Analyst, January 1996, Vol. 121 (79-81) 79 Electrochemical Trace Analysis of Gold in Ore Jyotsna Shukla and K. S. Pitre Department of Chemistry, Dr. Hari Singh Gour University, Sagat (M.P.) 470 003, India Differential-pulse polarography (DPP) has been successfully used for the simultaneous determination of some trace metal ions in ore. Ammonium tartrate (0.1 mol 1-1) was used as supporting electrolyte and 0.01% gelatin as maximum suppressor. The sample produced eight well defined polarographic peaks. The first signal with E , = -0.08 V versus SCE in the polarogram may be due to Agl or Ad1'. To confirm the presence of Agl and/or Ad1' in the sample, EDTA was used as a complexing agent, which resulted in this single peak splitting into two, with E , equal to -0.04 V and -0.2 V versus SCE, confirming the presence of both Agl and AulI1 in the sample. The observed voltammetric results were compared with those obtained by using AAS.Keywords: DifSerential pulse polarography; trace metal; chalcopyrite; gold Introduction Chalcopyrites are a known source of gold.'-2 Often, these ores are also associated with galena and sphalerite.34 Galena is a known source of silver.5-6 Thus, the ore may contain silver and gold and also other trace metals. In order to determine Au and Ag at trace levels, various electrochemical methods have been employed.7-11 The accuracy and reliability of trace analysis in chalcopyrite by using voltammetric methods has been investi- gated. Experimental Apparatus Polarographic measurements were made on an Elico Model CL- 90 (Hyderabad, India) pulse polarograph.The electrode system consisted of a dropping-mercury electrode (DME) as working electrode, a coiled platinum wire as auxiliary electrode and a saturated calomel electrode (SCE) as reference electrode. All pH measurements were made on an Elico Model LI- 120 digital pH-meter. Chemicals and Reagents Merck (Darmstadt, Germany) analytical-reagent grade chem- icals were used. Stock solutions of ammonium tartrate (1 mol 1-I), Ag, Au, Cu, Cd, Pb, Zn, Fe and Cr (0.01 mol 1-l) and EDTA Na2 (0.01 mol 1-l) were prepared by dissolving their requisite amounts in doubly distilled water. Gelatin solution (0.01%) was prepared in hot (5040°C) distilled water. The solutions were standardized and diluted as required. Preparation of the Sample Solution Chalcopyrite ore (containing sphalerite and galena) was obtained from Rajpur Dariba, Udayapur, Rajasthan, India.Finely pulverized ore sample (1 g) was dissolved in 10 ml of hydrobromic acid and evaporated to dryness. The dry residue was dissolved in 10 ml of 16 mol 1-1 nitric acid and again evaporated to dryness. This dry residue was dissolved in 10 ml of ammonium hydroxide containing 3 g of tartaric acid. The final volume was made up to 100 ml with distilled water.12 Preparation of the Analyte and Recording of the Voltammogram The sample solution (10 ml) was mixed with 10 ml of 1.0 moll-' ammonium tartrate as supporting electrolyte and 10 ml of 0.01% gelatin as maximum suppressor in a polarographic cell. The final volume was made up to 100 ml with distilled water.The pH of the test solution was adjusted to 9 k- 0.02, with ammonium hydroxide. The analyte was placed in a polarographic cell. Pure hydrogen gas was bubbled through the test solution for 15 min and the pH of the test solution was checked before recording the polarograms. Results and Discussion The differential pulse polarograms of the sample solution (Fig. 1) showed eight well defined peaks with Ep = -0.08; -0.28; -0.4; -0.63; -0.92; - 1.14; -1.34; and -1.44 V versus SCE indicating the presence of Ag1/Au1I1; Cu"; Pb"; Cd"; Nil'; Zn"; Fell1; and Cr"', respectively. Table 1 Instrumental parameters Parameter Value Parameter Value Initial applied voltage Sensitivity Charging current compensation IR compensation Height of Hg column Time constant Pulse amplitude Temperature 0.0 V versus SCE 10 PA 3.5 5.0 140.0 cm 5.0 ms 50 mV 25 f 2 "C Drop time 1 s Scan rate 12 mV s-1 Aquisition Fast (6 mV s-I) Output Zero* 5.0 PH 9.00 * 0.02 On Polarocard x-Axis 200 mV cm-1 y-Axis 200 mV cm-' * The value was fixed according to the instruction manual.200 mV H Fc c C 3 p! 0 Voltage Fig. 1 Differential-pulse polarogram of 100 mg per 100 ml of ore sample (including chalcopyrite, galena and sphalerite) in 0.1 mol 1-l ammonium tartrate + 0.01% gelatin at pH 9 k 0.02.80 Analyst, January 1996, Vol. 121 To confirm the presence of the above-mentioned metal ions in the sample, a known quantity of standard solution of each metal ion was added to the analyte and the resulting polarogram was recorded (which increased the observed wave height of each metal ion signal without any change in E , values).After confirming the presence of these metal ions in the sample, some synthetic samples of various ion concentrations were prepared (Table 2) and their polarograms were recorded under identical conditions. The results indicated no change in Table 2 Analysis of synthetic samples* Composition of synthetic sample- Cu 0.47 0.63 0.95 1.90 Pb 2.33 3.10 4.66 9.32 Cd 0.01 0.02 0.03 0.07 Ni 0.07 0.09 0.14 0.29 Zn 0.10 0.14 0.21 0.43 Fe 0.43 0.48 0.72 1.45 Cr 0.06 0.07 0.10 0.21 Amount found by using DPP- Cu 0.46 0.62 0.96 1.89 Pb 2.32 3.11 4.65 9.31 Cd 0.01 0.02 0.03 0.07 Ni 0.07 0.09 0.15 0.29 Zn 0.10 0.14 0.23 0.43 Fe 0.42 0.48 0.71 1.45 Cr 0.05 0.07 0.11 0.20 * Average of four determinations ( 10W2 pg I-').Voltage Fig. 2 Ag in 0.1 mol 1-1 ammonium tartrate +0.01% gelatin at pH 9 f 0.02. Differential-pulse polarogram of Na2EDTA-Au and Na2EDTA- Table 3 Minimum investigated detection limits Metal ion DPP/pg ml-1 Individual Combined Individual Combined Individual Combined Individual Combined Individual Combined Individual Combined Individual Combined Individual Combined Individual Combined 0.10 0.10 0.19 0.19 0.063 0.063 0.26 0.26 0.06 0.06 0.06 0.06 0.64 0.64 0.56 0.56 0.52 0.52 half wave potential (Fig. 3) of the above-mentioned metal ions. The linear relationship between the concentration of each metal ion and the corresponding wave height/peak height was also unchanged confirming the possibility of accurate simultaneous, multi-element qualitative and quantitative determination of the metal ions in the sample.The concentrations of each metal ion (taken/found) in synthetic samples by using DPP is given in Table 2. Minimum Investigated Detection Limit The minimum investigated detection limits (smallest quantities that have been used) for measurement of the individual and combined metal ions are given in Table 3. Except for Ag' and Au"', all metal ions in the sample could be determined in one run. For Ag' and Au"' differential complexation of the metals with Na2EDTA was used. The detection limits were examined by preparing synthetic samples. Analysis of Gold and Silver The peak potential at -0.08 V may have been due to either Agl or Au"'. According to the literature13-14 this value is neither that of Ag and Au. To confirm the presence of Ag and Au, differential complexation of the metals with 10 ml of 0.01 moll-' Na2 EDTA, was used.As a result, the single peak split into two, with E, equal to -0.04 V and -0.2 V versus SCE, indicating the presence of both ions, i.e., both Ag and Au were present in the sample (Fig. 2). Ag-Na2EDTA (1 + 1) and Au- Voltage Fig. 3 Differentialpulse polarogram of a synthetic sample containing Cu" (1.26 mg); Pb" (6.15 mg); Cd" (0.05 mg); Nil' (0.67 mg); Zn" (0.22 mg); Fern (0.95 mg); and Cr"' (0.19 mg) per 100 ml, in 0.1 mol I-' ammonium tartrate +0.01% gelatin at pH 9 f 0.02. Table 4 Results of ore sample (mixture of chalcopyrite, galena and sphalerite) analysis for metal ions (pg 1-1). Metal Standard ion Added Found Recovery (%) deviation Ag Au c u Pb Cd Ni Zn Fe Cr - 2.16 5 .OO 190.50 828.80 6.70 29.30 45.00 140.00 20.70 - - - - - - - - 2.40 4.54 4.00 8.95 190.50 380.00 932.24 1760.00 7.00 13.65 29.70 59.00 43.20 88.10 145.00 284.00 21.30 42.00 99.6 99.4 99.7 99.9 99.6 100 99.8 99.6 100 0.02 0.01 0.01 0.02 0.04 0.08 0.03 0.02 0.02Analyst, January 1996, Vol.121 81 NaZEDTA (1 + 1) complexes were separately prepared and their polarograms were recorded. They produced well defined peaks with E , equal to -0.04 V and -0.2 V versus SCE, respectively. Similar signals were also observed using a mixture of the two complexes. After confirming the presence of Ag and Au in the sample, quantitative analysis was carried out on these two ions. Quantitative Analysis of the Sample by Using DPP Quantitative analysis of Ag, Au, Cu, Pb, Cd, Ni, Zn, Fe and Cr in the sample at pH 9 A- 0.02 was carried out by using DPP.Spiked samples were prepared in order to evaluate the concentration of each metal ion. The results are given in Table 4. The results indicate that the percentage recovery is over 99% for all metal ions, with high accuracy and precision of determination. Table 5 shows the final analysis results for the sample. These results were compared with those obtained by using AAS. The comparative data are also shown in the table. Although the Table 5 Comparison of AAS and voltammetric trace analysis data on the ore sample (mixture of chalcopyrite, galena and sphalerite) Amount found/lO-2 pg g-l Metal ion Au"' CU" Pb" Cd" Nilf Zn" Fell1 Cr"' Ag' AAS Not reported Not reported 186.00 922.00 5.00 28.00 39.00 139.00 21.00 Voltammetry 2.40 4.00 190.50 932.24 7.00 29.70 43.20 145.00 21.30 polarographic and AAS data are in good agreement for the Cu, Pb, Cd, Ni, Zn, Fe and Cr content of the sample, AAS failed to determine Ag and Au.Statistical data supports the superiority of the polarographic method for such an analysis. The authors thank Professor S. P. Banerjee, Chemistry Department, Dr. Hari Singh Gour University for providing laboratory facilities. References 1 2 3 4 5 6 7 8 9 10 11 12 13 14 Rao, S. K. L., Ind. Miner., 1974, 15, 28. Khetegurov, G. V., Geokhimiya, 1969, 2, 1362. Poddar, B. S., Econ. Geol., 1965, 60,636. Rao, R., Poddar, B. C., Mathur, R. K., and Dhara, M. K., GSI Miscellaneous Publication, 1977, 27, 347. Ganzha, T. I., Tarasenko, Pilat, V. Z., Panchenko, B. V., and Sondar, V. M., Tsvetn. Met. (Moscow), 1972, 45(6), 67. Khetreyarv, K. K., and Tallinsk, T. V., Politekhn. Inst. Ser., 1965, 228, 141. Verma, N., and Pitre, K. S., Indian J. Chem., Sect. A: Inorg. Bio- inorg., Phys., Theor. Anal. Chem., 1992, 31, 210. Verma, N., and Pitre, K. S., Analyst, 1993, 118, 65. Bharathibai, J., Rajagopalan, S. R., and Padma, D. K., Indian J . Chem., Sect. A: Inorg. Bio-inorg., Phys., Theor. Anal. Chem., 1995, 34, 320. Verma, N., and Pitre, K. S., J. Indian Chem. Soc., 1994, 71, 129. Shukla, J., and Pitre, K. S., Indian J. Chem., Sect. A: Inorg. Bio- inorg., Phys., Theor. Anal. Chem., 1996, in the press. Babaeva, Z. E., Issled. 061. Neorg. Fiz. Khim., Chem. Abstr., 91348W. Vogel, A. I., Text Book of Quantitative Inorganic Analysis, ELBS Longman, London, 1978,4th edn., p. 324. Meites, L., Polarographic Techniques, International Scientific, N.Y., 1965, 2nd edn., p. 661. Paper 51029.581 Received May 10, 1995 Accepted August 17, I995