrochloric acid in excess. In the course of 

 rive or ten minutes the precipitate had completely settled and 

 was found to be in a distinctly crystalline condition and much 

 coarser than the usual form of precipitated barium sulphate. 



At the time, it was thought that the cause of this rapid 

 subsidence was the alkaline salt present, and, accordingly, a 

 series of experiments was made in which potassium, sodium 

 and ammonium hydroxides were added in varying amounts to 

 about 400 cm 3 of water, hydrochloric acid added to more than 

 acidity (but not in measured amount), 0'5 grm. of barium chlor- 

 ide introduced, and precipitation brought about by adding 

 dilute sulphuric acid. Sometimes these precipitates settled 

 rapidly, but as often cauie down in the familiar milky condi- 

 tion. Later, another series of experiments, in which the dif- 

 ferent conditions were more carefully regulated, was made 

 thus : in 400 cm 3 of water were dissolved 0*5 grm. of barium 

 chloride, 10 cm 3 of strong hydrochloric acid, and amounts of 

 the alkaline chlorides varying from 5 grms. to 0*05 grm., the 

 whole being precipitated with 10 cm 3 of a solution of sulphuric 

 acid made by diluting the concentrated acid with three parts 

 of water. These precipitates all settled rapidly, and the varia- 

 tion in the amounts of alkali seemed to exert no very marked 

 infln ence. Finally, these experiments seeming to point to 

 hydrochloric acid as the influential factor, a series of experi- 

 ments was made to test the effect of varying the amount of 

 this acid. From a solution containing in 400 cm 3 0'5 grm. of 

 barium chloride and amounts of hydrochloric acid varying 

 from 1 cm 3 to 50 cm 3 , the barium was thrown out by means of 

 10 cm 3 of dilute sulphuric acid. This series showed that the 

 hydrochloric acid had a very marked effect upon the precipi- 

 tation of the barium sulphate. When only one or two cubic 

 centimeters of hydrochloric acid were present, the precipitate 

 appeared immediately, in a milky condition, and settled slowly ; 

 as the amount of acid was increased, a point was soon reached 

 where the precipitate was not so quickly apparent, but settled 

 out much more quickly and in a coarser condition. With 

 10 cm 3 to 15 cm 3 of strong hydrochloric acid in the solution, 

 the precipitate settled clear in ten or twelve minutes and was 

 in excellent condition for filtration. When the solution con- 

 tained 50 cm 3 of the acid, the precipitate settled clear in five 

 minutes. Upon adding the sulphuric acid to such very acid 

 solutions, no precipitate shows for a moment, but then it separ- 

 ates in beautiful crystalline condition and falls almost immedi- 



