176 G. Edgar— Iodometric Estimation of Vanadic Acid. 



(III). In the second half of the original solution the chromic 

 acid was estimated by adding sulphuric acid to slight acidity, 

 three cubic centimeters of syrupy phosphoric acid and an 

 amount of standard arsenious acid in excess of that required to 

 effect the reduction of the chromic acid. After standing for 

 fifteen to twenty minutes the solution was made alkaline with 

 sodium bicarbonate, and an excess of standard iodine solution 

 was added. This was allowed to stand in a stoppered flask for 

 from one half hour to an hour, the excess of iodine being then 

 removed with arsenious acid and the solution titrated to color 

 with iodine after the addition of starch. 



The use of phosphoric acid causes the iron to be precipitated 

 as phosphate, and thus the difficulty mentioned by Browning* 

 in observing the end-point, due to the reddish-brown ferric 

 hydroxide, is in large measure obviated, the blue of the starch 

 iodide being quite clear against the pale green chromic hydrox- 

 ide. The results are given in (III), Table (II). 



The first step in the calculation is the reduction to terms of 

 tenth normal solution of the figures under (I), (II) and (III). 

 It is evident that the subtraction of (III) from (I) gives the 

 number of cubic centimeters corresponding to reduction of the 

 vanadium pentoxide to tetroxide, while the subtraction of this 

 result from (II) gives the number equivalent to the reduction 

 of the ferric salt. By multiplying these figures by the solutions' 

 equivalents of vanadic acid, chromic acid and ferric oxide, the 

 necessary data are obtained. An example will make this 

 clearer : 



cm 3 N/10 factor cm 3 



(No. 1) (I) 31-15 X l'lOO = 34-26 (N/10) =-== V 2 5 + Cr0 3 



(II) 23-4 X MOO = 25-74 (N/10) =^= V 2 6 + Fe 2 3 



III 30-00-8-74 (N/10) = 21-26 (N/l 0) -*c= OrO, 

 (I)-(III) (34-26 - 21-26) = 13*00 cm 3 =o= V 2 5 



(II)-13-00 (25-74 - 13-00) = 12-74 cm 3 =-j= Fe 2 3 



21-26 X 0-003334 (factor for Cr0 3 ) = 0*0709 grm. Cr0 3 found. 

 13-00 X 0-00912 (factor for V 2 5 ) = 0*1185 grm. V 9 B found. 

 12*74 X 0*00799 (factor for Fe 3 O s ) = 0-1018 grm. ¥e.O s found. 



The distillation flask used in this work will be briefly de- 

 scribed. It consisted of a 100 cm3 pipette modified as shown in 

 fig. (I), the inlet tube being bent upwards and having a separ- 

 atory funnel sealed to its end, while 'the outlet tube was bent 

 upwards and then down to enter the absorption flask ; a small 

 bulb being blown in it to prevent mechanical loss during distil- 

 lation. In carrying out the process a slow current of hydro- 

 gen from a Kipp generator was kept up through the apparatus, 

 and this entering near the bottom of the flask enabled distil- 

 lation to be carried to very small volume without danger of 



* Loc. cit. 



