452 Gooch and Ward — Copper Oxalate in Analysis. 



If any part of the apparent loss of copper oxalate- precipi- 

 tated from solutions of oxalic acid is due to hydrolysis of the 

 normal oxalate, and formation of a basic oxalate as the product 

 of hydrolytic action, it should be possible to obviate such 

 apparent loss by increasing the active acidity of the solution 

 and thus inhibiting hydrolysis, providing that the solubility of 

 the normal oxalate is not made greater thereby. The experi- 

 ment shows that beyond a reasonable degree of concentration 

 the results are not affected by the use of oxalic acid up to the 

 point of saturation of the solution. It is worth while there- 

 fore to look somewhat more carefully into the effect of stronger 

 acids present at the time of precipitation. In Table III are 

 shown the details of experiments in which the active acidity 

 was increased by the addition of either free sulphuric acid or 

 free nitric acid to the solution of the copper salt before 

 precipitation w T as brought about by oxalic acid. These experi- 

 ments were made under conditions otherwise similar to those 

 of Table I. The copper sulphate was used in standard solu- 

 tions. The copper nitrate was prepared in solution for each 

 experiment by dissolving weighed electrolytic copper in nitric 

 acid, evaporating the solution to dryness, moistening the residue 

 with a few drops of nitric acid and dissolving in water. 



A comparison of the results of Table III with the results of 

 corresponding experiments in Table I brings out the facts that 

 the apparent error is actually diminished by the presence of 

 even very small amounts of sulphuric acid or nitric acid in the 

 liquid, while, within reasonable limits, the addition of more 

 acid produces no further effect. At the higher dilution, the 

 effect of the active acid is marked. At a volume of 100 cm3 the 

 average error of deficiency shown in Table I is cut in two by 

 the addition of 0-l c,u3 to 5 cm3 of nitric acid and of 0'5 cni3 to 

 2 cm3 of sulphuric acid. At smaller volume of 50 cm3 the effect 

 is not so marked, but it is still obvious. These results favor 

 strongly the hypothesis that copper oxalate is increasingly 

 subject to hydrolysis as dilution increases, and that the 

 tendency to form a basic salt may be checked by the presence 

 of the stronger acids in suitable amounts. Even very large 

 amounts of nitric acid produce a surprisingly small increase in 

 the apparent solubility of the oxalate. 



Losses due to solubility of copper oxalate may evidently be 

 kept at low limits by restricting the volume of the solution of 

 oxalic acid in which precipitation takes place; but too much 

 concentration is likely to introduce error due to mechanical 

 inclusion of oxalic acid in the precipitates. The natural 

 alternative to a close restriction of the volume of the aqueous 

 solution is the limitation of the solvent power of a larger 

 volume of liquid by partially substituting for water some other 



