Cuproine

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,... 

The submitters used butyllithium in hexane available from Janssen Chimica and standardized before use by titration against a solution of benzylic alcohol in toluene and cuproine (2,2 -biquinoline). The checkers used butyllithium (1.6 M in hexane) available from Aldrich Chemical Company, Inc., which was standardized before use by titration with diphenylacetic acid in THF. 

H. Diehl and G. F. Smith, The Copper Reagents-Cuproine, Neocuproine and Bathocuproine. Smith Chem. Co., Columbus, Ohio, 1958. 

Rehorek D, Jarzynowski T, Stasicka Z. Uber die Photoreduktion von Carboxylato (cuproin)kupfer(II) Chelaten in Alkoholen. J prakt Chem 1980 322 411-16. 

In die Gruppe der Diamine, die Chelatkationen bilden, gelioren zwei hoch selektive Verbindungstypen, die Ferroine und die Cuproine. 

Cuproine (6,6 -substituierte Dipyridj le, 2,9-Methyl- und -Phenyl-dcrivate des 1,10-Phenanthrolins und 2,2 -Bichinolin) cnthalten die gleiche chelatbildende Gruppierung, geben aber nur noch Chelate mit Kupfer(I), da die Bildung von entsprechenden Eiscn(Il)-Chclaten ste-risch gehindert ist (314). 

There are very few specific reagents and reactions. Among the scarce examples are cuproine for Cu(l) and bathophenthroline for Fe(II). 

The separation of copper by extraction is usually connected with spectrophotometric determination with the use of dithizone, dithiocarbamates, cuproine and related compounds, as well as other reagents. Sometimes copper is first preconcentrated by extraction as the dithizonate or dithiocarbamate before its determination by other methods [1-3]. Crown ethers [ 1 ] and porphyrin compounds [2] have also been used for extracting Cu(II). 

A great variety of spectrophotometric methods for determination of copper has been developed. Among the most widely used are the dithizone, dithiocarbamate, cuproine, and cuprizone methods described below in more detail. 

Cuproine (2,2 -biquinolyl, formula 19.2), neocuproine (2,9-dimethyl-1,10-phenanthroline, formula 19.3), and bathocuproine (2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline, formula 19.4) react with copper(I) to form coloured cationic complexes. The methods using these reagents are specific for copper, but are of relatively low sensitivity. 

Hoste et al. [26] have thoroughly investigated the conditions for the extractive spectrophotometric determination of copper with cuproine. Amyl alcohols and n-hexanol are most suitable for the extraction of the Cu complex from aqueous solutions. A reducing agent (usually NH2OH.HCI), is added to the sample solution, the pH is adjusted to the optimum range of 4-7, and the sample shaken with a colourless isoamyl alcohol solution of cuproine. 

At Xmax 546 nm, the molar absorptivity of the copper(I)-cuproine complex in isoamyl alcohol is 6.4-10 (specific absorptivity 0.10). 

Figure 19.2 shows the absorption spectra of the copper(I) complexes with cuproine, neocuproine, and bathocuproine in isoamyl alcohol. 

Fig. 19.2. Absorption spectra of copper(l) complexes with cuproine (1), neocuproine (2), and bathocuproine (3) in isoamyl alcohol...

From among other cations, only Ti " gives a greenish-coloured complex with cuproin, but its colour is much weaker than that of the purple copper(l) complex. Of the anions, cyanide, thiosulphate, oxalate, and EDTA interfere in the determination of copper [27,28]. 

Neocuproine is as specific for copper(l) as cuproine is. Substitution of methyl groups at the 2- and 9-positions in 1,10-phenanthroline renders the reagent unreactive towards iron(Il). 

The bathocuproine method for the determination of copper(I) [30,31 ] is about twice as sensitive as those based on cuproine and neocuproine. The molar absorptivity of the copper-bathocuproine complex in isoamyl alcohol at max = 479 nm is 1.42-10 (a = 0.22). 

Cuproine, 0.03% solution. Dissolve 30 mg of reagent in 100 ml of amyl alcohol. 

To an acidic solution containing not more that 150 pg of Cu, add about 0.2 g of NH2OH.HCI, ammonia to pH 3, and 10 ml of acetate buffer. Transfer the solution to a separating funnel, and shake for 3 min with each of 2 or 3 portions of the cuproine solution. The last portion should remain colourless. Dilute the combined extracts with isoamyl alcohol to the mark in a 25-ml standard flask, and measure the absorbance at 546 nm, using the solvent as reference. 

Copper can be determined by use of ion associates, formed by the cationic complexes of Cu(I) with cuproine [63-65], neocuproine [65], bathocuproine [66] and thio-crown ethers [67,68], associated with the acid dyes such as Rose Bengal (e = 7.8-10 ) [63,65,66], the ethyl ester of eosin (e = 9.4-10 ) [64], and Erythrosin [63]. These ion-associates are extracted into chloroform [65,66], 1,2-dichloroethane [64,67,68], and other solvents. The ion-associates of cyanide [69] and chloride [70,71] complexes of Cu(I) with Methylene Blue (1,2-dichloroethane, = 9.8-10 ) [69], and Ethyl Violet (toluene, = 9.6-10 ) [70] are also worth mentioning. The halide complexes of Cu(I) with azo dyes have also been extracted. 

Copper was determined with cuproine in silicate rocks, biological materials, and sea water [112,113], in environmental samples [113], steel and cast iron [114]. Neocuproine was used for determination of copper in biological materials [12,115], foods [116], sea water [117], beryllium [118], arsenic and gallium [119], tungsten [120], aluminium alloys [117], plutonium [121], tellurium [122], and fertilisers [123]. Bathocuproine was applied in determinations of copper in blood serum [124,125], water [126,127], niobium, tantalum, molybdenum, and tungsten [128], lead and nickel [129], cast iron and steel [66]. 

Nitrogen dioxide absorbed in alkali can react to form azo dyes [117-120]. These diazotization reactions are suitable for continuous automatic monitoring of atmospheric NO [117]. Oxides of nitrogen in cigarette smoke have been determined with brucine [121]. The amount of NO in liquid N2O4 (0°C) can be determined from the green colour of the N2O3 [122]. NO2 in the atmospheric air has been absorbed in Cu(DDTC)2 in toluene and determined from the deerease in the initial absorbance at 437 nm [123]. In a proposed indirect method, NO reduces Cu(II) to Cu(I), which is then determined with the use of cuproine [124]. 

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