Indian Journal of Chemistry . Vo1.39A, May 2000, pp. 560-563 Trace metals analysis in molybdenite mineral sample Praveen Kumar Tamrakar & K S Pitre* Department of Chemistry Dr Hari Singh Gour University Sagar 470 003 (M P) India Received 12 October 1999; revised 2 February 2000 DC polarography and other related techniques, viz., DPP and DPASV have been sucessfully used for the simultaneous determination of trace metals in.molybdenite mineral sample. The polarograms and voltammograms of sample solution have been recorded in 0.1 M (NH 4 )2 tartrate supporting electrolyte at two different pH values i.e., 2.7 ±n.1 and 9.0 ± 0.1. The results indicate the presence of CI12~, Mot'+ ,Cd 2+Ni 2+,In)+, Fe)+ and WM metal ions in the sample. For the determination of tungsten(VI), II M HCI has been used as supporting electrolyte. Thngsten(VI) produces a well defined wave/peak with E.,,/Ep=-0.42V/-O.48V vs SCE in II M HCI. The quantitative analysis by the method of standard addition shows the mineral sample to have the following composition, Cu 2+(14.83), MoM (253.70), Cd 2+ (41.36), Nj2+ (16.08), In)+(3.06), Fe)+ (83.00) and WM (4.14)mglg of the sample. Statistical treatment of the observed voltammetric data reveals high accuracy and good precision of determination. The observed voltammetric results are comparable with those obtained using AAS method. Molybdenite mineral, a source of molybdenum, is associated with a large number of metals 1.3, such as Si, Mn, Fe, Zn, Pb, Ni, Co, Ca, Cd, Sn, and W. Among these trace elements the most significant is tungsten, because it finds wide applicability for industrial purposes . There are many spectral and electroanalytical methods used for the determination of tungsten and other trace metals in natural origin samples 4 •9 • The present' note describes the trace analysis of molybdenite mineral sample using DCP, DPP and DPASV. Special attention has been given for the determination of tungsten(VI) in the sample. Voltammetric results have been compared with those obtained using AAS method. Experimental Polarographic and voltammetric measurements were made on an Elico (India) pulse polarograph (model CL-90) coupled with a X-Y polarocard (model LR-108). The electrode system consisted of a dropping mercury electrode as a working electrode, a coiled platinum wire electrode as an auxiliary electrode and a saturated calomel electrode as a reference electrode. The electrochemical cell used had provision for inserting a bubbler for deaerating the solution. A carbon fibre electrode (NFI2, Sigitii Eletitiogitit, UK) was used for stripping voltammetric analysis (DPASV). All pH measurements were made on a Systronics digital pH meter (model-335). All the chemicals used were of AnalaR I BDH grade. Stock solutions of ammonium tartrate (I M), Cu 2+, Mofi+,Cd 2+, Ni 2+, In 3+, Fe3+ and W 6+ all O.OIM solutions were prepared by dissolving the required amounts of their soluble salts in doubly distilled water. Gelatin (0.1 %) solution was prepared in hot distilled water.· The molybdenite sample was collected from Malajkhand area of Balaghat district, Madhaya Pradesh, India. Procedure Finely pulverised molybdenite mineral sample (I g) was boiled gently with 60ml of conc. HNO" until the volume was reduced to about 15 ml. Conc. HCI (40ml ) was added to the solution and evaporated to a small volume. After cooling the sample solution was transferred to a 250 ml beaker. The residue that was not decomposed by this treatment was placed in a platinum crucible and heated with lOml of conc . HCI0 4 for about 10 min . and then with lOml of conc. HF to dissolve the residue. Both the sample solutions were mixed and total volume of the sample solution was made up to 100 ml with doubly distilled water. To 10 ml of the above molybdenite sample solution, 10ml of 1M (NH4)2 tartrate and 1 ml of 0.1% gelatin solution (maximum suppressor) were added and the final volume was made up to 100 ml with distilled water. The pH of the test solution was first adjusted to 2.7 ± 0.1 and then to 9.0 ± 0.1, as the analysis of the sample is carried out at two different pH values. The analyte was placed in a polarographic cell equipped with the electrode assembly specified above. Pure nitrogen gas was passed through the test solution for 15 min. The polarograms and 561 NOTES \ (a) Fe , Jl (b) F, Ni+ln t r f- ~I cr~ a: a: f- ~ I'~. a: ~ Cd . Cd :) u N 0 CU Mo u I -O.OV 200 my I----t -VOLTAGE_ -O.OV 200my f----I VOLTAGE- Fig. I-(a) Direct current polarogram; (b) Differential pulse polarogram of the molybdenite sample in O.IM ammonium tartarate + 0.001 % gelatin , [PH =2.7 ± 0.1] voltammograms were then recorded with the following instrumental parameter: sensitivity 11lA/ V, C C compensation 5, I R compensation 4, pulse amplitude 50 mY, drop time 0.5 s, time constant 10 ms and .scan rate 12m V S-I • For DPASV the deposition potential was fixed at -2.0 V in the case of 0.1 M (NH 4)2 tartrate and -1.2 V in the case of 1 I M HCI as supporting electrolyte. The deposition time was 60 s and 10 s resting period. Results and discussion The direct current polarogram and differential pulse polarogram (Fig. 1 a and b) of the sample solution in 0.1 M (NH 4)2 tartrate (pH= 2.7± 0.1) showed three distinct polarographic waves/peaks with E,i Ep values =-0.09V/-0.lOV, - 0.37V/-0.36V and - 0.60V/-0.68 V vs SCE, indicating the presence of Cu2+, MoM and Cd2+, respectively. In addition to this, two more peaks were observed with E p=-1.32V and -1.S8V vs SCE, which by changing the pH of the test solution to 9.0 ± 0.1 was split into three waves/peaks with E,iEp=1.1OV/-1.16V,-1.32V/-1.38V and -1.63V/-1.64V vs SCE 10, corresponding to the presence of Ni 2+, In 3+ and Fe 3+ respectively in the sample. . The differential pulse anodic stripping voltammograms were recorded for the sample solution under identical experimental conditions, using glassy carbon fibre electrode. The voltammograms showed well defined peaks with Ep values =-0.04V, -0.38V, -0.66V, -1.26V,-1.34V and - 0.65V vs SCE, indicating the presence of Cu 2+, M 0 6+'Cd 2+, Ni 2+, In 3+ and Fe 3+ respectively in the sample. To investigate the presence of tungsten(VI) in the sample, 11M HCI was used as supporting electrolyte. INDIAN J CHEM, SEC. A, MAY 2000 562 Table I ~ Results on molybdenite mineral sample analysis for metal ions (mgtl) Metal ions DCP Parameter Added Found Amount 14.62 Cu 2+ R% C.V Amount R% S.D. C.V 255.20 99.53 0.08 0.03 Amount 41.58 Cd 2+ R% CV Amount R% S.D. CV 15.85 97.24 0.04 0.25 Amount 3.44 In 3+ R% CV Amount Fe 3+ R% S.D. cv. 83.77 99.47 0.05 0.06 4.04 W 6+ R% S.D. CV 253.70 506.55 99.02 0.05 1.20 * Average of four determinations Recovery, % R% = Standard Deviation S.D. Coefficient of Variance , % C.V. = Added 41.36 82.12 99.52 0.04 0.00 16.08 31.45 15.85 98.49 0.05 0.31 3.06 6.42 3.44 98.61 0.05 1.63 83.00 165.90 83.77 99.59 0.03 0.04 4.14 8.12 99.26 0.06 1.44 83.00 166.10 83.77 99.47 0.02 0.02 4.04 3.06 6.41 3.44 98.76 0.Q2 0.65 83.00 165.90 16.08 31.45 15.85 98.49 0.05 0.31 3.06 6.30 41.36 82.55 41.58 99.37 0.04 0.00 16.08 31.05 253.70 507.10 255.20 99.64 0:04 O.oI5 41.36 82.42 41.58 14.83 29.40 99.83 0.05 0.34 253.70 506.90 255.20 Found 14.62 99.W 0.08 0.03 4.14 8.10 Amount Found 99.83 0.Q2 0.13 96.92 0.Q2 0.65 S.D. DPASV 14.83 29.40 14.62 99.oI 0.03 0.07 S.D. Ni 2+ 14.82 29.38 Added 99.76 0.05 0.34 S.D. M 0 6+ DPP * 4.14 8.14 4.04 99.5 0.04 0.96 NOTES The dc polarogram and differential pulse polarogram of the sample solution in 11M HCI produced well defined wave/peak with E,;/Ep value= -0.42V/-0.48V vs SCE 11, indicating the presence of tungsten(VI) in the sample. The differential pulse anodic stripping voltammogram of the sample solution in 11 M HCl also showed a well defined peak with Ep=-0.44V vs SCE. Based on the presence of the said metal ions in the sample, some synthetic samples with varying concentrations of the said metal ions were prepared and their polarograms and voltammograms were recorded. The concentration of each metal ion (taken/ found) in synthetic samples using DPP method were found to be in excellent agreement. The minimum tried detection limits of the DCP, DPP and DPASV techniques for the measurement of the individual and combined metal ions were determined. Of these, Cu 2 +, Mo6+ and Cd 2 + were determined in one run at pH = 2.7 ± 0.1 and other metal ions, viz., Ni 2+, In 3+ and Fe3+ at pH = 9.0 ±O.l. while for W 6 +, 11M HCl was used as supporting electrolyte. The detection limits were examined by preparing synthetic samples. The data shows that the DPASV method using glassy carbon fibre electrode is highly sensitive in determining the reported metal ions down to nanogram. level, i.e., for Cu 2+(6.3), Mo6+(9.6),Cd 2+(6.0), Ni 2+(6.2)and Fe 3+(2.8)ngl· 1 in both cases individual and combined. For In 3+ (5.8) and W6+ (9.5)ngl- 1 in case of individual and 11.6 ngl- I and 19.0 ngl- I respectively, in case of combined were detected. After ascertaining the presence of the said metals, the quantitative analysis of the sample for its metals content was carried out using the method of standard addition. It is quite clear from the data in Table 1 that the recovery is over 99% for most of the metal ions, with high accuracy and precision of determination. 563 Analysis of molybdenite mineral sample show the presence of Cu 2 + (14.83), M0 6 + (253.70), Cd 2 + (41.36), Nj2+ (16.08), In 3+(3.06), Fe3+ (83.00) and W6+ (4.14)mg/g in the sample. These results were compared with those obtained using atomic absorption spectroscopic method which revealed the following results, i.e., Cu 2+ (14.60), Mo6+ (249.52), Cd 2+ (40.98), Nj2+ (16.02), In3+(3.1O), Fe 3+ (82.50) and W 6+ (4.09) mg/g in the sample. · The agreement between two data demonstrates the utility of voltammetric methods for such an analyses. Further, the voltammetric techniques are simple, fast and economic. Acknowledgement The authors are highly grateful to Prof S P Banerjee, Head, Department of Chemistry, Dr. H. S. Gour University, Sagar, for providing the necessary laboratory facilities. References I 2 3 Orloy V G, SOy Geol, 5 (1989) 43. Bhargava M & Pal A B, J Geol Soc India: 53 (1999) 675 . 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