Neo-cuproin

Discussion. Neo-cuproin (2,9-dimethyl-l,10-phenanthroline) can, under certain conditions, behave as an almost specific reagent for copper(I). The complex is soluble in chloroform and absorbs at 457 nm. It may be applied to the determination of copper in cast iron, alloy steels, lead-tin solder, and various metals. 

Procedure. To 10.0 mL of the solution containing up to 200 fig of copper in a separatory funnel, add 5.0 mL of 10 per cent hydroxylammonium chloride solution to reduce Cu(II) to Cu(I), and 10 mL of a 30 per cent sodium citrate solution to complex any other metals which may be present. Add ammonia solution until the pH is about 4 (Congo red paper), followed by lOmL of a 0.1 per cent solution of neo-cuproin in absolute ethanol. Shake for about 30 seconds with 10 mL of chloroform and allow the layers to separate. Repeat the extraction with a further 5 mL of chloroform. Measure the absorbance at 457 nm against a blank on the reagents which have been treated similarly to the sample. 

Discussion. Small quantities of copper may be determined by the diethyldithio-carbamate method (Section 6.10) or by the neo-cuproin method (Section 6.11), an extraction being necessary in both cases. In another somewhat simpler procedure, the copper is complexed with biscyclohexanone oxalyldihydrazone and the resulting blue colour is measured by a suitable spectrophotometer within the range 570-600 nm (orange filter). The solution measured should contain not more than 100 /tg of copper. 

Neo-cuproin (i.e., 2,9-dimethyl-1 10-phenathroline) under specific experimental parameters almost behaves as a critical reagent for copper (I). The resulting complex is freely soluble in chloroform and absorbs at 457 nm. 

Add ammonia solution gradually until the pH is about 4.0 (use Congo Red) followed by 10 ml neo-cuproin solution,... 

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