THE DETERMINATION OF MOLYBDIC TRIOXID. 



By B. HERSTEIN. 



The quantitative estimation of molybdenum and its compounds 

 offers a number of difficulties. Of the titrimetric methods proposed for 

 this purpose those based on the reduction with zinc and subsequent 

 titration with permanganate solution give persistently, though not 

 consistently, too low results, while those based oniodometric methods 6 

 require great familiarity with the subject, scrupulous observations 

 of empirical limitations, the concurrence of many favorable condi- 

 tions, and the exclusion of all chance combinations which might 

 interfere with the correctness of the results, which even then are not 

 uniform. Of the gravimetric methods known, some are untrust- 

 worthy, as, for instance, precipitation as lead melybdate, while 

 others require several repetitions of the operations to insure constant 

 results. If a series of determinations is to be made, one must neces- 

 sarily familiarize himself with the best conditions essential for the 

 greatest degree of accuracy possible, but where molybdenum, or its 

 compounds, is to be determined only occasionally, as is the case in 

 most laboratories, a method is needed which, with ordinary skill and 

 precaution, will give accurate results. 



The method described in this paper is believed to meet this need. 

 It involves the precipitation of molybdenum by means of thioacetic 

 acid in a strongly acid solution, and the transformation of the pre- 

 cipitate so obtained into molybdic trioxid by mere ignition and weigh- 

 ing as such. Thioacetic acid under these conditions precipitates 

 molybdenum quantitatively and within a short time, while hydrogen 

 sulphid and similar reagents, as is well known, precipitate only par- 

 tially and require several repetitions to complete the reaction. The 

 precipitate formed by the thioacetic acid does not seem to be merely 

 a combination of molybdenum with sulphur, as one would at first 

 suppose, but apparently contains thioacetic acid within the molecule, 

 since on ignition of the purified precipitate the penetrating odor of 

 this acid is strong and unmistakable. The precipitate also seems to 

 be less dense than the sulphid ordinarily obtained which greatly facili- 

 tates its transformation into molybdic acid when ignited. 



Euler and Friedheim c in 1895 recommended that the molybdenum 

 sulphid be transformed into molybdic trioxid and weighed as such. 

 They, however, used paper filters which were incompletely ignited, 

 and to separate the charred particles it was necessary to dissolve the 



a V. d. Pfordten. Ber. d. chem. Ges., 1882, 16: 1927. 



6 Euler and Friedheim. Ber. d. chem. Ges., 1895 (2), 28: 2061; Gooch and Fairbanks. Amer. J. Sci., 

 1896 (4) 2: 156; and Gooch and Pulman. 1901 (4), 12: 449. 

 cLoc. cit. 



44 



