24 Organic Acids in the Examination of Minerals. 



Of the eighteen sulphides selected for examination, all but 

 two, molybdenite and cinnabar, are readily decomposed by 

 heating with citric acid and potassium nitrate ; stibnite and 

 bournonite yield clear and colorless solutions ; argentite and 

 galenite, turbid and colorless solutions; bornite, sphalerite, 

 chalcocite, pyrrhotite, niccolite, smaltite, pyrite, chalcopyrite, 

 ullmannite, marcasite, arsenopyrite, and tetrahedrite give 

 colored solutions not particularly characteristic. The copper 

 minerals, heated with a large proportion of the organic acid, 

 give precipitates of red oxide of copper. The sulphur in these 

 minerals is not completely oxidized by the potassium nitrate 

 and a part floats in the solutions. 



Parallel experiments with citric acid and potassium chlo- 

 rate gave similar results, but the action of the chlorate is 

 slower. Tartaric acid may be substituted for citric, and ap- 

 pears to differ little in its solvent power. 



20. Although oxalic acid decomposes potassium nitrate, 

 only in the most concentrated solutions, yet the presence of an 

 oxidizable body like pyrites incites a reaction which effects 

 complete decomposition of the sulphide, precisely as with citric 

 acid. Potassium nitrate and oxalic acid heated together give 

 the following reaction : — 



2KN0 3 +5H 2 C 2 4 =2KHC 2 4 +4H 2 0+E 2 2 +6 CO, ; 

 and on the addition of pyrites we have : 



2FeS 2 + 12KNO,+ 13H 2 C A=Fe 3 (S0 4 ) 3 + K 2 S0 4 + 8H 2 0+ 

 10KHC 2 O 4 + 6N A+ 6C0 3 . 



Since, however, the nitric oxide is evidently evolved in 

 much greater proportion than the carbonic anhydride, the 

 following equation probably expresses the actual reaction with 

 greater accuracy : — 



6FeS i +32KN0 3 +29H 2 CA = 3Fe 3 (S0 4 ) 3 +3K 2 S0 4 +26KHC 2 4 

 +16H 2 0+16NA+6C0 2 . 



To decompose sulphides with oxalic acid and potassium 

 nitrate, the best results are secured by boiling the two reagents 

 together for a short time and then adding the finely pulverized 



