R. W. Langley — Determination of Barium in Hocks. 123 



Art. XIY. — The Determination of Small Amounts of 

 Barium in Rocks ; by Ralph W. Langley. 



In the usual methods of rock analysis, barium is deter- 

 mined in a separate sample, and no correction is made for its 

 possible interference in the main analysis. I wish to show in 

 this article that it is possible to determine barium as sulphate 

 in the main analysis for silica and bases, by precipitating with 

 sulphuric acid immediately after the separation of silica. 

 The details of this method are as follows : 



The filtrate from the silica is diluted to a volume of about 

 4:00 cm3 , and ammonium hydroxide added until most of the 

 hydrochloric acid is neutralized. The solution is then heated 

 to boiling, and about 2 cm3 of 25 per cent sulphuric acid is 

 added and the solution allowed to stand for about ten hours. 

 Barium sulphate separates and is filtered on a small filter, the 

 filter paper burned in a platinum crucible, and the residue 

 dissolved in about 5 cm3 of concentrated sulphuric acid by 

 warming over a free flame. As soon as all or nearly all of 

 the barium sulphate has dissolved, the solution is cooled and 

 poured into about 100 cm3 of water. When the barium sul- 

 phate has precipitated, it is filtered and weighed as usual. 

 It is best to allow the solution to stand for ten hours before 

 filtering. The second filtrate from barium sulphate usually 

 contains iron and should be added to the first filtrate. From 

 this point the subsequent determinations may be made as 

 usual. 



To test the method, an analysis was first made on a gabbro 

 rock free from barium. The methods used were those of the 

 geological survey. The results are the average of two or 

 more determinations. One gram of material was used for 

 each analysis. The high figure for titanium oxide led to its 

 repeated determination, using two standard solutions, one pre- 

 pared from titanium oxide and one from a crystal of rutile. 

 The results are as follows : 



Si0 o 50-75 



H o 0-56 



FeO 8-13 



Fe o 3 2-08 



TiO„ 3-05 



A1„0, .-.. 17-31 



Cat) _ 6j77 



MgO 3-48 



K o ' 2-87 



Na 2 4-14 



.99-14 



