F. W. Mar — Estimation of Barium as the Sulphate. 295 



salt, as is well known, is deposited in large crystals, which can 

 be filtered off as readily as sand. The following series of ex- 

 periments show the result of an attempt to utilize this property 

 of comparatively slow and large crystallization in purifying 

 the precipitate Solution of the precipitate was effected as in 

 the experiments described above, and the evaporation of the 

 acid was effected over a matting of asbestos, or by means of a 

 ring burner, in porcelain, which is preferable to platinum when 

 the evaporation is carried on as slowly as is necessary. After 

 the acid was completely evaporated, the crystals were washed 

 upon a felt of asbestos, ignited and weighed. Five grams of 

 potassium chloride and 10 cm 3 of hydrochloric acid were added 

 to the solution of barium chloride in each case. 



BaCl 2 . 2H 2 0taken. 



BaS0 4 found. 



Error. 



(35) 0-5029 grm. 



0-4796 grm. 



0-0006 grm.— 



(36) 0-5008 " 



0-4783 " 



0-0001 " + 



(37) 0-5038 " 



0-4810 " 



0-0001 " — 



(38) 0-5087 " 



4861 " 



0-0003 " + 



(39) 0-5025 " 



0-4795 " 



0*0006 " + 



These results are plainly good and satisfactory so far as con- 

 cerns the purification of the salt, but, when the evaporation is 

 conducted in the manner described, several hours are needed 

 for the evaporation, and great care must be exercised to obviate 

 the danger of snapping which becomes manifest in the later 

 stage of the evaporation. 



By the aid, however, of a Hempel evaporating burner* the 

 operation can be finished safely, and with but little care, in the 

 course of half an hour. The following determinations were 

 made exactly like those of the last series, with the exception 

 that the evaporation was effected by means of the Hempel 

 apparatus. 



BaCl 2 . 2H 2 taken. 



BaS0 4 found. 



Error. 



(40) 0-5050 grm. 



(41) 0-5069 " 



(42) 0-5041 " 



(43) 0-5021 " 



(44) 0-4033 " 



0-4824 grm. 

 0-4838 " 

 0-4825 " 

 0-4812 " 

 0-4801 " 



0*0002 grm. + 



o-oooo " 



0-0021 " + 

 0-0018 " + 

 0*0005 " - 



Though not an absolutely perfect process, the purification of 

 barium sulphate by this method of solution and evaporation is 

 evidently better, by far, than the old method of solution and 

 reprecipitation by dilution. 



The writer gratefully acknowledges his indebtedness to 

 Prof. F. A. (looch for many helpful criticisms and suggestions 

 during the course of this investigation. 



*Ber. d. deutsch. cliem. Ges., xxi, p. 900. 



