66 Goocli and Heath — lodometnc Determination of Copper. 



outspoken in praise, to the extent of declaring a preference 

 for this method in the most accurate technical work over all 

 other methods, even the electrolytic method. 



According to Low's earlier modification, metallic copper is 

 dissolved in nitric acid, the solution is freed from nitrogen 

 oxides by boiling, a considerable amount of zinc acetate is 

 added, and in the solution having a volume of 50 crnS an excess 

 of solid potassium iodide is dissolved. Zinc acetate is pre- 

 ferred to sodium acetate to take up the free nitric acid. It 

 is said that an excess of potassium iodide is necessary to insure 

 rapidity of- action and is harmless. According to the later 

 modification of this method Low prepares the cupric salt by 

 dissolving the metal in nitric acid (sp. g. about 1'20), boils the 

 solution, adds bromine water to destroy the nitrogen oxides, 

 boils to expel the bromine, treats with ammonium hydroxide 

 in excess, adds acetic acid and boils again if necessary to get a 

 clear solution. The advantage of using an excess of potassium 

 iodide is emphasized, and the statement is made that unless an 

 excess of this reagent is present the reaction does not proceed 

 to completion until the titration of the free iodine takes place. 

 Low recommends the use of 1 gm. of potassium iodide, an 

 excess of 0*6 grm., for every 0*075 grm. of copper. 



Various criticisms have also been made of the reaction when 

 employed in gravimetric estimations of the enprons iodide pre- 

 cipitated. Pisani * notes that potassium iodide can be used to 

 effect the precipitation of cuprous iodide and that satisfactory 

 separations may thus be brought about. 



Flajolotf states that potassium iodide cannot be used as the 

 precipitant since it dissolves cuprous iodide, and recommends 

 the precipitation of cuprous iodide from the solution of copper 

 sulphate slightly acidified with sulphuric acid, by treatment 

 with sulphurous acid and hydriodic acid; KohnerJ affirms 

 that cuprous iodide is soluble both in hydriodic acid and in 

 potassium iodide. 



Browning§ has shown that cuprous iodide may be satisfac- 

 torily precipitated and separated from a cadmium salt by add- 

 ing to a solution of cupric sulphate a moderate excess of 

 potassium iodide (1 grm. to -i grm. in all), expelling iodine and 

 hydriodic acid by evaporating the solution to dryness, and 

 treating the residue with water, filtering off the precipitate and 

 weighing upon asbestos in the perforated crucible. 



As a result of elaborate study Moser | has reached the con- 

 clusion that the reaction by which cuprous iodide is formed 



*Conipt. rend., xlvii, 294. 



f Journ. prakt. Cheni., xi, 105. 



% Ztschr. anal. Chem., xxvii, 213. 



§This Journal [4], xlvi, 280, 1893. 



|Zeitschr. anal. Chern., xliii, 597, 1904. 



