DETERMINATION OF MOLYBDIC TBIOXID. 



45 



molybdenum compound with ammonium hydroxid, evaporate, and 

 ignite again in order to obtain the molybdic trioxid in a pure condition. 

 By using a Gooch crucible these complications and potential sources 

 of error are eliminated. The procedure is as follows : 



The solution containing not more than 0.5 gram (preferably between 0.2 and 0.3 

 main of molybdic trioxid) is placed in a pressure flask or a thick-walled Erlenmeyer 

 flask of about 200 cc capacity and acidified with 5 cc of concentrated hydrochloric 

 acid. A freshly prepared aqueous solution of thioacetic acid containing 0.75 cc of 

 the pure acid for each 0.1 gram of molybdenum trioxid is then added, the flask securely 

 stoppered, the mixture digested on a water bath for from forty to sixty minutes, then 

 cooled somewhat, filtered through a tared Gooch crucible covered with asbestos, and 

 thoroughly washed with water containing about 2 per cent of hydrochloric acid. The 

 use of too strong a vacuum is to be avoided. The particles which have crept up the 

 sides of the crucible are washed down with a little alcohol, which at the same time 

 replaces the water and facilitates the drying. The precipitate is dried at about 120 

 C. in the crucible, which is then set to half its depth in a close-fitting hole in a stout 

 asbestos board and heated over a bunsen flame. The crucible is covered in such a 

 way as to permit a little air to enter. The flame of the burner is to be kept very low 

 at first until the thioacetic and sulphurous acids have disappeared, then it is gradually 

 increased until the submerged part of the crucible shows dull redness and the heating is 

 discontinued when the originally brown precipitate has assumed the yellow or slightly 

 grayish color of molybdenum trioxid. The crucible is weighed after cooling in a 

 desiccator. As a precaution the crucible may be reheated and reweighed. 



Molybdic trioxid, as is well known, volatilizes in the open air at a 

 bright red heat, but no loss need be apprehended if the directions 

 given are followed. The protruding part of the crucible under these 

 conditions remains sufficiently cool to condense any molybdic trioxid 

 which might be vaporized. For the purpose of hastening the oxida- 

 tion the cover may be removed after the volatile gases have ceased to 

 come off, but it has been found that by allowing the oxidation to take 

 its natural course a more uniform product is obtained. 



The following figures, obtained on ammonium molybdate whose 

 composition was carefully determined in various ways, show the 

 availability of the method which, though not very rapid (requiring 

 from 3 to 4 hours for a determination) has the advantage of giving 

 reliable results : 



Determinations of molybdic trioxid in known samples by proposed method. 



o Thioacetic acid dissolves in water to the extent of about 6 per cent, but such a solution deteriorates very 

 rapidly and after standing about one week is worthless. It is, therefore, necessary to work with a freshly 

 prepared solution only. 



b Owing to insufficient drying and the premature removal of the crucible cover some loss by decrepitation 

 was sustained. 



