THE STUDY OF CU IONS EXTRACTION WITH BATHOPHENANTHROLINE FROM WATER SOLUTIONS

Transcription

1 ACTA UNIVERSITATIS PALACKIANAE OLOMUCENSIS FACULTAS RERUM NATURALIUM 2000 CHEMICA 39 THE STUDY OF CU IONS EXTRACTION WITH BATHOPHENANTHROLINE FROM WATER SOLUTIONS Libor Kvítek, a Dana Sichertová, a David Milde, b and Jana Skopalová b a Department of Inorganic and Physical Chemistry, Palacký University, Svobody 8, Olomouc, Czech Republic. kvitek@aix.upol.cz b Department of Analytical Chemistry, Palacký University, Svobody 8, Olomouc, Czech Republic. Abstract Received July 10, 2000 Accepted August 30, 2000 The study of the complex compound [Cu I (baphen) 2 ] + extraction to many organic solvents from water was accomplished. Dichloromethane, butyl alcohol, n-amyl, and i-amyl alcohols were suitable solvents for extraction and sensitive determination of the Cu ions with bathophenanthroline. Key Words: Copper, bathophenanthroline, extraction, spectrophotometric determination. Introduction Heavy metal ions trace impurities reduce the quality of production in pharmacy, food industry etc. They are one of the most trace pollutants in ecology, too. Therefore, new effective, sensitive, and economic methods for determination and removing of the harmful impurities are developed. 1 Extraction is broadly used methods for concentration of the impurities in order to their determination or removing. This separation method has more advantages it s very simple, quick, and efficient in comparison with another methods used for estimation of the impurities concentration. 2 Many more or less selective species are used for extraction and spectrophotometric determination of copper ions. For selective Cu II ions determination bis-cyklohexanoneoxalyldihydrazone (cuprizone) is used, 3 but less selective sodium diethyldithio- 53

2 carbamate (cupral) or diphenylthiocarbazone (dithizone) are used too. 4 1,10- phenanthroline and its derivatives are important group of the reagents that create complex compounds with Cu I ions that can be extracted into organic solvents. 5 2,9- dimethyl-1,10-phenanthroline (neocuproine) and 2,9-dimethyl-4,7-diphenyl-1,10- phenanthroline (bathocuproine) with two methyl groups in 2,9 position are probably the most selective reagents for Cu I ions with good sensitivity (e 479 = 1, l mol -1 cm -1 in i-amyl alcohol). There exist modifications of the neocuproine or bathocuproine method that can lead to more sensitive Cu I ions determination. These methods are based on ion-associate formation of Cu I complex cation with appropriate dye anion. 6 4,7-difenyl-1,10-phenanthroline (bathophenanthroline, baphen) is used for spectrophotometric or extraction spectrophotometric determination of Fe II ions especially. Bathophenanthroline creates with this ion red coloured complex cation [Fe II (baphen) 3 ] 2+ in the wide range of ph. It is one of the most sensitive reagents for Fe II ions because molar absorption coefficient of the complex compound is very high (e 538 = 2, l mol -1 cm -1 in nitrobenzene). 4 However bathophenanthroline can be used also for extraction and determination of Cu I ions. In neutral or weak alkaline solutions bathophenanthroline forms with Cu I ion orange complex cation [Cu I (baphen) 2 ] + that can be extracted into organic solvents 7 but only colourless [Cu I (baphen)] + complex cation is formed in acidic solution (ph<4). Molar absorption coefficient of [Cu I (baphen) 2 ] + is reported 1, l mol -1 cm -1 at wavelength of 457 nm. 8 Bathophenanthroline is examined as reagent for spectrophotometric determination of Fe II and Cu I ions in mixtures, 9 but there are lack of details about [Cu I (baphen) 2 ] + complex compound extraction from water solutions. The aim of this work is study optimal conditions for extraction of this complex compound with consideration of common sensitive determination of Cu and Fe in a real samples. Materials and methods For preparation of compound [Cu I (baphen) 2 ] + was used the same experimental method as for [Fe II (baphen) 3 ] 2+ complex ml of the reduction substance (10% solution of hydroxylamine hydrochloride), 7 ml of the ammonium solution (c = 2 mol l -1 ), and 2 ml of the 0.1% ethanolic bathophenanthroline solution was added to appropriate amounts (0.1 to 1.0 ml) of the standard Cu solution (0,1 mg Cu/ml). Ammonium solution was used instead of acetate buffer in order that ph had to be maintained in the weak alkaline range (ph 7,5), where colour complex cation [Cu I (baphen) 2 ] + is formed. Because wrong separable emulsion was formed in extraction process, 7 ml of the ethanol was added to initial solution. Then volumetric flasks were filled up to final volume 25 ml with distilled water. The water solution was extracted by 2x 10 ml of the organic solvent after 15 min standing. Both extracts were transferred into volumetric flask and filled up to 25 ml with ethanol. The absorption of the extracts was measured against reagents blank solution at 460 nm (maximum of the complex compound absorption). 54

3 The standard solution of Cu was obtained by tenfold dilution of the stock solution Cu (concentration 1 mg Cu/ml) that it was prepared by dissolving of g CuSO 4.5 H 2 O (p.a., Lachema) in 500 ml distilled water with 2 drops of conc. HNO 3. All the used chemicals were p.a. grade, Lachema a.s. Spectrophotometric measurements were performed with the UV-VIS spectrophotometer Helios a (Unicam, GB). The measurements of the precise concentration Cu in stock solution and in mineralized extracts were performed with the flame AAS spectrometer Avanta S (GBC, Australia), and verified electrochemically by the anodic stripping voltammetry (ASV) with the EcoTribo Polarograph (PolaroSensors, Czech Republic). Also complexometric titration was used for concentration control of Cu in studied samples. Results and discussion The absorption spectrum of the [Cu I (baphen) 2 ] + in water solution was measured (Fig. 1). The measurement was done in a mixed solvent water-ethanol (1:1) because studied complex compound is not soluble in pure water. The concentration dependence of [Cu I (baphen) 2 ] + complex compound absorption was studied at l max = 460 nm. This dependence was linear (correlation coefficient 0.999) in the range of studied concentrations (from to mol l -1 ), and the value of the molar absorption coefficient e 460 = l mol -1 cm -1 was obtained from this measurement. Fig. 1: Absorption spectrum of the [Cu I (baphen) 2 ] + complex cation in mixed solvent water-ethanol (1:1). 55

4 The initial study of [Cu I (baphen) 2 ] + complex cation extraction from water solution was performed with 17 organic solvents from several basic groups of organic compounds immiscible with water. Hexane, heptane, isooctane, and cyclohexane were used from group of aliphatic and cyclic carbohydrates, benzene, toluene, and xylene from group of aromates, n-butyl, n-amyl, and i-amyl alcohols, and cyclohexanole from group of alcohols. Diethyl ether, cyclohexanone, and n-amyl acetate from group of oxygen derivatives of carbohydrates, and dichloromethane, chloroform, and carbon tetrachloride from group of halogen derivatives were used. Extraction efficiency, colour intensity, and stability of the complex compound were studied in used solvents. The experiments showed that carbohydrates are not useable for [Cu I (baphen) 2 ] + complex cation extraction from water because it is fully unstable in this group of solvents. Oxygen and halogen derivatives can be used for [Cu I (baphen) 2 ] + extraction, very good results could be obtained especially with alcohols. Six solvents were chosen for further study of extraction process n-butyl alcohol, n-amyl alcohol, i-amyl alcohol, diethyl ether, dichloromethane, and chloroform. The absorption spectrums of extracts in all six solvents had identical shapes as in waterethanol mixture. The relationship of extracts absorption on the amount of the complex compound in starting water solution were measured at l max = 460 nm. They were linear (correlation coefficients were from to 0.999) in the range of used concentrations (from to mol l -1 ), and they were well reproducible. The values of the apparent molar absorption coefficients were calculated for every one of chosen solvents, their values are referred in Tab. 1 (they are denominated by e 460). So that extraction efficiency can be determined, the real values of absorption coefficient of the complex compound in all the used solvents had to be found. The effort to isolate the pure complex compound from solution was unsuccessful. There was fundamental change of the absorption spectrum shape when the isolated compound was dissolved again. Therefore, the exact concentrations of complex compound in the extracts had to be measured. 10 ml of extracts in all the used solvents were mineralised in the hot mixture of concentrated sulphuric and nitric acids. The concentration of Cu in mineralizates was determined by the AAS method after evaporating and redissolving of it in distilled water. The determined values of concentrations were verified electrochemically by the ASV method and by complexometric titration. The acquired concentration data were used in recalculation of the molar absorption coefficients to the exact values (they are denominated by e 460 in Tab. 1), and in extraction efficiency determination. The values of the distribution coefficient (determined from the Nernst distribution law) depended on the complex cation concentration in a water solution. Reason for this behaviour is the ion-associate formation in the case of extraction process. The value of the stoichiometric ratio of the complex cation [Cu I (baphen) 2 ] + to anion 2:1 was found on the base of the ion-associate formation equilibria effect on the complex cation distribution. Therefore, sulphate anion was probably the counter-ion in the extracted ion-associate. 56

5 Table 1: The values of the apparent (e ) and real (e) molar absorption coefficient (460 nm), extraction efficiency, and complex compound stability for solutions of the [Cu I (baphen) 2 ] + in six studied organic solvents. solvent ε 460 ε 460 extraction efficiency [%] stability [l mol -1 cm -1 ] [l mol -1 cm -1 ] water-ethanol (1:1) min. 2 hours butyl alcohol min. 2 hours n-amyl alcohol min. 2 hours i-amyl alcohol min. 2 hours diethyl ether min. 20 minutes dichloromethane min. 2 hours chloroform under 20 minutes The stability of the extracted complex compound [Cu I (baphen) 2 ] + in six solvents was also studied. The absorbance of the complex compound solutions in dichloromethane and butyl alcohol was stable for two hours. The variations from the absorbance starting value were smaller than 1.5 % of the starting value. The stability of the extracted complex compound in n-amyl, and i-amyl alcohols was not so good as in last two solvents but it was sufficiently higher. Decreasing from the absorbance starting values was about 3 % of the starting value. The studied complex compound was in these four solvents practically stable. However, the stability of the [Cu I (baphen) 2 ] + complex cation after extraction to diethyl ether was poor (especially in low concentration range). There was decline in the absorbance starting value about 20 % of the starting value. The lowest stability of the extracts was observed in chloroform. The high concentrated solutions showed particularly low stability. In this case absorbance lost 2/3 of the starting value after 20 minutes. Conclusion Presented study showed that dichloromethane, butyl, n-amyl, and i-amyl alcohols are useful solvents for extraction and sensitive photometric determination of the Cu ions with bathophenanthroline. Diethyl ether and chloroform are not suitable because the [Cu(baphen) 2 ] + complex cation is not stable in these two solvents. Also aliphatic and aromatic carbohydrates are not generally useable for extraction Cu-ions with the bathophenanthroline from water. 57

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