Gooch and Ward — Copper Oxalate in Analysis. 449 



involving separations Peters used volumes as high as 85 cm3 , of 

 which concentrated nitric acid made up 5 cm3 , with 3 grin, of 

 oxalic acid. 



With regard to the time required for completing the pre- 

 cipitation, Peters showed that when no added nitric acid is 

 present precipitates formed in the hot solutions at a volume of 

 50 cm3 may he filtered, either at once or after cooling, without 

 loss ; hut that when the nitric acid is added the mixture must 

 stand before filtration, best over night. 



The fact that small amounts of precipitated copper oxalate 

 may he redissolved in a sufficient excess of the precipitant points 

 to an appreciable degree of solubility of the precipitate in the 

 solution of oxalic acid. The observation that very considerable 

 amounts of copper oxalate fail to come down at all until a 

 certain minimum of the copper salt is present, while precipi- 

 tation is nearly complete when that minimum is reached, 

 indicates supersaturation of the precipitant bj copper oxalate ; 

 while the capacity of the liquid for supersaturation is appar- 

 ently limited to some extent by increase in concentration of 

 the oxalic acid. The solubility coefficient of the cojDper oxalate 

 under the conditions is made up, therefore, of at least two 

 factors, of which one depends upon the normal solubility in the 

 solution of oxalic acid which constitutes the medium of pre- 

 cipitation, while the other depends upon the solubility due to 

 supersaturation. In order that small amounts of copper may 

 be precipitated it is necessary to find means of eliminating or 

 at least limiting the capacity of the medium for supersaturation ; 

 and in order that large amounts, as well as small amounts, of 

 copper may be determined with the highest degree of accuracy 

 it is necessary to reduce to the low T est point the normal solu- 

 bility of the oxalate under the conditions of precipitation. 

 The present paper is an account of the experimental study of 

 conditions under which small as well as large amounts of copper 

 may be determined by the oxalate method. 



The Normal Solubility of Copper Oxalate. 



It is to be noted in the first place that the character of pre- 

 cipitated copper oxalate depends upon the conditions of pre- 

 cipitation. When oxalic acid is added to a cold concentrated 

 solution of a salt of copper the copper oxalate precipitated is 

 of extreme fineness and tends to pass through the closest filters. 

 The precipitate formed in hot solution is, on the other hand, 

 crystalline and easily separated by filtration of this liquid. 

 The solubility of the precipitate, as well as the ease with which 

 it may be separated from the liquid, turns upon the conditions 

 of precipitation and treatment. In the experiments to be 

 described, attention is first called to the degree of insolubility to 



