536 CHAPTER XXVI 



connects with a platinum sealed into the tube ; b is a glass tube through 

 which a copper wire extends and connects with a platinum wire e sealed 

 into this tube. The tube b may be slipped up and down, thus regulating the 

 distance between the wires e and a, and regulating the current. The twin 

 wire m is separated, severed, and one end d connected with the platinum 

 dish in which the copper is to be deposited, and the other with the regulator 

 b, thence through the acidulated water, and a with the platinum cylinder 

 suspended in the copper solution. Sufficient current for a large number of 

 dishes arranged in sets of four will pass through a 16 C.P. or 32 C.P. lamp. 

 The copper should be deposited very slowly. Usually, if the apparatus 

 be connected when the lights are turned on in the evening, all the copper will 

 be deposited before they are turned off in the morning." 



By the Permanganate Process. In this process the cuprous oxide is 

 dissolved in a concentrated solution of ferric sulphate in 25 per cent, sul- 



FIG. 346 



phuric acid ; the ferric oxide is reduced by the cuprous oxide according to 

 the equation: 



5 Cu 2 O +5 Fe 2 (SO 4 ) 3 +5 H 2 SO 4 = TO Cu SO 4 + 10 FeSO 4 + 5H 2 0, 

 and the ferrous sulphate formed is estimated by titration with potassium 

 permanganate. 



The exact copper value of the permanganate should be determined by 

 direct assay against a pure preparation of a copper salt. 



A solution of a ferric salt will always decolorize a few drops of deci- 

 normal permanganate, and hence a fixed quantity of the ferric solution should 

 be adhered to ; by standardizing the permanganate under the conditions 

 of the subsequent assays, this source of error is automatically removed. 



lodometric Process. The reactions involved are : 



2 Cu (CH 3 COO) 2 + 4 KI=2CuI + 4 KCH 3 COO + I 2 

 2 Na 2 S 2 O 3 -f-I 2 = 2 NaS 2 O 3 +2 Nal. 



From the above equations it follows that 126 8 parts of iodine are equival- 

 ent to 63-5 parts of copper. 



