Cadmium by the Iodide Method. 281 



made to show the effect of varying amounts of potassium 

 iodide, and were all conducted in the same manner, the stan- 

 dard copper solution being measured from a burette into a 

 counterpoised beaker and weighed, the potassium iodide in 

 solution added, the whole evaporated to dryness to insure the 

 expulsion of the iodine, and water added to the amount indi- 

 cated in the table. The filtration was made under gentle 

 pressure, by means of a Bunsen filtering flask, on an asbestos 

 felt contained in a perforated platinum crucible, the whole 

 having been previously ignited and weighed. It is advisable 

 to use a moderately thick felt and not to allow the felt to run 

 dry nor to remove the pressure during the filtration, on account 

 of the tendency of the cuprous iodide to pull through under 

 these conditions. The precipitate was washed thoroughly with 

 either hot or cold water, the washings never amounting to less 

 than 100 cm 3 . The crucible containing the cuprous iodide was 

 placed in an air bath and the precipitate dried at a tempera- 

 ture between 120° C. and 150° C, and weighed. The first 

 weight was uniformly constant. It would seem from experi- 

 ments (4) to (12) that a moderate excess of potassium iodide 

 has no great solvent effect upon the precipitated cuprous 

 iodide, while free iodine and large amounts of hydriodic acid 

 have a decided solvent effect as shown in experiments (1), (2), 

 (3). In two cases where 1 grm. and 2 grm. respectively of 

 potassium iodide were used to precipitate about 012 grm. of 

 copper the filtrate from the first gave no reaction with either 

 ammonia or acetic acid and potassium ferrocyanide, and the 

 second gave no reaction with ammonia and with potassium 

 ferrocyanide only a trace. 



These filtrates before testing for copper were treated with 

 potassium nitrite and sulphuric acid to remove the iodide and 







Series ] 



■si- 











Copper found 



Error in 





Final volume 



Copper taken. 



(weighed as Cu a I 2 ). 



Copper. 



KI used. 



of liquid. 





grm. 



- grm. 



grm. 



grm. 



cm 3 . 



(1) 



0-1189 



0-1093 



0-0096- 



l 



300 



(2) 



0-1197 



1050 



0-0147 — 



" 



" 



(3) 



0-1191 



0-1056 



0-0135 — 



<l 



" 



(4) 



0-1194 



0-1189 



0-0005- 



" 



" 



(5) 



0-1194 



0-1196 



0-0002 + 



u 



100 



(6) 



0-1191 



0-1194 



00003 + 



" 



" 



(?) 



0-1193 



0-1193 



o-oooo 



2 



" 



(8) 



0-00-J9 



0-0045 



0-0004— 



" 



" 



(9) 



00051 



0-0047 



0-0004- 



" 



c< 



(10) 



0-1195 



0-1195 



o-oooo 



o 



" 



(11) 



01192 



0-1188 



0-0004- 



4 



" 



(12) 



0-1191 



0-1189 



00002- 



il 



200 



(13) 



0-1193 



0-1182 



o-ooii 



5 



" 



(14) 



0-1193 



0-1179 



0014- 



" 



'• 



The copper is calculated as the metal in this table. 



