Determination of Molybdenum. 173 



at the end of the operation the color of the liquid was an olive 

 brown which grew browner on cooling. The addition of con- 

 siderable hydrochloric acid to the residual liquid restored the 

 clear green color, while water changed the olive brown to 

 reddish yellow, the tint varying with the dilution. The 

 results of these experiments are recorded in (i) to (5) of the 

 accompanying table. In division A are given the weights of 

 molybdenum trioxide corresponding to the amounts of iodine 

 found in the three stages of distillation ; in division B, the 

 molybdenum trioxide corresponding to the iodine evolved from 

 the beginning of the process to the end of each stage. 



The mean error of the indications taken during the period 

 of distillation advocated by Friedheim and Euler is 0-0045 

 grm. — ;* that of the period of concentration from 40 cm3 to 25 cm3 

 is 0-0008 grm.+ ; and that of the full period of distillation is 

 0'0014-K It is plain beyond a peradventure that in the pro- 

 cess as conducted by Friedheim and Euler, excepting the pro- 

 tection against atmospheric action, the theoretical reduction of 

 the molybdic acid does not take place. The best results are 

 obtained when the distillation is prolonged until the original 

 volume of 40 cm3 has been diminished to 25 cm3 . Concentration 

 beyond the limit of 25 cm3 tends to develop a tendency toward 

 over-reduction, especially when the amount of potassium iodide 

 is increased beyond about 5 grm. in excess of that theoreti- 

 cally required. This is shown in experiments (6) and (7), con- 

 ducted otherwise similarly to those described above, in which 

 the amount of potassium iodide was increased to 1 grm. and 

 2 grm. The error after distilling from 40 cra3 to 10 cm3 , the lowest 

 limit of the preceding experiments, was 0*0053 grm.+ and 

 0*0040 grm. 4-, and the latter error was increased to 0*0074 

 grm.+ on repeating the distillation with a fresh portion (30 cm3 ) 

 of the acid. It is interesting to note incidentally that in the 

 experiment in which the largest amount of iodide (2 grm.) was 

 used the solution did not take the green color at any stage of 

 the distillation, probably because the large excess of iodide 

 held the free iodine and so masked the color until the degree 

 of concentration was reached at which the olive brown color 

 displaces the green. 



The possibility of the interaction of atmospheric oxygen and 

 gaseous hydriodic in the analytical process, even to the extent 

 of producing errors of from one to three per cent reckoned as 

 molybdenum trioxide, was recognized by Friedheim and Euler ; 

 and it was to obviate this difficulty that the recommendation 

 was made by them to warm very gradually the distillation 

 flask filled two-thirds with the mixture of iodide, molybdate 



* Even this figure does not disclose the full error, which is partly counter- 

 balanced, as will appear later, by the effect of oxygen dissolved in the acid used 

 in the process. 



