CANE SUGAR. 



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 he connected when the lights are turned on in the evening, all 

 the copper will he deposited before they are turned off in the morning." 



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

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

 acid ; the ferric oxide is reduced by the cuprous oxide under the equation 



5 Cu 2 + 5 Fe 2 (S0 4 ) 3 + 5 H 2 S0 4 = 10 CuS0 4 + 10 Fe S0 4 + 5 H 2 

 and the ferrous sulphate formed is estimated by titration with potassium per- 

 manganate. 



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 decolourize 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. 



F. Sidersky's Process. 5 In Sidersky's method the precipitated cuprous 

 oxide is dissolved in a measured quantity of standardized sulphuric acid to which 

 a little potassium chlorate has been added. The reaction proceeds as follows : 



3 Cu 2 + 6 H 2 S0 4 + K Cl 3 - 6 Cu S0 4 + K Cl + 6 H 2 0. 

 The excess of sulphuric acid is then titrated with standard alkali, or an 

 excess of standard alkali may be added and titrated back with standard 

 sulphuric acid ; in the presence of free ammonia the solution is blue changing 

 to green when all the ammonia is saturated. It is advised that half normal 

 ammonia be employed. The amphotere in this method appears to the writer's 

 eye inconveniently large, the change from blue to green not being at all sharp. 

 The ammonia and sulphuric acid solutions should be standardized against each 

 other, using 2 c.c. of a concentrated solution of copper sulphate as an indicator. 



G. lodometric Process. The reactions involved are 



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

 2 Na 2 S 2 3 + I 2 = 2 ]S T aS 2 3 -f 2 Nal 



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

 lent to 63 -5 parts of copper. 



The precipitated cuprous oxide is dissolved in nitric acid, the excess of 

 acid partly removed by evaporation, neutralized with a slight excess of sodium 

 carbonate, and the precipitate redissolved with acetic acid. A slight excess of 

 potassium iodine over that indicated as necessary from the above equation is 

 added, and the iodine determined in the usual way with sodium thiosulphate, 

 using starch as an indicator. 



The thiosulphate solution should be standardized against pure electrolytic 

 copper or a pure preparation of a copper salt. 



462 



