1904.] SMITH AND EXNER — ATOMIC WEIGHT OF TUNGSTEN. 133 



filtering out and washing the calcium tungstate it was boiled with 

 hydrochloric acid. Tungstic acid of a rich yellow color separated. 

 It was washed and dissolved in ammonia water. There was a very 

 slight bluish-colored residue. The ammonium paratungstate, which 

 crystallized out, was perfectly white in color, but a portion of it 

 ignited and the resulting oxide, digested with a dilute sodium car- 

 bonate solution, disclosed the usual white residue in which tungsten 

 and iron were found. 



The salt, purified in this way, was fully as free from impurities 

 as a salt which had passed through six crystallizations from water. 

 This fact led us to prepare pure calcium carbonate and hydrochloric 

 acid before repeating the method with another portion of ammo- 

 nium paratungstate. 



Commercial calcium carbonate was boiled with a solution of 

 pure ammonium chloride. Iron and other impurities remained 

 with the excess of calcium carbonate. The filtrate from the latter 

 was precipitated with pure ammonium carbonate. The resulting 

 calcium carbonate was thoroughly washed and then dried. 



Ordinary hydrochloric acid was saturated with calcium chloride 

 and after the addition of phosphorus pentoxide was distilled with 

 water. This gave pure acid. With these purified reagents ammo- 

 nium paratungstate, which had passed through several crystalliza- 

 tions, was subjected to the treatment outlined in the beginning of 

 this section. The purified salt, when tested, showed but traces of 

 impurities and it is very probable thai these, after several repeti- 

 tions, would disappear entirely. The actual trial was not made, 

 because another course seemed to lead to the desired end in a 

 much shorter period. Experience also showed that nitric acid was 

 preferable. 



Method 4. — In this procedure a boiling solution of ammonium 

 paratungstate was decomposed with hydrochloric acid. The pre- 

 cipitated tungstic acid was again dissolved in ammonia and the 

 decomposition repeated. Repeating this process several times 

 yielded tungstic acid which might be asserted to be quite pure, 

 although when the mother liquors from the several fractions of the 

 ammonium salts were reduced to a small volume the dark color 

 appeared. A white residue, although slight, was also obtained 

 from the ignited tungsten trioxide. Wyman (Thesis, University 

 of Pennsylvania, 1902) found, after twenty-five decompositions 

 with hydrochloric acid, evidences of similar contamination. This 



