214 IHllebrand — Vanadium and Molybdenum in Rocks. 



mate dryness. Care should he taken to avoid overrunning 

 neutrality because of the reducing action of the nitrous acid 

 set free from the nitrite, hut when chromium is present it has 

 been my experience that some of this will invariably be 

 retained by the precipitated silica and alumina, though only in 

 one case have T observed a retention of vanadium, it being 

 then large. The use of ammonium nitrate instead of nitric 

 acid for converting the sodium carbonate into nitrate did not 

 seem to lessen the amount of chromium retained by the silica 

 and alumina. 



As a precautionary measure therefore, and always when 

 chromium was to be estimated also, the silica and alumina pre- 

 cipitate was evaporated with hydrofluoric and sulphuric acids, 

 the residue fused with a little sodium carbonate and the 

 aqueous extract again nearly neutralized with nitric acid and 

 boiled for a few moments, the filtrate being added to the main 

 one. 



Mercurous nitrate was now added to the alkaline solution in 

 some quantity so as to obtain a precipitate of considerable 

 bulk containing chromium, vanadium, molybdenum, tungsten, 

 phosphorus, and arsenic, should ail happen to be in the rock, 

 and also an excess of mercurous carbonate to take up any 

 acidity resulting from the decomposition of the mercurous 

 nitrate. Precipitating in a slightly alkaline instead of a neu- 

 tral solution renders the addition of precipitated mercuric oxide 

 unnecessary for correcting this acidity. If the alkalinity, as 

 shown by the formation of an unduly large precipitate, should 

 have been too great, it may be reduced by careful addition of 

 nitric acid until an added drop of mercurous nitrate no longer 

 produces a cloud. 



After heating and filtering, the precipitate is ignited in a 

 platinum crucible after drying and removing from the paper to 

 obviate any chance of loss of molybdenum and of injury to the 

 crucible by reduction of phosphorus or arsenic. The residue is 

 fused with a very little sodium carbonate, leached with water, 

 and the solution if colored yellow filtered into a graduated 

 flask of 25 or more cubic centimeters capacity. The chromium 

 is then estimated accurately in a few minutes by comparing 

 with a standard alkaline solution of potassium monochromate.* 

 Then, or earlier in absence of chromium, sulphuric acid is 

 added in slight excess and molybdenum and arsenic together 

 with occasional traces of platinum are precipitated by hydrogen 

 sulphide, preferably in a small pressure bottle. f If the color 

 of the precipitate indicates absence of arsenic, the filter with its 



* Jour. Am. Chem. Soc, xx, 454, 1898. 



•f From a sulphuric solution the separation of molybdenum by hydrogen sul- 

 phate is much more rapid and satisfactory than from a hydrochloric solution. 



