Polarographic analysis of lead(IV) species in solutions containing sulfuric and phosphoric acids

Electroanalytical Chemistry and Interracial Electrochemistry Elsevier Sequoia S~A., Lausanne

427

Printed in The Netherlands

POLAROGRAPHIC ANALYSIS OF LEAD(IV) SPECIES IN SOLUTIONS CONTAINING SULFURIC A N D PHOSPHORIC ACIDS

R. F. AMLIE AND T. A. BERGER*

Globe-Union, Inc., P.O. Box 591, Mihvaukee, Wise. 53201 (U.S.A.) (Received 1st October 1971)

INTRODUCTION

The possible existence of soluble tetravalent lead, Pb(IV), species in sulfuric acid solution is of particular interest with regard to the mechanism of the lead dioxide electrode in the lead acid battery 1. Much of the work reported on stability and solubility of Pb(IV) species in sulfuric acid solutions was carried out around the turn of this century and is contradictory 2. A relatively recent study by Vanyukova et al. 3 reports the determination of fl-PbO2 dissolved in 10M0 wt ~o sulfuric acid with a concentration of almost 0.2 mM in the more concentrated acid. While it is not known whether unstable Pb(IV) species are generated in H z S O 4 at the PbO2 electrode surface, the oxygen anion equilibrium mechanism proposed for nonstoichiometric metal oxide electrodes 4 does not require the existence of such species and it is now generally accepted that these ions are not detected in sulfuric acid solution 5. The existence of soluble, relatively stable lead(IV) phosphate species and compounds in phosphoric acid and in sulfuric/phosphoric acid mixtures was also reported 2 and has been confirmed by more recent studies 6- s. These plumbic phosphate solutions and compounds were prepared by chemical and electrochemical procedures. Bode and Voss 7 report the isolation of two distinct lead(IV) phosphate compounds from sulfuric/phosphoric acid mixtures. They also determined the solubility of Pb(IV) as a function of the mole ratio of sulfuric/phosphoric acids at two acid concentrations, but do not give the measurement technique. Huber and E1-Maligy s identified and characterized two phosphatolead(IV) acids which were not very sensitive to hydrolysis at room temperature, but they do not report solubility determinations. In order to follow more readily the generation of Pb(IV) species in sulfuric and sulfuric/phosphoric acid solutions, the electroanalytical procedures described were evaluated in our laboratory to determine its presence qualitatively and to provide a quantitative analysis of concentration. Since alteration of the electrolyte (e.g. making it less acidic or alkaline) modifies or destroys the complexed species s, the analyses were made directly on the sampled solutions. EXPERIMENTAL

Solutions

All solutions were prepared with doubly distilled water and reagent grade * Present address : Department of Chemistry and Surface Studies Laboratory, University of Wisconsin, Milwaukee, Wisc. 53201

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R. F. AMLIE, T. A. BERGER

chemicals unless otherwise noted. Only those solutions showing no trace of contaminants by a polarographic method were used. Lead(IV) phosphate solutions were prepared chemically by adding Pb304 to suitable sulfuric/phosphoric acid mixtures 7. Electrochemical preparations of lead(IV) phosphate solutions were made by anodizing a lead dioxide porous plate electrode (similar to the positive plate in a lead-acid storage battery) which was first partially discharged in the presence of phosphoric acid. Standard Pb(IV) solutions in sulfuric/phosphoric acid mixtures could not be prepared due to their instability toward hydrolysis. Apparatus

Polarographic measurements were made using a Heath polarography system (model EUW-401) which incorporates four chopper-stabilized operational amplifiers. The module was operated in a three-electrode mode with a platinum wire counter electrode, and either a mercury-mercurous sulfate or lead-lead sulfate reference electrode. The dropping mercury electrode (DME) was operated with a mercury column height of 45 cm and a drop time of 2.5-3.5 s in 1.260 specific gravity (35 %) sulfuric acid. Polarograms were recorded on a Moseley model 7000AM X-Y recorder. The reference electrode was mounted in a Pyrex test tube which was connected to the test cell by a capillary side-arm bridge. A Pb/PbSO4 electrode was used as a reference in some measurements to prevent the introduction of any soluble mercury by the use of a H g / H g z S O 4 reference electrode. The Pb/PbSO4 reference electrode was prepared by abrading the surface of a 0.5 cm diameter lead rod which was mounted in a vial cap. Freshly prepared Pb/PbSO4 electrodes usually gave potentials of - 0.968 _ 0.005 V vs. the Hg/Hg2SO 4 electrodes in 1.260 specific gravity HzSO 4 at 25° C and were checked prior to each use. It was found that the Pb/PbSO4 potentials were often sufficiently stable for about two weeks before re-treatment was required. Since the soluble Pb(IV) species were found to readily oxidize free mercury metal, a simple Pyrex test cell as shown in Fig. 1 was constructed to separate the mercury pool from the solution. This was accomplished by the addition of a small

Fig. 1. Polarographic Pyrex test cell: (a) DME, (b) purge tube, (c) referenceelectrodebridge, (d) test soln., (e) chloroform,(0 residual mercury. J. Electroanal. Chem., 36 (1972)

Pb(IV)

POLAROGRAPHY o r

IN H 2 S O 4 AND

429

H3P04

volume of chloroform (density = 1.50 g ml 1) which remained as a layer over the bottom mercury. The tip of the DME was placed within 3 mm of the chloroform layer to minimize contact time of detached mercury with the solutions. No mercury was dissolved prior to detachment since the DME is polarized negative to the mercury dissolution potential.

Procedures Dissolved oxygen was removed from the test solutions by bubbling dry nitrogen gas through a small purge tube (see Fig. 1) for at least 5 min, with care taken not to agitate the chloroform layer. The stream of nitrogen bubbles also removed any droplets of chloroform which may have adhered to the DME tip. All experiments were conducted in a room maintained at 25 _+I°C. Most of the linear varying potential (LVP) scans were carried out at 0.50 V min- 1 RESULTS AND DISCUSSION

Pb(IV) polarography Polarograms were obtained for many sulfuric/phosphoric acid solutions which contained soluble lead(IV) phosphate species produced both chemically and electrochemically. A typical polarogram is shown in Fig. 2 for an acid mixture containing 1.6 x 10- 3 M Pb(IV) which was chemically prepared. This characteristic curve is obtained for both the clear and the yellow solutions prepared by the addition of reagent grade Pb30 4 to the acid mixtures as reported by Bode and Voss v. The mole ratio of sulfuric to phosphoric acid is designated as ~ in conformity with Bode and Voss. Lead(IV) solutions generated electrochemically in 35 ~o total acid mixtures containing I

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4

Fig. 2. Polarogram of a soln. containing 1.60 x 10 3 M Pb(IV) phosphate species; 35 % total acids, c~(mole ratio H2SO4/H3PO4) = 50.

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99-90 acid mol % sulfuric and 1-10 acid mol % of phosphoric acid exhibited similar results. The lead(IV) phosphate polarogram in Fig. 2 exhibits a wide, nearly flat diffusion plateau extending from the dissolution potential of mercury (0.97 V) to about 0.2 V vs. the Pb/PbSO4 reference potential. This diffusion plateau is due to the reduction of the soluble Pb(IV) complex to Pb(II). Its half-wave potential cannot be determined since it is positive to the mercury dissolution potential. Another well-defined diffusion plateau with a half-wave potential (E~) of 0.15 V extends from + 0.1 to -0.45 V (H2 evolution) and exhibits a current magnitude essentially twice that of the more positive diffusion plateau. This indicates that the diffusion current arises from the reduction of the Pb(IV) complex to the metal (amalgam). Since lead does not exist in either the +1 or +3 oxidation state and Pb(II) ions would not be reduced at potentials considerably positive to the Pb/PbSO4 electrode, the plateau anodic to 0.2 V can only be due to the soluble Pb(IV) species. Moreover, the solubility of lead(II) sulfate should not exceed 10- s M in 35 ~ I--IzSO4 at 25°C I° and only a small cathodic current (~0.02 pA) with a half-wave potential (E~) at 0.0 V vs. Pb/PbSO 4 was measured in a 35 % H z S O 4 solution saturated with PbSO 4 (see Fig. 3). These polarograms corroborated other evidence that the lead(IV) phosphate complex species in acid solutions are unstable 6- s and also showed that both diffusion plateaux decreased proportionally with time. In one experiment it was found that the 4 x 10 -3 M Pb(IV) content in a 35 % (~= 50) acid solution decayed 50~ in 4.2 days at 26_+ I°C and that the rate of this decay was not dependent on the Pb(IV) concentration (i.e. zero order). This result is not regarded as descriptive of this complex system since at least two Pb(IV) phosphate species may be involved and the nature of the system is affected by composition as well as method of preparation. No detectable soluble Pb(IV) species were generated either chemically or electrochemically with a pure 35 % sulfuric acid solution, in agreement with Burbank 5. Electrochemical experiments included the cycling and overcharging of PbO 2 porous plate electrodes. Potentials are given with reference to the Pb-PbSO 4 electrode which is -0.97 V vs. the Hg/HgzSO 4 reference electrode in 35 % sulfuric acid. Potentials could also have been referred to the normal hydrogen electrode (NHE) which has been calculated to be +0.35 V vs. Pb/PbSO 4 in 33% (4.2 M) HzSO 4 solution9 or the saturated calomel electrode (SCE) which is + 0.242 V vs. NHE and may be calculated to be 0.59 V vs. Pb/PbSO4 in 35 % H z S O 4. A m p e r o m e t r i c titration o f P b ( I V )

The magnitude of the Pb(IV) diffusion current did not provide a satisfactory quantitative determination of the soluble Pb(IV) complex concentration since the DME characteristics were altered by the HzSO4/H 3PO 4 acids ratio and the total acid content. A quantitative amperometric titration procedure was therefore investigated in which the total Pb(IV) species concentration might be accurately determined. The amperometric determination of Pb(IV) content of alkaline plumbate solutions by arsenite titration has been reported 11. Addition of a sodium arsenite solution was found to progressively diminish the intensity of the yellow lead(IV) phosphate species in the acid solutions with the simultaneous formation of a lead(M) sulfate precipitate. J. Electroanal. Chem.. 36 (1972)

POLAROGRAPHY OF Pb (IV) IN H 2 S O 4 AND I-I3PO 4

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Titration of a known Pb(IV) solution with standardized N a A s 0 2 solution showed that the overall process can be described by the two-electron exchange reaction Pb 4 ÷ + A s O 2 + H 2 0 + H 2 S O 4 -+ PbSO4 + A s O 3 + 4H +. Since the arsenate oxidation product should not be reducible at the D M E in the acid solution employed 12-14 and the Pb(II) ion concentration is limited to about 6.7 x 10 -6 M (35 ~o H2SO4) by the low solubility of lead sulfate 1°, an amperometric titration of the Pb(IV) acid solution seemed feasible. Polarograms of each of the reactant and product ionic species in acid mixture supporting electrolytes were obtained to predict a potential range most suitable for end-point detection. These results are illustrated by the curves in Fig. 3 for Pb(IV), Pb(II), As(IV) and As(III) ion species in a 35 % acid solution in which ~ is 50. Curve A is a polarogram for a 3 x 10 -5 M Pb(IV) solution and the diffusion currents are 3 - 4 % of those shown in Fig. 2 for a 1.6 × 10 - 3 M Pb(IV) solution. The E~ value (+0.15 V vs. Pb/PbSO4) was not significantly altered by change in the Pb(IV)~concentration. Saturation of the acid solution with lead(II) sulfate produced only a small cathodic diffusion current in agreement with the literature 15 with an E~ value essentially the same as the Pb/PbS 04 reference electrode (curve C). It is evident from curve D that a 10- 6 M sodium arsenate solution is not reduced at the DME. No cathodic current was evident even when the arsenate concentration was increased to 3 x 10-3 M. C o m p o u n d s of trivalent arsenic exhibit complex polarographic behavior,

0.3,

A

0.1

0,20

0.10

0

-0.I0

POTENTIAL/V

-0.20

-0.30

-0.40

vs. Pb/PbSO4

Fig. 3. Polarograms in 35 % acid (~ = 50) soln, ; (A) 3 x 10 5 M Pb(IV) phosphate, (B) 2 x 10- 5 M NamsO 2, (C) ~1 x 10-5 M Pb(II), (D) 1 x 10-6 M NaAsO3. J. Electroanal. Chem.,

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R. F. AMLIE, T. A. BERGER

particularly in acid solutions where up to four waves have been observed, and widely different reduction mechanisms have been advanced 16-19. A comprehensive polarographic investigation of As(III) in sulfuric acid solutions (0-16 M) is reported by Vasileva et al. 19. Curve B in Fig. 3 for a 2 × I0-5 M NaAsO2 addition is similar to the initial portion of their (ref. 19) polarogram for 10-3 M [AsO2] in 4.6 M (36 %) sulfuric acid. We found the height of the initial plateau to be essentially proportional to the arsenite concentration when measured by the vertical distance of this plateau to a parallel base-line slope. Arsenite concentrations were therefore more reliably determined from LVP scans rather than constant potential measurements. Amperometric titration curves at three potentials taken from 0.50 V rain-1 LVP polarograms, which were run after each addition of a standardized arsenite titrant, are shown in Fig. 4. In this experiment, 10.0 ml of 1.60 M Pb(IV) in a 35 % acid solution (e = 50) was titrated with a 5.053 × 10- 3 M NaAsO2 solution and the recorded currents were corrected for the dilution effect. At +0.35 V vs. Pb/PbSO4 only the Pb(IV)/Pb(II) diffusion current is obtained, as seen in Fig. 3. The + 0.50 V curve includes diffusion currents for both the Pb(IV)/Pb(0) and the first arsenite plateau reductions. At -0.12 V the total i a for the Pb(IV)/Pb(0) reduction is added to that of the second arsenite wave and possibly Pb(II) ions. Well-defined slopes were obtained as seen in Fig. 4 which illustrates the usual experimental deviation. I

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°1o TITRATION

Fig, 4. Amperometric titration curves of 1.60 x 10-3 M Pb(IV) with 5.05 x 10-3 M NaAsOz; 35 % total acids, ~ = 50.

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POLAROGRAPHY OF

Pb(IV)

IN H 2 S O 4 AND

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The titration curves plotted at - 0.12 V and + 0.05 V exhibit well-defined and reproducible intercepts, whereas the currents recorded at +0.35 V give a relatively poorly defined end-point. It was also found that the titration curves plotted at + 0.05 V provided the most accurate determination of the equivalence point, as illustrated in Fig. 4. SUMMARY

The existence of soluble, relatively stable Pb(IV) species in sulfuric/phosphoric acid mixtures has been demonstrated by polarographic measurements. Lead(IV) species could not be detected in 35 °/o sulfuric acid solution following the identical preparation procedures. An amperometric titration technique was developed to determine quantitatively the Pb(IV) species concentration in sulfuric/phosphoric acid solutions. REFERENCES 1 G. VINAL,Storage Batteries, Wiley, New York, 4th ed., 1951, pp. 179 183. 2 J. P. MELLOR, A Comprehensive Treatise on hlorganic and Theoretical Cl~emist13', Vol. VII, Longmans, Green and Co., London, 1930, pp. 822-823. 3 L. V. VANYUKOVA,M. M. ISAEVAAND B. N. KABANOV,Dokl. Akad. Nauk, SSSR, 143 (1962) 377. 4 K. J. VETTER, Electrochemical Kinetics--Theoretical and Experimental Aspects, Academic Press, N e w Y o r k , 1967, pp. 717 721. 5 J. BURBANK, Naval Res. Lab. Report #6859, (Feb. 18, 1969). 6 E. Voss, Proc. Secondlntern. Syrup. Batteries, Bournemouth, 1960; Paper No. 16. 7 H. BODE AND E. VOSS, Electrochbn. Acta, 6 (1962) 11. 8 F. HUBER AND M. S. A. EL-MELIGY, Z. Anorg. Allg. Chem., 367 (1969) 154. 9 P. RUETSCrn AND R. T. ANGSTADT, J. Electrochem. Soc., 111 (1969) 1323. 10 D. N. CRAIG AND G. W. V1NAL, J. Res. Natl. Bur. Stand., 22 (1939) 55. 11 D. M. KERN, Collect. Czech. Chem. Commun., 25 (1960) 3159. 12 I. M. KOLTHOFFAND J. J. LINGANE,Polarography, Interscience, New York, 1946, p. 262. 13 K. BAMBAC'H,hTd. Eng. Chem., Anal. Ed., 14 (1942) 265. 14 J. J. LINGANE, Ind. Eng. Chem., Anal. Ed., 15 (1943) 583. 15 A. C. WALKER, Bell Lab. Rec., 22 (1944) 349. 16 L. MEITES, J. Amer. Chem. Soc., 76 ((954) 5927. 17 K. KACIRKOVA, Collec. Czech. Chem. Commun., 1 (1929)477. 18 E. G. VASILEVA.S. I. ZHDANOVAND T. A. KRYUKOVA, Elektrokhimo'a, 4 (1968) 19. 19 E. G. VASILEVA, S. I. ZrtDANOV AND T. A. KRYUKOVA, Elektrokhimiya, 5 (1969) 1218.

J. Electroanal. Chem., 36 (1972)