Metal chelates of phosphonate-containing ligands

MICROCHEMICAL

JOURNAL

Metal Chelates

30, 6- 11 (1984)

of Phosphonate-Containing

VII. Analytical 1-Hydroxyethane-1

Ligands

Applications of ,l-diphosphonic Acid

M. T. ZAKI, M. I. ISMAIL, AND E. N. RIZKALLA~ Faculty

of Science, Ain Shams Universiry,

Abbassia,

Cairo, Egypt

Received January 1.5, 1982

Organic phosphonates have found widespread applications in many aspects (7). Particular emphasis was given to the use of these reagents in metal ion sequestration and complex formation (I, 3, 4). The use of these reagents for the analytical determination of metal ions have attracted little attention (IO). Pribil and Veseley (8) were able to use lhydroxyethane- l,l-diphosphonic acid (HEDP) as a titrant for thorium. Quantitative determinations of the phosphonate groups by procedures based upon the complexing ability of these compounds with different metal ions were also reported (5). Recently (9), a detailed potentiometric study of the complexes formed between HEDP and some metal ions including the alkaline earths, transition and nontransition mono-, bi-, and tervalent metal ions was reported. The titration curves revealed that the formation of bis complexes are limited only to zinc, mercuric, and tervalent metal ions and that copper, cobalt, and nickel are bound to form 1:1 complexes. Both protonated and unprotonated species were inferred. Increasing the pH of the medium to a value >lO is always accompanied by the complete deprotonation of these species. In the present investigation, an attempt is made to study the polarographic behavior of copper, zinc, antimony, and lead and to analyze these elements either individually in the presence of cobalt and nickel ions or simultaneously in admixture. EXPERIMENTAL Reagents and procedure. Approximately 60% (w/w) aqueous solution of HEDP has been kindly donated by Monsanto Chemicals Europe-Brussels-Belgium. Stock solutions of this reagent were prepared by the proper I Present address: Chemistry Department, Florida State University, Tallahassee, Florida 32306. 6 0026-265X/84 $1.50 Copyright 0 1984 by Academic Prcsr, Inc. All rights of reproduction in any form reserved.

ANALYTICAL

APPLICATIONS

OF HEDP

7

dilution and the exact molarity of these solutions were measured potentiometrically (9). Metal salt solutions were standardized with EDTA (12). Iodometric procedure was followed in the case of antimony (Ii). The polarograms were recorded with a radiometer polarograph Model PO 4. The electrolysis cell has been described before (2). The dropping mercury electrode has the open-circuit characteristics, m = 2.074 mgi set and t = 4.1 set at a mercury height of 50 cm. An external saturated calomel electrode was used as a reference electrode and the cell was thermostated at 25 + 0. 1°C. Prior to each run, nitrogen gas was bubbled through the polarographic cell. The hydrogen ion concentration was recorded with a radiometer pHmeter Model M 62 fitted with a combined glass-calomel electrode. RESULTS AND DISCUSSION

Polarographic Behavior of Metal Ions in the Presence of HEDP The HEDP complex of Cu(I1) ion gave a well-defined single wave of a diffusion-controlled nature. Plots obtained for E vs log (i/(id - i)) showed curvature with a nonintegral values of n. Consequently, it is possible to assume that the process involved is not a reversible one, and in such cases predicting an II value from the slope of the plots is impossible. The polarograms shows also the presence of a second reversible wave (inverse slope = 0.024) at a less negative potential. To study the effect of concentration of free hydrogen ion on the wave parameters E+ and id, solutions of Cu(II) and HEDP at a constant molar ratio of M:L (1:2.3) and at varying pH values (ca. 1.8-12.0) were polarographed under similar conditions. The results obtained, clearly indicate an increase in id value of the irreversible wave at the expense of the reversible one and limiting values are obtained at pH values 1.8 and 11.8 The exclusive appearance of the reversible wave in strong acidic media and its complete suppression in strongly alkaline solutions, where the metal ion is completely sequestered by the phosphonate ligand, lead to the assignment of the reversible and irreversible waves to the reduction of the free metal ion and its complex(s), respectively. Further evidence to this assignment was obtained from the relative id values obtained for solutions of Cu(II) with variable contents of HEDP at a fixed pH value (ca. -6). The increase in ligand contents was accompanied by an increase in the ratio id(irrev.)/id(rev.). In the case of Co(H) and Ni(II) complexes, only one wave is obtained, the height of which is a decreasing function of the pH and HEDP concentration (Table 1). At pH X3 and metal to ligand molar ratio 1:2.3, the waves are completely suppressed. The E,,?of the waves was found to be dependent on both ligand concentration and the pH. An increase in either,

8

ZAKI, ISMAIL,

EFFECTOF pH ON &

AND RIZKALLA

TABLE 1 VALUES OFTHE ANODICPOLAROGRAPHIC WAVESOF Co(I1) AND Ni(I1) IONSIN THE PRESENCEOF HEDP

Co(I1) PH 4.3 5.2 6.0 6.5

Ni(I1)

ida 6.32 4.40 2.72 1.56 7 to -12 residual

EV2

PH

-1.28 - 1.38 - 1.41 - 1.44

4.0 5.0 5.6 6.9

Note. Test solutions: (A) [Co(II)], 59 &ml, [HEDP], 2.3 rnkf; and pH as indicated. a In microamperes. b In volts vs WE.

i,” 6.72 6.40 3.20 1.60 8.1 to -12 residual

EI,~ -1.05 - 1.09 - I .07 - 1.13

[HEDP], 2.3 mM; (B) [Ni(II)], 58 kg/ml,

causes a negative shift in the half-wave potential. This behavior is indicative of the presence of equilibria involving the electroactive species. In the region of excess HEDP concentration and at pH >8, more than 90% of the metal ion is in the form of MHL species (9), which suggests that this species and its deprotonated form are inactive toward reduction. These findings lead to a study of the experimental conditions which would enable copper to be determined polarographically with HEDP serving as a masking agent for both Co(H) and Ni(I1) ions. Similar studies were carried out to demonstrate the validity of this method to analyze for mixtures of M Zf, Co(II), and Ni(I1) with M being Pb(II), Sb(III), or Zn(I1) ion, since mixtures of these elements, including copper, constitutes the base of different commercially available alloys. Lingane (6) has shown that the reduction of the biplumbite ion from strongly alkaline media is reversible at the dropping mercury electrode and the half-wave potential was found to increase linearly with the concentration of hydroxide ion according to the relationship:

E = - 0.765- 0.083 log COH = - 0.574 (at pH 11.8) In the presence of HEDP and at pH 11.8, lead was found to produce a quasi-reversible diffusion-controlled wave with an inverse slope of 0.041 and E,,?value of - 0.83 V vs SCE. The observed differences in the nature of the waves and the shift in El,* values (ca. 256 mV) suggests that the electroactive species is a lead-phosphonate complex rather than the simple HPbO; ion with the former being more stable toward reduction compared to the hydrogen plumbite ion. Potentiometric titration curves obtained by titrating mixtures of zinc

ANALYTICAL

APPLICATIONS

9

OF HEDP

TABLE 2 CURRENT-CONCENTRATION DATA (A) Analysis of the individual elements in the presence of 0.5 mM Co(B) and Ni(II) ions Zn(I1) Sb(III) Cu(I1) Pb(I1) Cont. wm

10.8 21.0 35.6 64.2 80.7 92.1 122.0 156.9 188.7 (id/C)O

Corr. coeff. (B) Analysis and 8.4 mM 31.8 45.1 66.7 110.6 129.0 183.6 251.0 c&m Con-. coeff.

id (1.4

0.85 2.25 3.78 6.80 8.20 10.18 12.68 16.50 19.56 6.65 0.9993

Cont. (wm)

12.8 34.1 53.4 76.8 107.8 133.4 168.6 220.9 308.4

id WV

0.40 1.70 2.80 4.00 6.00 7.68 9.56 12.60 17.28 6.14 0.9997

Cont. (ppm)

id (PA)

1.44 9.2 14.1 2.00 31.6 4.76 54.1 7.90 66.0 9.70 74.5 10.90 113.8 16.10 142.5 19.70 167.4 23.84 9.13 0.9996

Cont. (mm)

id (14

15.8 1.00 40.2 2.50 79.1 3.20 108.4 7.24 141.2 9.40 169.3 11.10 216.7 14.10 270.3 18.20 326.3 21.56 8.31 0.9958

of the quaternary mixtures in the presence of 0.5 mM of all other elements HEDP 31.3 4.70 60.9 3.98 3.50 52.3 3.10 46.4 6.64 99.8 6.48 4.90 82.2 4.78 60.8 8.80 130.3 8.50 6.00 123.8 6.10 71.3 10.30 153.4 9.90 9.95 172.1 10.00 95.5 13.48 175.3 11.52 13.38 226.3 12.90 117.0 16.22 222.8 14.72 18.46 281.7 16.08 148.4 21.00 284.9 18.50 25.68 348.9 19.86 9.03 7.96 6.48 6.13 0.999 0.999 0.997 0.998

Note. The id was measured at E = -0.70 V Cu(I1); E = - 1.00 V Pb(II); E = - 1.40 V Zn(II); and E = - 1.80 V Sb(III) vs PA mmol-’ I-‘. ” Current sensitivity.

nitrate and HEDP solution in a molar ratio of 1:2 shows that at the working pH of 11.8, the completely deprotonated ZnL8,- species is dominant. The polarograms obtained for this system appeared to show the conversion of ZnLi- to Zn” and 2L5-. The irreversible wave observed has an E,, value of - 1.55 V vs SCE. Antimony(II1) followed essentially the same pattern as zinc, a simple irreversible wave was obtained in strongly alkaline HEDP medium with an inverse slope of 0.186 and Eli2 of - 1.17 V vs SCE. Analytical Applications Suitable aliquots of the ternary mixtures copper (lead, antimony, or zinc), cobalt, and nickel were mixed with a large excess of HEDP solution

10

ZAKI, ISMAIL,

AND RIZKALLA

Ev. vs. S.C.E.

FIG. 1. Effect of Sb(II1) concentration on the polarographic behavior of the mixture in the presence of 8.4 mM of HEDP and at pH 11.8. [Sb(III)]: (1) 0.50; (2) 0.81; (3) 1.06; (4) 1.25; (5) 1.44; (6) 1.68; (7) 1.85; and (8) 2.34 mM.

to ensure the formation of ML species (ML, in the case of zinc and antimony). The pH of the mixture was then adjusted to 11.8 by adding a predetermined quantities of sodium hydroxide. Under these conditions, cobalt and nickel ions form a stable irreducible phosphonate species. The polarograms of these mixtures were then recorded and the limiting currents were measured at -0.70, - 1.00, - 1.40, and - 1.80 V for copper, lead, antimony, and zinc, respectively. The results obtained are summarized in Table 2A. Plots of id values vs metal ion concentration are invariably linear with a correlation coefficient >0.99 in all cases. The simultaneous analysis of the quaternary mixtures of Cu(II), Pb(II), Sb(III), and Zn(I1) ions in the presence of Co(I1) and Ni(I1) ions were carried out by fixing the concentrations of all the elements (at 0.5 mM) except for the element under test which was successively increased. Figure 1 represents a typical example of the polarograms obtained. As is shown, in each case four well developed waves at half-wave potential values, -0.51, -0.83, -1.17, and -1.55 V vs SCE are obtained corresponding to the reductions of Cu, Pb, Sb, and Zn species respectively. The results obtained are summarised in Table 2B. Again the currentconcentration plots are linear, however, the current sensitivity factor (idI C) gets slightly lower compared to that obtained in the individual determinations. The data indicate that the procedure presented is an accurate and simple method to determine the considered elements in the presence of each other. The fact that cobalt and nickel phosphonate complexes are electrically inactive, as stated before, renders the determination of the active phosphonates of copper, lead, zinc, and antimony a selective one in the presence of these elements.

ANALYTICAL

APPLICATIONS

OF HEDP

11

SUMMARY The polarographic behavior of copper, lead, antimony, and zinc ions in the presence of the title ligand, HEDP, is discussed. In highly alkaline solutions, the reversible wave of copper splits into two components, reversible and irreversible, the first of which is attributed to the reduction of the free cupric ions and the second wave was assigned to the reduction of the copper phosphonate species. The polarograms of zinc and antimony showed the presence of a single irreversible wave. In the case of lead, a quasi-reversible wave is observed. Cobalt and nickel forms inactive phosphonate chelates as inferred from the suppression of the wave heights with increasing of both the concentration of the ligand and the pH of the medium. In the presence of sufficiently excess HEDP buffered to pH 11.8, mixtures of the six elements showed the presence of four reasonably separated waves at half-wave potential values - 0.51, -0.83, - 1.17, and - 1.55 V vs SCE corresponding to the reductions of copper, lead, antimony, and zinc complexes, respectively. The possibility of the individual and simultaneous analysis of these elements in their ternary mixtures with cobalt and nickel is discussed.

REFERENCES 1. Choppin, G. R., and Rizkalla, E. N., Complexation by Organic Phosphonates. Rev. Iflorg. Chem, in press. 2. Issa, I. M., and Tharwat, M., Effect of Surfactants on the Polarographic Reduction of Hexavalent Uranium in Sulphuric Acid Solutions. Electrochim. Acta 17, 343-349 (1972). 3. Kabachnik, M. I., Medved, T. Ya., Dyatlova, N. M., Arkhipova, 0. G., and Rudomino, M. V., Organophosphorus Complexones. Russ. Chem. Rev. 37(7), 503-518 (1968). 4. Kabachnik, M. I., Medved, T. Ya, Dyatlova, N. M., and Rudomino, M. V., Organophosphorus Complexones. Russ. Chem. Rev. 43(9), 733-744 (1974). 5. Liggett, S. J., and Libby, R. A., Spectrotitration of Ethane- I-hydroxy-1, I-diphosphonic Acid (EHDP) with Thorium-Diaminocyclohexanetetraacetate. Talanta 17, l1351140 (1970). 6. Lingane, J. J., Interpretation of the Polarographic Waves of Complex Metal Ions. Chem. Rev. 29, 1-35 (1941). 7. Monsanto Technical Bulletins No. IC/SCS-320, 321, 322, and 323. 8. Pribil, R., and Veseley, V., I-Hydroxyethylidene-1, I-diphosphonic Acid as a Titrimetric Agent. 14, 591-595 (1967). 9. Rizkalla, E. N., Zaki, M. T. M. and Ismail, M. I., Metal Chelates of PhosphonateContaining Ligands. V. Stability of Some 1-Hydroxyethane-l,l-diphosphonic Acid Metal Chelates. Tdanta 27, 715-719 (1980). 10. Szczepaniak, W., Siepak, J., and Kuczynski, K., Phosphoorganic Complexones in Chemical Analysis. Chem. Anal. (Warsaw) 23, 211-223 (1978). 11. Vogel, A. I., “Text Book of Quantitative Inorganic Analysis,” p.362. Longmans, London, 1951. 12. West, T. S., “Complexometry with EDTA and Related Reagents.” Broglia Press, London, 1969.