One method for determining copper is the “Neocuproine Method”. In this method, copper in a +1 oxidation state reacts with neocuproine (2,9-dimethyl-1,10-phenanthroline) to form a complex of Cu(neocuproine)2. The complex is extracted into a chloroform-methanol mixture, giving a yellow solution with a molar absorptivity of 8000 M-1 cm-1 at 457 nm. Beer’s law is obeyed up to a concentration of 0.2 mg Cu/25 mL of extraction solvent. Full color development occurs when the sample’s pH is between 3 and 9. A typical procedure is provided here:
A 100.0-mL sample is placed in a 250-mL beaker, acidified with 1 mL of H2SO4 and 5 mL of HNO3, and boiled to destroy any traces of cyanide, sulfide, or organic material that may be present. The remaining sample is transferred to a 100-mL volumetric flask and diluted to volume. A 50-mL portion of this sample is transferred to a 250-mL separatory funnel and 5 mL of with hydroxylamine hydrochloride is added to reduce Cu2+ to Cu+. A 10-mL portion of a sodium citrate solution is added to complex any metal ions in the sample that might precipitate when the sample’s pH is adjusted. A solution of 5 M NH3 is added in 1-mL increments until the pH is between 4 and 6. A 10-mL portion of neocuproine is added along with 10 mL of CHCl3. The contents of the separatory funnel are shaken and the layers allowed to separate. The CHCl3 layer is drained into a 25-mL volumetric flask and diluted to volume with methanol. The absorbance of the CHCl3-CH3OH solution is measured at 457 nm in a 1.00-cm cell.
(a) Explain how you would prepare an appropriate blank for this analysis.
(b) The minimum absorbance that can be measured with confidence is 0.010. To what mass of Cu in the original sample does this correspond?