ANALYST, AUGUST 1984, VOL. 109 Spectrophotometric Determination of Trace Amounts of Copper Steel, Stainless Steel and Aluminium Alloys With 2,2’-Biquinolyl Tetra bromophenolphthalein Ethyl Ester 1043 in and Tadao Sakai, Noriko Ohno and Shigekazu Tsurubo Department of Chemistry, Gifu College of Dentistry, 1851- I Hozumi, Hozumi-cho, Gifu 501-02, Japan and Masaya Tanaka and Masahiro Horigome Department of Industrial Chemistry, Faculty of Engineering, Tottori University, Ko yama-cho, Tottori-shi, Tottori 680, Japan An extraction - spectrophotometric determination of copper with 2,2‘-biquinolyl and tetrabromophenolph- thalein ethyl ester (TBPE) is described. The copper - 2,2’-biquinolyl complex in 1,2-dichloroethane forms a blue ion complex with TBPE in an aqueous layer buffered at pH 6.0.The molar absorptivity of the ion complex is 9.4 x l o 4 I mol-1 cm-1 at 610 nm. The concentrations of 2,2’-biquinolyl, TBPE, hydroxylammonium chloride and tartaric acid available for 0.1 pg ml-1 of copper are 5 x 10-4 M, 8 x I O - ~ M , 1% and 1%, respectively. This method can be applied to the determination of trace amounts of copper in steel, stainless steel and aluminium a I loys. Keywords: Copper determination; 2,2’-biquinolyl; extraction - spectrophotometry; steel, stainless steel and aluminium alloy analysis 2,2’-Biquinolyl and its analogues have been used as selective reagents for the spectrophotometric determination of cop- per,’ but the molar absorptivities of these chelate compounds are very small (about 6200 1 mol-1 cm-1). Recently, many ligands have been synthesised for the sensitive and selective determination of c0pper,2-~ but the molar absorptivities are low (about 2 x l o 4 1 mol-1 cm-1) and the sensitivity is poor.Many studies on the solvent extraction of anionic and cationic chelates with a counter ion have been made for the determination of trace amounts of copper.5.6 For example, Sekine and Ohnishi7 have reported that copper can be extracted into 3-methylbutan-1-01 as CuL2X (X=anion, L= ligand, 2,2’-biquinolyl) and X can be easily replaced by a strongly coloured anion dye; Sekine and Ohnishi chose bromophenol blue. However, bromophenol blue, being a diprotic acid dye and hydrophilic, was not satisfactory because of its poor extractability and sensitivity and compound formation between the copper complex and anion varied at different pHs.On the other hand, Poe et a1.8 have reported a sensitive determination of copper using triphenylmethane dyes and quaternary ammonium salts, but the optimum pH range is very narrow compared with the method of Motomizu and Toei.6 In this work, tetrabromophenolphthalein ethyl ester (TBPE), being hydrophobic and having a larger molar absorptivity and a wider pH range, was used, allowing a more sensitive and selective spectrophotometric determination of copper. The original method1 was modified for application to the determination of trace amounts of copper in steel, stainless steel and aluminium alloy samples. Experimental Apparatus and Reagents A Hitachi Model 101 spectrophotometer and a Hitachi Model 100-50 double-beam spectrophotometer were used for record- ing spectra and absorbance measurements in a quartz cell of 1-cm path length.A Toa Electrics Model HM-5B pH meter was used for pH measurements. Extractions were carried out by shaking with an Iwaki Model KM shaker. All reagents were of analytical-reagent grade unless stated otherwise. Stock copper solution, 100 p.p.m. Prepared by dissolving 0.1 g of pure copper metal in nitric acid (1+2) and diluting to 1 1 with distilled water. The stock solution was not standar- dised. Working solutions were prepared by accurate dilution of the stock solution. M. Prepared by dissolving 0.064 g of 2,2’-biquinolyl (obtained from Nakarai Chemicals) in 500 ml of distilled l12-dichloroethane. TBPE solution, 4 X 10-3 M. Prepared by dissolving 0.7000 g of tetrabromophenolphthalein ethyl ester potassium salt (Nakarai Chemicals) in 250 ml of ethanol by heating on an electrical heater.Buffer solution, pH 6.0. Prepared from equal volumes of 0.3 M potassium dihydrogen phosphate solution and 0.1 M sodium tetraborate(II1) solution, the pH being adjusted with 1 M sodium hydroxide solution and 1 N sulphuric acid. Tartaric acid solution , 10% mlV. Prepared by dissolving 25 g of tartaric acid in 250 ml of water. 2,2’-Biquinolyl solution, 5 X Recommended Procedure Pipette 5 ml of a sample solution containing about 1 pg of copper into a 50-ml beaker and subsequently pipette 5 ml of 10% tartaric acid solution and 5 ml of 10% hydroxylammo- nium chloride solution. Adjust the pH with 1~ ammonia solution to 5.7.Transfer the solution into a 50-ml calibrated flask and dilute to 50 ml with distilled water. Transfer the solution into a 100-ml separating funnel and shake the mixture for 2 min with 10 ml of l12-dichloroethane containing 5 x l O W 4 ~ 2,2’-biquinolyl. Allow to stand for 5 min. After separation of the two layers, discard the aqueous phase completely using a dropping pipette. Prepare 10 ml of a mixture made by adding 5 ml of buffer solution (pH 6.0), 2 ml of TBPE solution and 3 ml of distilled water into a 10-ml calibrated flask. To the organic phase in the separating funnel add 10 ml of this mixture and shake for 5 min. After separation of the layers, transfer the organic phase into a test-tube through a filter-paper to remove the water droplets. Measure the absorbance of the organic phase at 610 nm against a reagent blank as reference.1044 8 0.6 C m 2 a I;: 0.4 0.2 ANALYST, AUGUST 1984, VOL.109 bance (0.04). An 8 X 10-4 M TBPE concentration was used for safety; however, the added volume of ethanolic TBPE solution was kept constant at 2 ml because the addition of an excess amount of ethanol gave positive errors in absorbance. - - - 0.8 I 1 2 * ~ 4 5 6 7 8 9 PH Fig. 1. Effect of pH on extraction. 1, Reagent blank; 2,0.1 pg ml-1 of copper(1). Conditions: tartaric acid, 1 o/o ; hydroxylammonium chloride. 1%; TBPE, 8 x 10-4 M ; reference, water 0.8 o, 0.6 0 m + 2 0.4 n a 0.2 0 1 - - _ - a - 0 = 7 - , I 1 2 4 6 8 10 Concentration of 2,2‘-biquinolyl/~ x Fig. 2. Effect of 2,2’-biquinolyl concentration on extraction.1, Reagent blank; 2,O.l pg m1-1 of copper(1). Conditions: tartaric acid, 1%; hydroxylammonium chloride, 1%; TBPE, 8 x M ; pH, 6.0; reference. water Results and Discussion Effect of pH on the Extraction of Ternary Complexes The effect of pH on the extraction of ternary complexes is shown in Fig. 1. The optimum pH range was 4.4-7, wider than that of the ion complex with bromophenol blue.7 In this work, the extraction was carried out at pH 6.0. At a pH from 7.5 to 10, the absorbances increased considerably because TBPE reacted with 2,2’-biquinolyl, being a ternary amine, to form a red charge-transfer c ~ m p l e x . ~ Also, at a pH above 10, the decomposition of the dyestuff caused a decrease in absorbance of the ion complex. Effect of 2,2’-Biquinolyl and TBPE Concentrations The effect of the concentrations of 2,2’-biquinolyl and TBPE in the extraction system was examined.2,2’-Biquinolyl (0-1 X 10-3 M) was added to 0.1 pg ml-1 of copper. As shown in Fig. 2, the extraction of the ion complex with 8 x l o - 4 ~ TBPE was approximately complete and constant at a concentration above 3 x 1 0 - 4 ~ of 2,2’-biquinolyl, but both the absorbances of the sample and reagent blank tended to be slightly greater in the presence of an excess amount of 2,2’-biquinolyl. In spectrophotometry , the smaller the absorbance of the reagent blank, the better the accuracy and the reproducibility, and so a 5 x l o - 4 ~ concentration of 2,2’-biquinolyl was chosen. On the other hand, when a TBPE concentration of above 1.6 x l o - 4 ~ was used a maximum and constant absorbance was obtained and the reagent blank also showed a small absor- Absorption Spectra The absorption maximum of the Cu(1) - 2,2‘-biquinolyl - TBPE complex was at 610 nm.In the absence of Cu(1) the organic phase was a faint-yellow colour 410 nm), and gave a very low absorbance at 610 nm. Therefore, the blue Cu(1) complex can be determined without any interference from the reagent blank. Effects of Experimental Variables Five solvents (1,2-dichloroethane, chloroform, o-dichloro- benzene, monochlorobenzene and benzene) were tested in the extraction system (Table 1). Of these solvents, 1,2- dichloroethane was the best. Both chloroform and o-dichloro- benzene showed high absorbances, but the colour stability of the organic phase was poor. On the other hand, TBPE potassium salt was extracted into nitrobenzene, isoamyl alcohol and isobutyl methyl ketone giving a blue colour even in the absence of copper and 2,2’-biquinolyl.A 1-ml volume of 10% hydroxylammonium chloride solution and 1 ml of 10% tartaric acid were used as a reductant or a masking reagent, but addition of tartaric acid and hydroxylammonium chloride solution, up to 15 mi, had no further influence. The colour intensity of the 1,2-dichloroethane layer was stable for at least 1 h. Shaking for above 30 s of the aqueous phase with organic phase sufficed for complete extraction of the ternary complex. In this work, the shaking time was fixed at 3 min. Calibration Graph and Molar Absorptivity The calibration graph with the reagent blank as a reference was a straight line passing through the origin over the range 0-0.1 pg ml-1 of copper.The apparent molar absorptivity at 0.1 pg ml-1 of copper was 9.4 x 104 1 mol-1 cm-1 and the Sandell sensitivity was 0.00068 pg cm-2. Most of the copper was transferred from the aqueous phase into the organic phase in the first extraction. The relative standard deviation for ten determinations of 0.1 pg ml-1 of copper was below 1%. Composition of the Extracted Species It is well known that copper reacts with 2,2’-biquinolyl to form a 1 : 2 complex, Cu(1) - (2,2’-biq~inolyl)~. Therefore, the composition of ion complexes in the organic phase was examined by the continuous variations method. The result obtained showed that the composition of the Cu(1) - R2 complex with TBPE was 1 : 1.Thus the extraction equilibria are as given in equations (1) and (2). Cu(I), + Xw- + R, Cu(1) - Rz - X, (1) Red TBPE,- + K,+ + Cu(1) - Rz - X, S Cu(1) - Rz - TBPE, + KXW Blue Blue (2) where X, R and K are a halide, 2,2’-biquinolyl and potassium ion respectively, and subscripts o and w refer to the organic phase and aqueous phase. Interferences For the determination of 0.1 vg ml-1 of copper by this method, the effect of co-existing ions was examined with the addition of 5 ml of 10% tartaric acid (Table 2). The addition of tartaric acid was very effective as a masking reagent. TheANALYST, AUGUST 1984, VOL. 109 1045 Table 1. Selection of organic solvents. Concentration of standard solution, 0.1 pg ml-I of copper Absorbance Dielectric Solvent constant (E) Standard solution Reagent blank A A Dichloroethane .. . . 10.36 0.770 0.041 0.729 o-Dichlorobenzene . . 9.93 0.585 0.030 0.555 Monochlorobenzene . . 5.62 0.415 0.008 0.407 Chloroform . . . . 4.81 0.576 0.014 0.562 Benzene . . . . . . 2.28 0.037 0.004 0.033 Table 2. Effect of diverse ions in the determination of 0.1 pg ml-l of copper(1) Concentration, Foreign ion p.p.m. Ba(I1) . . . . 100 Bi(II1) . . . . 100 Ca(I1) . . . . 100 Cd(I1) . . . . 100 Cd(I1) . . . . 10 Fe(I1) . . . . 100 Mg(I1) . . . . 100 Mn(I1) . . . . 100 Mo(V1) . . . . 100 Ni(I1) . . . . 100 Pb(I1) . . . , 100 Sr(I1) . . . . 100 Ti(1V) . . . . 100 Zn(I1) . . . . 100 Al(II1) . . . . 10 Cu(1) found/ Pg 0.100 0.099 0.100 0.106 0.103 0.098 0.101 0.102 0.102 0.103 0.100 0.100 0.100 0.102 0.136 Concentration, Foreign ion p.p.m.Al(II1) . . . . 5 As(II1) . . . . 5 As(1II) . . . . 1 Co(I1) . . . . 10 Co(I1) . . . . 5 Cr(II1) . . . . 10 Cr(II1) . . . . 5 Sn(I1) . . . . 5 Sn(I1) . . . , 1 Ag(1) . . , , 5 Ag(1) . . . . 1 Hg(I1) . . . . 5 Hg(I1) . . . . 1 Br- . . . . . . 100 I- . . . . . . 100 Cu(1) found/ 0.102 0.096 0.099 0.106 0.101 0.091 0.101 0.083 0.100 0.105 0.101 0.109 0.102 0.100 0.100 Table 3. Determination of trace amounts of copper in steel, stainless steel and aluminium alloys Certificate value for Copper Recovery, Sample* copper, % found,? ‘/o Yo Aluminium alloys: NBS 2024 . . . . . , 4.41 4.35 98.6 NBS 85b , , . . . . 3.99 4.04 101 NBS 101e , . . . . . 0.359 0.353 98.3 Stainless steel: Steels: NBS 19g . . . . . . 0.093 0.094 101 NBS 126b . . . . . .0.082 0.0821 100 NBS 55e . . . . . . 0.065 0.0646 99.4 JSS 501-1 . . . . . . 0.1 0.103 103 * Other components (YO) as follows. NBS 2024: Fe, 0.25; Al, 93.6; Mg, 1.49; Si, 0.12; and Zn, 0.04. NBS 85b: Mn, 0.61; Ni, 0.084; Cr, 0.211; Fe, 0.24; Mg, 1.49; and Al, 93.1. NBS 101e: Ni, 9.48; Co, 0.18; Cr, 17.98; and Mn, 1.77. NBS 19g: Ni, 0.066; Co, 0.012; Cr, 0.374; and Mn, 0.554. NBS 126b: Ni, 35.99; Co, 0.032; Cr, 0.066; and Mn, 0.380. NBS 55e: Ni, 0.03; Co, 0.007; Cr, 0.006; Mn, 0.035. JSS 501-1: Cr, 1.04; Mo, 0.17; and Mn, 0.74 t Average of 3 determinations. tolerance limits of various ions are as follows: Ba(II), Bi(III), Ca(II), Fe(II), Mg(II), Mn(II), Mo(VI), Ni(II), Pb(II), Sr(II), Ti(1V) and Zn(II), 100 p.p.m.; Cd(II), 10 p.p.m.; Al(III), As(III), Co(II), Sn(1I) and Cr(III), 5 p.p.m.; Ag(1) and Hg(II), 1 p.p.m.Other metal and non-metal ions commonly present in samples of steel, stainless steel and aluminium alloys do not interfere. Determination of Trace Amounts of Copper in Steel, Stainless Steel and Aluminium Alloys Micro-amounts of copper in standard steel, stainless steel and aluminium alloys were determined by the following proce- dure. Weigh accurately 0.1 g of the steel, stainless steel or aluminium alloy sample and dissolve in 10 ml of aqua regia. Evaporate the resulting solution to dryness and add 1 ml of hydrochloric acid (1 + 1), then dilute with distilled water to 100 ml. Take a suitable aliquot for the copper determination. The results obtained for some standard samples are shown in Table 3. The copper contents obtained by the proposed method are all in good agreement with the certified values for copper. In conclusion, the proposed method has the advantages of high sensitivity, selectivity, wide optimum pH range and low absorbance of the reagent blank and it can be applied to the determination of trace amounts of copper in a wide range of complicated materials. 1. 2. 3. 4. 5. 6. 7. 8. 9. References Diehl, H . , and Smith, G . F., “The Copper Reagents: Cuproine, Neocuproine, Bathocuproine,” G. Frederick Smith Chemical Co., Columbus, OH, 1958. Kato, T . , and Hasegawa, A . , Bunseki Kagaku, 1982, 31, 579. Isagai, K., and Isagai, K., Bunseki Kagaku, 1982, 31, 565. Silva, M., and Valcarcel, M., Mikrochirn. A m , 1977, 11, 121. Ishii, H . , Koh, H., and Satoh, K., Bunseki Kagaku, 1982,31, E389. Motomizu, S., and Toei, K., Bunseki Kagaku, 1978, 27, 213. Sekine, K., and Ohnishi, H., Anal. Lett., 1974, 7, 187. Xi-Wen, H., and Poe, D. P . , Anal. Chirn. Acra, 1981,131,195. Sakai, T., and Ohno, N., Analyst, 1982, 107, 634. Paper A31447 Received December 20th, 1983 Accepted February 20th, 1984