F. W. Mar — Estimation of Barium as the Sulphate. 293 



highly advantageous to have the solution strongly acid with 

 hydrochloric acid ; that it is not necessary to add the reagents 

 drop by drop, but that the whole quantity required to complete 

 the reaction may be added at once ; that ordinary quantities of 

 barium salts, in presence of a considerable excess of sulphuric 

 and hydrochloric acids, are precipitated completely and at 

 once, but that when only a few milligrams are present, the 

 precipitate requires more time to separate uuder the same con- 

 ditions. Two or three hours are, however, sufficient, and in 

 no case is the excessive time of twelve hours required. 



In the light of the fact demonstrated in the preceding 

 account, that hydrochloric acid may be introduced freely, and 

 without detriment to the quantitative exactness of the precipi- 

 tation of barium in the form of sulphate from pure solutions, 

 it seemed desirable to look somewhat into the question as to 

 what the influence of a large excess of hydrochloric acid might 

 be upon the well known contaminating effect of alkaline salts 

 present during precipitation, especially as it is customary to 

 attempt the purification of barium sulphate thrown down in 

 the reverse of this process — the determination of sulphuric 

 acid by means of a soluble barium salt — by digestion of the 

 washed precipitate in hydrochloric acid. The following series 

 of experiments was undertaken with this end in view. The 

 details are shown in the tabular statement, precipitation being 

 effected in the presence of free acid and the alkaline salt. 



BaCl 2 . 2H 2 



BaS0 4 





HC1 in 



Alkaline Salts 



taken. 



found. 



Error. 



solution. 



present. 



(23) 0-5092 grra 



. 0-5032 grin. 



0-0169 grm 



. + 110 cm 3 



KC10.3 grm 



(24) 0*5027^" 



0-4907 " 



0'0107 " 



+ 10 " 



a a 



(25) 0-5026 " 



0-4944 " 



0-0154 " 



+ 100 " 



KC1 5 grm 



(26) 0-5045 " 



0-4939 " 



0-0122 " 



+ 10 " 



a a 



(27) 0-5020 " 



0-4931 " 



0-0137 " 



+ 10 " 



u a 



(28) 0-5013 " 



0-4849 " 



0-0061 " 



+ 10 " 



NaCl 



From the results it is plain that, whatever may be the effect 

 of digesting the washed precipitate in hydrochloric acid, the 

 presence of this acid in large excess during precipitation in the 

 presence of alkaline salts, does not prevent contamination of 

 the precipitate. On the contrary, the greatest contamination 

 seems to have occurred in those cases in which the acid was 

 present to the largest degree, but, in view of the slight varia- 

 tion in contamination as compared with the great differences 

 in the amount of acid employed, it d»es not appear probable 

 that the increase of acid has very much to do with the amount 

 of contamination. 



It likewise seemed to be a matter of some interest in this 

 connection, to investigate the process by which it is currently 



