158 Gooch and Fairbcmks — lodometric 



hydrochloric acid, one atom of iodine is distilled for every 

 molecule of molybdic acid present. The process according to 

 their directions consists in treating in the Bunsen apparatus 

 from 0*2 to 0*3 grm. of a soluble molybdate, or molybdic acid 

 dissolved in sodium hydroxide, with 0*5 grm. to 0*76 grm. of 

 potassium iodide and enough hydrochloric acid (sp. gr. 1'12) to 

 two-thirds fill the flask. The solution is warmed for sometime 

 and only heated to boiling when the flask is wholly filled with 

 the heavy vapor of iodine. It is boiled until no more iodine 

 is visible and the liquid is of a clear green color. The iodine, 

 collected in a receiver previously charged with a solution of 

 potassium iodide, is then titrated with a standard thiosulphate 

 solution. Results, varying between -05 per cent and 1*3 per 

 cent on amounts, ranging from 0*2 grm. to 0*4 grm., of the 

 molybdic acid present, were obtained by them. 



Our experience fully confirms the usefulness of the method 

 of Friedheim and Euler, but we have found it necessary to 

 define more exactly the conditions of distillation. It is not 

 sufiicient to say that the boiling shall be stopped when a clear 

 green color appears and when the steam is no longer colored by 

 iodine ; for the green color comes very gradually and we have 

 repeatedly found iodine in the residue after the green color 

 had developed distinctly. We have found it safer and more 

 convenient to start the distillation with a definite volume of 

 liquid and boil until the volume is reduced to a definite point. 



Our work has shown that if the initial volume is about 40*^°, 

 no iodine remains in the flask after the liquid has been boiled 

 down to 25 ''^j and that at that degree of concentration the 

 molybdic acid shows the theoretical reduction ; but if the con- 

 centration is pushed beyond this point, a tendency to further 

 reduction of the molybdic acid becomes evident. 



We find it necessary to carry on the distillation in an atmo- 

 sphere of carbon dioxide, inasmuch as the hydriodic acid freed 

 by the action of hydrochloric acid of the strength employed 

 upon the potassium iodide, is decomposed by distillation in 

 contact with air, with liberation of iodine. As even a trace of 

 oxygen will immediately set free iodine from boiling hydriodic 

 acid, the carbon dioxide must be as free as possible from oxygen. 

 The carbon dioxide which we used was evolved from boiled 

 marble by the action of boiled acid to which a little cuprous 

 chloride had been added, and was finally passed through a solu- 

 tion of iodine in potassium iodide to free it from any reducing 

 substance which might be present. 



The apparatus which we found most satisfactory was con- 

 structed with sealed and ground joints exclusively, and is 

 shown in the accompanying figure. The distillation takes place 

 in the first flask, and the iodine collects in the second flask and 

 trap, which hold a solution of potassium iodide kept cool by 

 immersion of the flask in cold water. 



