ANALYST, APRIL 1989, VOL. 114 521 Polarographic Study of Cobalt(l1) and Manganese(l1) at a Dropping Mercury Electrode Using 2-Amino-3-hydroxypyridine as a - Complexing Agent Ashok Kumar, Ashok Joshi and Rama Kant Shukla Department of Chemistry, University of Indore, lndore 452 00 I , India The polarographic behaviour of cobalt(l1) and manganese(l1) has been studied using 2-amino-3-hydroxy- pyridine as a complexing agent at a constant ionic strength, 1.1, of 0 . 6 ~ NaC104 and a t pH 6.0 & 0.5. Well defined diffusion-controlled irreversible waves were obtained for both metals. The forward rate constant (@f,h) and the charge-transfer coefficient (an) were calculated. The diffusion current constants were 3.83 for cobalt and 3.16 for manganese, and were constant over the concentration ranges 0.50-6.00 and 0.50-8.50 mM, respectively.Based on the large difference in their half-wave potentials, a method is proposed for the simultaneous determination of these metals when they occur together in pure solutions. The method was applied to the determination of these metals in a number of standard alloys. Keywords: 2-Amino-3-hydroxypyridine; polarograph y; alloys; cobalt and manganese determination The polarographic reduction of cobalt and manganese at a dropping mercury electrode has been studied in the presence of various complexing agents such as rn-aminobenzoate, 4aminosalicylate, 2-aminopyrimidine , I sulphothalein deriva- tives,' sulphosalicylate ,3 ~-tryptophan,4 thiodiethanol,5 glycineh and E-caprolactam .7.8 In most instances mixed elec- trolytes were used and in some instances a maximum suppressor was also required. Hence, the sensitivity and accuracy of the diffusion current measurements were low.2-Amino-3-hydroxypyridine (AHP) has been used as a sensitive and selective reagent in several analytical tech- niques, e.g., spectrophotometry,g potentiometry") and polar- ography.11 However, few polarographic studies using the complexes of cobalt(I1) and manganese(I1) with AHP have been reported. One advantage of using this complexing agent is that a maximum suppressor is not required. In this work the method was applied to the determination of cobalt and manganese in various reference materials. The kinetic parameters are reported. Experimental Reagents Stock solutions of the metal ions were prepared by dissolving their analytical-reagent grade nitrate salts in doubly distilled water.A solution of 2-amino-3-hydroxypyridine (Aldrich) was prepared in purified ethanol. A NaC104 solution was used to keep the ionic strength (p) constant at 0 . 6 ~ . All chemicals were checked polarographically before use. Apparatus and Procedure For each polarographic measurement, the volume of the solutions was kept constant (20 ml) by adding the necessary volumes of doubly distilled water. Purified nitrogen was also passed through the solutions to effect deaeration. The polarograms were recorded at 298 K with a Toshniwal manual polarograph. A saturated calomel electrode (SCE) was used as the reference electrode, which was connected to the polarographic cell by means of a potassium chloride agar - agar bridge.The dropping mercury electrode had the follow- ing characteristics: mass of mercury flowing th<o;igh the capillary, rn = 2.80 mg s-1; drop time, t = 2.9 s; rn Itz = 2.43 mg + S-f ; and height of the mercury column, h,,,, = 58.8 cm. The number of electrons ( n ) involved in the reduction processes was determined by the millicoulometric method of Devries and Kroon. 12 This gave a value for n of two at pH 6.0 k 0.5 for both metals. Results and Discussion Effect of pH The polarograms for a solution containing 1 mM cobalt(I1) or manganese(I1) and 0.1 M AHP were recorded between pH 1.5 and 11.0. It was observed that the negative shift in the half-wave potential (EB) reached a maximum at pH 6.0 in both instances. Hence, this pH was chosen for all subsequent work.Effect of the Pressure of Mercury Polarograms for 0.5 mM metal and 0.1 M A€1P solutions were recorded at various heights of the mercury column. The linear dependence of the limiting current on the square root of the height of the mercury column indicates that the reduction of the metal ion is diffusion-controlled. Effect of AHP The shape of the wave does not change when the concentra- tion of AHP is increased from 0.1 to 0.5 M. The formation of the complex is evident, as the half-wave potential becomes more negative (Table 1) with increasing concentrations of AHP. The various criteria for irreversibility'3J4 indicate that the electrode reactions are irreversible at the dropping mercury electrode. Hence, kinetic parameters such as the charge-transfer coefficient (an) and forward rate constant (lcof,J were calculated by Koutecky's theoretical treatment as developed by Meites and Israel.15 The value of an was calculated by equating the slope of the graph of Ed,, versus log i/(id - i) to -0.0542/an, where id is the diffusion current and Ed,,, is the potential corresponding to this current; the value of E+ was calculated from the intercept of this graph and was used to calculate kOf,h.The data presented in Table 1 show that there is a decrease in the charge-transfer coefficient (an) and the forward rate constant (kOf,h) with an increase in the concentration of AHP, indicating the tendency of the wave to move towards irreversibility. Because of this irreversibility, no deduction could be made regarding the composition of the complexes.However, on the basis of the values of n (determined coulometrically) , the electrode reaction may be represented as M2+ + 2e- + MO(Hg), where M = cobalt(I1) or manganese(I1). The stability of the complexes is reflected in the difference between the half-wave potentials obtained for a simple metal ion and a complex metal ion. Therefore, a comparison of the values for E+ may yield information about the relative522 ANALYST, APRIL 1989, VOL. 114 Table 1. Polarographic data for the cobalt(I1) and manganese(I1) systems. Cobalt(I1) or manganese(I1) concentration = 0.5 x 10 -3 M; pH = 6.0 k 0.05; p = 0 . 6 ~ NaClO, - E; vs. Slope of AHPi iCll SCE/ logplot/ D,+/lO--? k", .h/ M pA V mV cm's-1 an cms-1 Cobalt(l1) - AHP- 0.1 4.65 1.110 81 2.71 0.669 1.77 x 10-13 0.2 4.45 1.145 84 2.59 0.645 1.58 x 10-11 0.3 4.15 1.175 86 2.42 0.630 1.20 x 10-13 0.4 3.90 1.200 88 2.27 0.616 9.80 x 10 14 0.5 3.75 1.250 91 2.16 0.590 6.48 x 10 IJ Manganese( / I ) -A HP- 0.1 3.85 1.368 86 2.29 0.630 9.76 x 10-16 0.2 3.55 1.400 88 2.07 0.616 7.77 x 10 16 0.3 3.40 1.435 90 1.98 0.602 6.15 x 10-16 0.4 3.25 1.484 93 1.89 0.583 4.67 x 10-16 0.5 3.10 1.518 95 1.81 0.571 3.75 x 10-lf~ Table 2.Polarographic determination of cobalt(I1) and manganese(I1) with AHP. [AHP] = 0.1 M ; pH = 6.0 k 0.05; sensitivity = 0.02 Amount of metalimg Relative standard Metal Taken Found* deviation, o/o Cobalt . . . Manganese . 0.590 1.179 1.768 2.358 4.715 0.549 1.098 2.196 3.794 4.392 0.585 1.188 1.784 2.380 4.675 0.552 1.105 2.210 3.752 4.370 1.02 0.76 0.91 0.93 0.85 0.55 1.01 0.64 1.10 0.50 * Average of ten replicate determinations.stabilities of the complexes under study. However, for the irreversible process the half-wave potential also depends on a, hence values of anAE, should be compared; it is found that the ZnIJ - AHP complex is stronger than the CO" - AHP complex. Effect of Metal Ion Concentration Polarograms over the concentration ranges 0.50-6.00 and 0.50-8.50 m~ CO" and Mn", respectively, were recorded in the presence of 0.1 M AHP at pH 6.0 k 0.05. The value of the diffusion current constant ( I ) was calculated using the Ilkovic equation,l-l I = i,/cm$tb , and was found to be 3.83 for COT' and 3.16 for MnII. These values were constant over the above concentration ranges.Polarographic results can, therefore, be used for the determination of these metals. General Procedure for the Determination of Cobalt(I1) and Manganese(I1) A calibration graph was constructed for both metals by recording the polarograms of each metal at various concentra- tions in a 0.1 M AHP solution at pH 6.0. The id values obtained from the polarograms were plotted against the concentration of the metal ions; the polarogram of a solution containing the metal in an unknown concentration was then recorded under identical conditions. The id values obtained from this wave were referred to the calibration graph and the concentration of the metal could then be determined. The results are given in Table 2. Ten replicate analyses of the same sample solution gave mean diffusion currents of 4.65 and 3.85 pA for Co" and Mn".respectively, with relative standard deviations of 0.95 and 1.02%, respectively. Table 3. Simultaneous polarographic determination o f cobalt(I1) and manganese(I1). [AHP] = 0.1 M ; pH = 6.0 -t 0 . 5 ; sensitivity = 0.02 Amount addedimg Amount found*/mg Error, "/o Sample Cobalt Manganese Cobalt Manganese Cobalt Manganese 1 0.560 1.605 0.554 1.620 -1.07 +0.93 2 1.585 1.900 1.598 1.916 +0.82 +0.84 3 2.575 2.225 2.562 2.205 -0.50 -0.90 4 3.540 2.870 3.560 2.892 +0.56 +0.77 5 4.126 4.118 4.160 4.148 +0.82 +0.72 * Average of seven replicate determinations. Table 4. Polarographic determination of metals in standard reference materials. [AHP] = 0.1 M ; pH = 6.0 k 0.05; sensitivity = 0.02. Alloy samples were supplied by The Iron and Steel Institute of Japan, Tokyo, Japan Amount of Relative metal/mg standard Certified composition, deviation, Sample YO Taken Found* NBS SRM 171 Magnesium Alloy .. Mn: 0.65;Zn:l.OS; Mn: 1.922 1.940 Pb: 0.0033; Ni: 0.009; Cu: 0.01 1 ; Si: 0.01 18; Al: 2.98; Fe: 0.0018 3A-30 A1 alloy (NiPPon Alumin- Mn: 0.042; Mg:O.Ol; Mn: 2.470 2.442 ium . . Zn: 0.043; Ni: 0.042; Ti: 0.638; Zn: 0.038; S: 9.96; Fe: 0.633 JSS, 607-6 High Speed Steel . . Co: 4.72; V: 0.86; Co: 1.050 1.042 W: 16.96; Mo: 0.30; Mn: 2.175 2.160 Cr: 4.14; Ni: 0.0158; Cu: 0.028; S: 0.006; P: 0.012; Mn: 0.30; Si: 0.32; C: 0.75 JSS, 655-4 Stainless Steel . . Co:0.28;Nb:0.60; Co: 2.58 2.52 Te: 0.03; W: 0.024; Mn: 1.55 1.52 Mo: 0.051; Cr: 18.54; Mn: 1 .58; Si: 0.060; S + P + C: 0.094 * Average of seven replicate determinations.Y O 0.94 1.13 0.86 0.95 0.98 1.02 Effect of Foreign Ions The following ions (in the amounts shown in parentheses) do not interfere: Pb", Cd", Hg", BPI, A+, Sb"1 and CrIII (80 mg each); Ba", Ca", Sr", Mg", RUT", Rh"*, WVI, MoVI, UVI, AuIII, PtIV, VV and Ag[ (100 mg each); Zn" (50 mg); and PdII (60 mg). Nickel(I1) interferes in both instances but can be masked by adding 15 ml of 10% sodium cyanide solution. Sodium chloride, sodium acetate, sodium sulphate, potassium bromide and potassium tartrate (350 mg each); sodium dihydrogen phosphate (400 mg) ; potassium thiocyanate and sodium citrate (850 mg each); sodium oxalate (230 mg); and EDTA, disodium salt (100 mg) are also tolerated. Mixed Polarograms of the Cobalt(I1) and Manganese(I1) Systems From the individual polarograms for cobalt(I1) and man- ganese(I1) it is apparent that it is possible to differentiate between the two metals when they are complexed with AHP as their E+ values differ by 0.3 V.Consequently, a series of polarograms were recorded for mixed, synthetic solutions ofANALYST, APRIL 1989, VOL. 114 523 these metals and the id values obtained for each metal were referred to the respective calibration graph and the metal concentrations calculated. Table 3 shows the concentrations of cobalt(I1) and manganese(I1) determined in the mixed solutions. The results obtained are accurate and reproducible. Analysis of Standard Reference Materials A 1.0-g sample of the standard reference material was decomposed with 30-40 ml of 1 + 1 hydrochloric acid and 10-20 ml of concentrated nitric acid.A 3.5-ml volume of 30% hydrogen peroxide was then added and the solution was heated on a hot-plate until the sample had dissolved com- pletely and until the volume had been reduced to about 5 ml. After cooling, the solution was diluted to 1 1 with doubly distilled water. An aliquot of this solution was taken and the metal was determined using the proposed method. The results obtained are given in Table 4. 1. 2. 3. References Corw, D. R., and Zanopoulos, J . , Anal. Chim. Acta, 1980,109, 231. Bhasin, S. K., Gaur, J . N., and Jain, D. S . . J. Electrochem. SOC., 1979, 20, 147. Ogura, K.. Murakani. S . , and Seno, K . , J. Inorg. Nucl. Chem., 1981, 43, 1243. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. Dubey, M. C., and Singh, M., Indian J. Chem., Sect. A , 1979, 18, 177. Khatri, K. C., Varshney, A., Shivahare, K., and Singh, M., Indian J. Chem., Sect. A , 1981, 20, 1144. Gritzner, C., and Redhenger, P., J. Electroanal. Chem. Interfacial Electrochem., 1980, 109, 334. Reddy, S . V. V.. Sethuram, B., and Rao, N. T., Indian J. Chem., Sect. A , 1981, 20, 1138. Puri, B. K., and Kumar, A . , Electrochim. Acta, 1984. 29, 345. Mehta, Y. L., Garg, B. S . , and Singh, R. P., Talanta, 1976,23. 53. Kalra, H. L., Malik, J. S . , and Gcra, V.. J. Indian Chem. SOC., 1982, 59, 1427. Swellen, R. S . , Pandega. K. B., and Singh, R. P., Indian J . Chem., Sect. A , 1976, 14,913. Devries, T., and Kroon, J . L., J . Am. Chem. Soc., 1953, 75, 2484. Kivaro, P., Oldham, K. B., and Laitenen, H. A., 1. Am. Chem. Soc., 1953, 75, 4146. Meites, L., “Polarographic Techniques,” Interscience, New York, 1965, p. 235. Meites, L., and Israel, Y. L., J. Am. Chem. Snc.. 1961. 83, 4403. Paper 8104504F Received November 11 th, 1988 Accepted December 5th, 1988