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Amperometric determination of copper with sodium picrate
ELECTROANALYTICAL CHEMISTRY AND INTERFACIAL ELECTROCHEMISTRY Elsevier Sequoia S.A., Lausanne - Printed in The Netherlands
151
Amperometric determination of copper with sodium picrate The polarographic study of picric acid and the indirect determination of scopolamine 1 showed that a solution of picric acid in buffer pH 12 gives two well defined polarographic waves (E½: 0.58 and 0.9 V). Recently, Ganichev and Dimitrova 2 used sodium picrate as a reagent for the photometric determination of copper. The present work investigates the polarographic behaviour of sodium picrate in the presence of different ions in ammoniacal media and assesses the potentiality for the amperometric titration of copper with picric acid or sodium picrate in this medium. Reagents and apparatus
The appropriate amount (5.726 g) of picric acid (Merck) was weighed and converted quantitatively to sodium picrate with standard NaOH solution using a potentiometric method. A standard solution of copper nitrate was prepared by dissolving accurately weighed electrolytic copper in dilute nitric acid, afterwards removing the excess acid. Standard solutions of others cations were prepared by dilution of standard salts solution. Aliquots of the copper solution were placed in the titration cell with the supporting electrolyte of 3 M N H 3 containing a drop of 10 M NaOH to prevent the formation of NH2 ion, and 1 ml of 0.2~ gelatin solution. The solution (20 ml total volume) was deoxygenated by passing pure nitrogen gas for 10 rain; the titration with sodium picrate was then carried out at an applied potential of - 0.4 V vs. SCE. A dropping mercury electrode was used as the cathode, and the titrant, 10 or 20 times stronger than the metal ion solution, was added in steps of 0.05-0.1 ml. The polarographic and amperometric curves were recorded with a P04 Polarograph (Radiometer, Copenhagen) using a DME as indicator and SCE as reference electrode. The characteristics of the capillary used were as follows: m, 2.15 mg s- 1 ; t, 8 s; h, 37 cm. Results and discussion
In a brief study of the polarographic behaviour of copper ammonia solution with sodium picrate, it was found that the well known diffusion-current curves for the metal ion were eliminated when the metal:reagent ratio reached 1:2. The precipitation during the titration suggests that a stable compound, Cu(NH3)4(pic)2, is formed by displacement of the phenolic sodium picrate and that 2 moles of picrate are involved in this precipitation (Fig. 1). From the current-voltage curves a potential of - 0 . 4 V, when other ions are present, was selected for the amperometric titration. A typical titration curve is shown in Fig. 2. During the titration, the current never decreased to zero but stabilized at a very low value (0.1-0.5 pA). Excellent titration curves were obtained at - 0 . 4 V. The range of metal ion concentration over which the method gives reproducible results is 0.016--0.9 mg/ml with an accuracy and precision of the order of 3%. The method was satisfactory in the presence of 10-20 fold (or more) amounts of foreign ions such as Cd(II), Zn(II), Ni(II), Co(II), Mn(II), Fe(III), As(III), Sb(III), J. Electroanal. Chem., 25 (1970) 151-154
152
SHORT COMMUNICATIONS
Fig. 1. Polarograms of 2 ml of ~.01 M copper as ammonia complex, and precipitating effect of 0.1 M sodium picrate. Initial voltage - 0 . 1 V, 0.1 V per cm chart paper, sensitivity 30, damping 5.
2B
2.0 <( 1.G
12
0.8
OA
0.1 ' £L~ ' Titiont in ml
i 0.5
,
Fig. 2. Amperometric titration curve of ammonia solution containing 1.00 mg of copper per ml, with 0.1 M sodium picrate. Voltage - 0 . 4 V.
Bi(III), U(VI), Mo(VI), operating always at - 0 . 4 V. The results and tolerance for different ions are summarized in Table 1. Ni z ÷ also gives a precipitate with picrate but only at very high concentrations. The polarographic wave in this medium is more negative than the polarographic wave of the reagent and there is, therefore, no interference. No precipitation occurs with C o 2+ but when Cu e+, or Cu z+ and Ni 2+, are present the titration of such compounds does not give satisfactory results. Ganichev et al. 3 using radioactive 6°C0 reported that coprecipitation of Co with Cu 2 + or Ni 2+ is produced in ammoniacal picric acid solution. Thus, in the presence of a 50-fold excess of Co there was no satisfactory recovery because cobalt co-precipitated with copper picrate. On the other hand, at - 0 . 4 V Sn 2+ and Pb 2+ interfered seriously. From the relative half-wave potentials, stannous tin ion should directly reduce cupric ion, but this reaction does not occur in this medium (NH3) at a measurable speed when J. l£leetroanal. Chem., 25 (1970) 151-154
153
SHORT COMMUNICATIONS TABLE 1 1.27 mg of copper in 20 ml of solution was placed in the titration cell Foreign ion added
Amount ion added~m9
Amount Cu found/m 9
Cd
2.24 44.96 1.307 13.07 1.174 23.476 1.179 11.79 1.098 10.98 1.177 5.580 1.498 14.98 2.435 121.76 4.18 20.90 4.76 23.8 1.91 9.50
1.278 1.278 1.262 1.306 1.325 1.325 1.325 1.371 1.184 1.122 1.340 1.277 1.340 1.309 1.325 1.216 1.260 1.090 1.220 1.370 1.278 1.278
Zn Ni Co Mn(II) Fe(III) As(III) Sb(III) Bi(III) U(VI) Mo(VI)
the solutions are mixed, At potentials between - 0.1 and - 0.3 V the cathodicdiffusion current of the cupric complex ion is compensated by the anodic diffusion current of the stannous ion as was observed by Lingane 4 in tartrate medium. The first cathodic copper wave occurs in the anodic part of the polarogram and the anodic stannous wave is shifted to the cathodic part (Fig. 3). However Sn can be easily separated during the
Fig. 3. Polarograms showing the interference of tin(II) in the polarographic wave of copper in ammonia media. (a) Cu alone, (b)Cu : Sn (1 : 0.5), (c) Cu : Sn (1 : 1), (d) Cu : Sn (1 : 2). Initial voltage - 0.1 V, sensitivity 30Z dampi.ng 5.
dissolution of copper-base alloy samples and the accurate amperometric titration of copper is thus possible. The metastannic acid is filtered off and determined in the usual way. J. Electroanal. Chem., 25 (1970) 151-154
154
SHORT COMMUNICATIONS
Pb 2 + interferes because the second wave of the copper complex precedes that of lead by about 0.09 V and consequently the copper first wave is completely displaced and diminished by the residual current of lead. A small excess of lead can be prevented from interfering by adding excess sodium chloride solution after the addition of the supporting electrolyte and then titrating the copper at - 0 . 4 V, i.e. a t the first wave which is not now affected by lead. Alternatively, more than a 10-fold excess of Pb can be previously separated as basic lead carbonate using concentrated a m m o n i u m hydroxide and a m m o n i u m carbonate. The precipitate is filtered off and washed with dilute a m m o n i u m hydroxide. The method was used to determine copper in copper-base alloys and in porphyric ores. Facultad de Filosofia y Educaci6n, Departamento de Quimica, Universidad de Chile, Santiago (Chile)
Alfonso Morales B. F. Gonz~ilez
1 J. P. E. WENGER,D. MONNERANDH. SCHMIDGALL,Helv. Chim. Acta, 35 (1952) 1108. 2 P. A. GANICHEVANDA. DIMITROVA,Mikrochim. Acta, 3 (1967) 507. 3 P. A. GANICHEV,I. N. BEREZINAANDA. I. GLOTOVA,Tr. po Khim. i Khim. Tekhnol., 2 (1953) 90. 4 J. J. LrNGANE,J. Am. Chem. Soc., 65 (1943) 866. Received September 29th, 1969 J. ElectroanaL Chem., 25 (1970) 151-154
151
Amperometric determination of copper with sodium picrate The polarographic study of picric acid and the indirect determination of scopolamine 1 showed that a solution of picric acid in buffer pH 12 gives two well defined polarographic waves (E½: 0.58 and 0.9 V). Recently, Ganichev and Dimitrova 2 used sodium picrate as a reagent for the photometric determination of copper. The present work investigates the polarographic behaviour of sodium picrate in the presence of different ions in ammoniacal media and assesses the potentiality for the amperometric titration of copper with picric acid or sodium picrate in this medium. Reagents and apparatus
The appropriate amount (5.726 g) of picric acid (Merck) was weighed and converted quantitatively to sodium picrate with standard NaOH solution using a potentiometric method. A standard solution of copper nitrate was prepared by dissolving accurately weighed electrolytic copper in dilute nitric acid, afterwards removing the excess acid. Standard solutions of others cations were prepared by dilution of standard salts solution. Aliquots of the copper solution were placed in the titration cell with the supporting electrolyte of 3 M N H 3 containing a drop of 10 M NaOH to prevent the formation of NH2 ion, and 1 ml of 0.2~ gelatin solution. The solution (20 ml total volume) was deoxygenated by passing pure nitrogen gas for 10 rain; the titration with sodium picrate was then carried out at an applied potential of - 0.4 V vs. SCE. A dropping mercury electrode was used as the cathode, and the titrant, 10 or 20 times stronger than the metal ion solution, was added in steps of 0.05-0.1 ml. The polarographic and amperometric curves were recorded with a P04 Polarograph (Radiometer, Copenhagen) using a DME as indicator and SCE as reference electrode. The characteristics of the capillary used were as follows: m, 2.15 mg s- 1 ; t, 8 s; h, 37 cm. Results and discussion
In a brief study of the polarographic behaviour of copper ammonia solution with sodium picrate, it was found that the well known diffusion-current curves for the metal ion were eliminated when the metal:reagent ratio reached 1:2. The precipitation during the titration suggests that a stable compound, Cu(NH3)4(pic)2, is formed by displacement of the phenolic sodium picrate and that 2 moles of picrate are involved in this precipitation (Fig. 1). From the current-voltage curves a potential of - 0 . 4 V, when other ions are present, was selected for the amperometric titration. A typical titration curve is shown in Fig. 2. During the titration, the current never decreased to zero but stabilized at a very low value (0.1-0.5 pA). Excellent titration curves were obtained at - 0 . 4 V. The range of metal ion concentration over which the method gives reproducible results is 0.016--0.9 mg/ml with an accuracy and precision of the order of 3%. The method was satisfactory in the presence of 10-20 fold (or more) amounts of foreign ions such as Cd(II), Zn(II), Ni(II), Co(II), Mn(II), Fe(III), As(III), Sb(III), J. Electroanal. Chem., 25 (1970) 151-154
152
SHORT COMMUNICATIONS
Fig. 1. Polarograms of 2 ml of ~.01 M copper as ammonia complex, and precipitating effect of 0.1 M sodium picrate. Initial voltage - 0 . 1 V, 0.1 V per cm chart paper, sensitivity 30, damping 5.
2B
2.0 <( 1.G
12
0.8
OA
0.1 ' £L~ ' Titiont in ml
i 0.5
,
Fig. 2. Amperometric titration curve of ammonia solution containing 1.00 mg of copper per ml, with 0.1 M sodium picrate. Voltage - 0 . 4 V.
Bi(III), U(VI), Mo(VI), operating always at - 0 . 4 V. The results and tolerance for different ions are summarized in Table 1. Ni z ÷ also gives a precipitate with picrate but only at very high concentrations. The polarographic wave in this medium is more negative than the polarographic wave of the reagent and there is, therefore, no interference. No precipitation occurs with C o 2+ but when Cu e+, or Cu z+ and Ni 2+, are present the titration of such compounds does not give satisfactory results. Ganichev et al. 3 using radioactive 6°C0 reported that coprecipitation of Co with Cu 2 + or Ni 2+ is produced in ammoniacal picric acid solution. Thus, in the presence of a 50-fold excess of Co there was no satisfactory recovery because cobalt co-precipitated with copper picrate. On the other hand, at - 0 . 4 V Sn 2+ and Pb 2+ interfered seriously. From the relative half-wave potentials, stannous tin ion should directly reduce cupric ion, but this reaction does not occur in this medium (NH3) at a measurable speed when J. l£leetroanal. Chem., 25 (1970) 151-154
153
SHORT COMMUNICATIONS TABLE 1 1.27 mg of copper in 20 ml of solution was placed in the titration cell Foreign ion added
Amount ion added~m9
Amount Cu found/m 9
Cd
2.24 44.96 1.307 13.07 1.174 23.476 1.179 11.79 1.098 10.98 1.177 5.580 1.498 14.98 2.435 121.76 4.18 20.90 4.76 23.8 1.91 9.50
1.278 1.278 1.262 1.306 1.325 1.325 1.325 1.371 1.184 1.122 1.340 1.277 1.340 1.309 1.325 1.216 1.260 1.090 1.220 1.370 1.278 1.278
Zn Ni Co Mn(II) Fe(III) As(III) Sb(III) Bi(III) U(VI) Mo(VI)
the solutions are mixed, At potentials between - 0.1 and - 0.3 V the cathodicdiffusion current of the cupric complex ion is compensated by the anodic diffusion current of the stannous ion as was observed by Lingane 4 in tartrate medium. The first cathodic copper wave occurs in the anodic part of the polarogram and the anodic stannous wave is shifted to the cathodic part (Fig. 3). However Sn can be easily separated during the
Fig. 3. Polarograms showing the interference of tin(II) in the polarographic wave of copper in ammonia media. (a) Cu alone, (b)Cu : Sn (1 : 0.5), (c) Cu : Sn (1 : 1), (d) Cu : Sn (1 : 2). Initial voltage - 0.1 V, sensitivity 30Z dampi.ng 5.
dissolution of copper-base alloy samples and the accurate amperometric titration of copper is thus possible. The metastannic acid is filtered off and determined in the usual way. J. Electroanal. Chem., 25 (1970) 151-154
154
SHORT COMMUNICATIONS
Pb 2 + interferes because the second wave of the copper complex precedes that of lead by about 0.09 V and consequently the copper first wave is completely displaced and diminished by the residual current of lead. A small excess of lead can be prevented from interfering by adding excess sodium chloride solution after the addition of the supporting electrolyte and then titrating the copper at - 0 . 4 V, i.e. a t the first wave which is not now affected by lead. Alternatively, more than a 10-fold excess of Pb can be previously separated as basic lead carbonate using concentrated a m m o n i u m hydroxide and a m m o n i u m carbonate. The precipitate is filtered off and washed with dilute a m m o n i u m hydroxide. The method was used to determine copper in copper-base alloys and in porphyric ores. Facultad de Filosofia y Educaci6n, Departamento de Quimica, Universidad de Chile, Santiago (Chile)
Alfonso Morales B. F. Gonz~ilez
1 J. P. E. WENGER,D. MONNERANDH. SCHMIDGALL,Helv. Chim. Acta, 35 (1952) 1108. 2 P. A. GANICHEVANDA. DIMITROVA,Mikrochim. Acta, 3 (1967) 507. 3 P. A. GANICHEV,I. N. BEREZINAANDA. I. GLOTOVA,Tr. po Khim. i Khim. Tekhnol., 2 (1953) 90. 4 J. J. LrNGANE,J. Am. Chem. Soc., 65 (1943) 866. Received September 29th, 1969 J. ElectroanaL Chem., 25 (1970) 151-154