156 Qooch and Fairhdriks — lodometric 



Akt. XIX. — The lodometric Estimation of Molyhdic Acid j 

 by F. A. GoocH and Charlotte Fairbanks. 



[Contribution 8 from the Kent Chemical Laboratory of Tale University — LII.] 



I. The Digestion Method. — Mauro and Danesi have shown* 

 that under carefully regulated conditions, hydrochloric acid, 

 potassium iodide and a soluble molybdate interact so definitely 

 that the iodine set free from an excess of potassium iodide 

 may be taken as a measure of the molybdic acid, assuming 

 that every molecule of molybdic acid sets free an atom of 

 iodine. They explain this action by the equation : 



' 2M0O3 + 4HI = 2M0O J + 1, + 2H3O 

 These results are obtained by acting upon a soluble molybdate 

 containing from O'l to 0*5 grm.. of molybdic acid with 1-5 grm. 

 of potassium iodide in 1-5°*' of water and 2*5" of strong hydro- 

 chloric acid, in an atmosphere of carbon dioxide, the whole 

 being heated an hour and a half in a sealed tube. The authors 

 point out that with prolonged heating the action proceeds a 

 little farther, and in the cold, under conditions otherwise simi- 

 lar, not quite so far as the theory of the equation would indicate. 



Upon following out exactly the directions of Mauro and 

 Danesi we obtained results similar in general to theirs. The 

 digestion in sealed tubes under the conditions indicated gave 

 results closely comparable with theirs, and these were in fairly 

 good accord with the theory of the reduction. Our results 

 obtained by digesting in the cold and in small volumes for a 

 long time, according to the second method of Mauro and Danesi, 

 were not equally favorable, and these results, as well as those 

 obtained by varying the treatment, indicated an increasing diffi- 

 culty in the reduction of the molybdic acid as its amount 

 increased. As will be seen from the table, the method of 

 Mauro and Danesi was varied in two ways : instead of only 

 treating in small volumes (4:^"^) at 100° C, we used larger vol- 

 umes (lo*^^ to 30*=°) at 100° C. with weaker acid and smaller 

 amounts of the iodide ; and instead of their treatment in the 

 cold of only small volumes (4<=°), large volumes (50" to 100") 

 with stronger acid were used. 



Treatment in sealed tubes at 100° for 1-J- hours. 

 (Method of Mauro and Danesi.) 

 M0O3, as ammonium 



HCl. H2O. KL molybdate. M0O3 found. Error, 



cc. cc. grm. grm. grm. grm. 



2-5 1-0 1-5 -1010 -1007 -0003 — 



2-5 1-5 1-5 -3015 -3004 '0007 — 



2-5 1-5 ]*5 -5006 '4795 '0031 — 



2'5 1-5 1-5 -5020 '4924 '0096 — 



* Zeitschr, fiir Anal. Chem., 1881. 



