April, 19661 HAMZA AND HEADRIDGE 237 The Polarographic Determination of Lead after Cation-exchange Separation BY A. G. HAMZA AND J. B. HEADRIDGE (Department of Chemistry, The University, Shefield 10) From ni hydrofluoric acid solution, lead, cobalt, copper(u), manganese(II), nickel and a small part of chromium(II1) are strongly adsorbed on a column of strongly acidic cation-exchange resin in the hydrogen form, while other elements present in steel are either not adsorbed or only weakly adsorbed, and are removed from the column on washing it with M hydrofluoric acid. On elution with 2 M hydrochloric acid, the lead is removed from the column and determined by d.c. polarography. This method is applied to the determination of lead (>0.01 per cent.) in steels. THE direct polarographic determination of lead in the presence of aluminium, chromium(m), cobalt, copper, iron(II), manganese, nickel, tin( 1v) and zinc is straightforward, and alloys containing these elements have been satisfactorily analysed for 1ead.l 9 2 9 3 However, the polaro- graphic determination of lead is more difficult in the presence of titanium(1v) and molyb- denum(vI), elements often present in high-alloy steels, because these species often produce reduction waves that interfere with the lead wave.Hamza and Headridge,* using M ammonium fluoride adjusted to pH 7 as the base electrolyte, obtained a reversible reduction wave for lead, E+ = -0.453 volt against a S.C.E., with which there is no interference from molybdenum(vI), titanium(1v) and vanadium(1v). However, if that base electrolyte was used for the direct determination of lead in steel, there would be interference from iron( HI), which produces an irreversible reduction wave, E, = -0-77 volt against a S.C.E., that interferes with the lead wave when the molar ratio of iron(II1) to lead exceeds 4 to 1.Although attempts were made to remove interference from iron(rI1) by reducing it quantitatively to iron(II), these were unsuccessful because iron(I1) is a powerful reducing agent in M ammonium fluoride. It was, therefore, decided to examine the possibility of separating lead from iron(m), molybdenum(v1) , titanium ( ~ v ) and vanadium(v) using a cation-exchange resin. Headridge and Dixon5 have reported that aluminium, iron(Ir1) and vanadium(v) are scarcely adsorbed on the cation-exchange resin, ZeoKarb 225, in the hydrogen form, from M hydrofluoric acid.On the other hand, cobalt, copper (11), nickel and manganese(I1) are strongly adsorbed. The behaviour of chromium( 111) was unusual. From boiled solutions, the chromium(II1) was obviously present in two complexes not in rapid equilibrium. The complex present in major amount was not adsorbed by the cation-exchange resin from M hydrofluoric acid, but the minor complex, possibly Cr(H,O) 4F,+, was strongly adsorbed. Nikitin6 has reported that lead is adsorbed by a cation-exchange resin from M hydro- fluoric acid. This is to be expected since lead, like copper(II), complexes only weakly with fluoride,' and copper(I1) is strongly adsorbed by ZeoKarb 225 from M hydrofluoric acid. arsenic(II1) and (v), antimony(II1) and (v), tin(Iv), titanium(Iv), zirconium, niobium(v), tantalum, molybdenum(v1) and tungsten(v1) are either not adsorbed or only weakly adsorbed by a cation-exchange resin from M hydrofluoric acid.6 8 9 9 A simple method is therefore available for separating lead from elements that interfere with its polarographic determination.The alloy is dissolved in a mixture of hydrofluoric and nitric acids, the excess of nitric acid is removed by evaporation, and a M hydrofluoric acid solution of the metallic ions is passed down a column of ZeoKarb 225 in the hydrogen form. Lead, cobalt, copper(rI), manganese(II), nickel and a small fraction of the chromium(II1) are adsorbed. On washing with 16 column volumes of M hydrofluoric acid, arsenic(v), antimony(v), aluminium, iron(m), tin(Iv), titanium(Iv), zirconium, vanadium(v) , niobium(v), tantalum, molybdenum(vI), tungsten(v1) and most of the chromium(II1) are removed from the column.Most, or all, of the copper(II), cobalt, chromium(m), manganese(I1) and nickel accompany the lead, but cobalt, manganese and nickel do not interfere with the polarographic determination of lead in a base electrolyte of M hydrochloric acid. Copper in amounts considerably in excess of the lead causes difficulties with the d.c. polarographic determination of lead in M hydrochloric Hydrochloric acid (2 M) was considered to be a suitable eluant for lead.238 HAMZA AND HEADRIDGE : POLAROGRAPHIC DETERMINATION [Analyst, Vol. 91 acid, but not with a differential cathode-ray or pulse polarographic determination. Chro- mium( 111) interferes with the polarographic determination of lead in M hydrochloric acid, and, if more than a trace of chromium(II1) is present, a base electrolyte of 0-5 hi acetic acid - 0.5 M sodium acetate - 0.5 M sodium chloride may be used.There is no interference from chro- mium(II1) in this base electrolyte. A polarographic method based on this scheme is now described for the determination of lead in steels. EXPERIMEKTAL APPARATUS- Polarograph-A Sargent model XV polarograph was used. Polarographic cell-This was a Meites-type H-cell with a saturated calomel electrode in the electrode compartment and an agar-saturated potassium chloride bridge. The volume of solution used in the solution compartment was 40 ml.The cell was immersed in a water tank thermostatically controlled at 25.0" C,. Oxygen-free nitrogen was used to free the solution from dissolved oxygen. Polythene column-This was constructed as follows. The bottom of part of a polythene specimen tube, of length 2.0 cm and internal diameter 1.3 cm, was drilled with eight holes of diameter 0.05 cm, and half-filled with polythene drillings. A polythene disc, of diameter 1-35 cm, was also drilled with eight holes of diameter 0.05 cm, and was forced into the specimen tube until it came into contact with the drillings. A piece of polythene tubing, 34 cm long with an internal diameter of 1.0 cm and external diameter of 1.3 cm, was then inserted into, and welded to, the specimen tube. A short piece of flexible plastic tubing was pushed over the lower end of the specimen tube and fitted with a screw clamp.An aqueous slurry of ZeoKarb 225 (SRC14), of mesh size 52 to 100, was added to the column to produce a resin bed 3.8 cm high and 3-0 ml in volume. The top 30 cm of the column acted as a reservoir. The resin was washed with 5 M hydrochloric acid to convert it entirely to the hydrogen form, and then by water until free from chloride ions. It was then ready for use. REAGENTS- Hydrochloric, hydrojluoric and nitric acids-These were of analytical-reagent grade. High-pzzrity iron-This was Specpure iron obtained from Johnson, Matthey and Company Limited. Standard lead solution-Prepare an exactly M solution from the appropriate weight of analytical-reagent grade lead nitrate crystals, the lead content of which has been previously determined by a complexometric titration with standard EDTA solution.The lead solutions used in obtaining the calibration graph are prepared from this standard lead solution by diluting with 2 M hydrochloric acid and water. METHOD- Dissolve 1 g of steel in 15 ml of 40 per cent. w/w hydrofluoric acid plus 1 ml of nitric acid, sp.gr. 1.42. Evaporate the solution just to dryness and dissolve the residue in 5ml of 40 per cent. w/w hydrofluoric acid. Re-evaporate just to dryness. Dissolve the residue in 25 ml of 10 M hydrofluoric acid and dilute the solution to 260 ml in a graduated flask. Immediately transfer the solution to a dry polythene bottle. Add a suitable aliquot of the M hydrofluoric acid solution to the column of ZeoKarb 225 resin at a flow-rate of approximately 2 ml minute-l, such that the quantity of lead, copper, cobalt, manganese, nickel and chromium(II1) does not exceed 0.8 millimoles (Notes 1 and 2).Then pass 50 ml of M hydrofluoric acid through the column at a flow-rate of approximately 2 ml minute-l, followed by 10 ml of water. NOTE 1. Only 2 per cent. of the total chromium(II1) is retained by the cation-exchange resin. Make allowance for this when calculating the quantity of adsorbable cations. NOTE 2. The column of cation-exchange resin has a total capacity of 3.2 millimoles for doubly charged ions. By restricting the total quantity of strongly adsorbed ions t o 0.8 millimoles, only the top 25 per cent. of thc column will be occupied by these ions. This is considered to be an adequate safety factor t o ensure that no lead is removed when 50 ml of M hydrofluoric acid are subsequently passed through the column.April, 19661 OF LEAD AFTER CATION-EXCHANGE SEPARATION 239 Elute all of the lead from the column with 50 ml of 2 M hydrochloric acid at a flow-rate of 2 ml minute-l, collecting the effluent in a 100-ml graduated flask.Dilute the solution to the mark with water. Place 40ml of this solution in the solution compartment of the polarographic cell and record a polarogram over the potential range of 0 to -1.0 volt against a S.C.E. When more than a trace of chromium(II1) is present in the alloy, transfer 50 ml of the M hydrochloric acid solution by pipette into a 100-ml graduated flask, make up to the mark with 2 M sodium acetate solution, and record a polarogram with this solution.Measure the lead diffusion current at -0.55 volt against a S.C.E. and determine the concentration of lead in the solution from a suitable calibration graph. Hence calculate the amount of lead in the alloy. The authors used a lead calibration graph prepared from six solutions in the concentration range of to M. The standard deviation of the error in diffusion current was 0.010 pA corresponding to a relative standard deviation of 1.4 per cent. at a lead concentration of M. The error in diffusion current is expressed by id (measured) - id (calculated), where the values of i d (calculated) are points exactly on the straight-line calibration graph of diffusion current versus concentration. RE su LTS ANALYSIS OF SYNTHETIC SOLUTIONS- Four synthetic solutions of iron(m) plus lead in M hydrofluoric acid were prepared and carried through the cation-exchange and polarographic procedures.Each synthetic solution contained 1 g of Specpure iron. The volume of each solution passed through the column was such that the lead concentrations of the solutions being polarographed were in the range of 4 x to 1 0 - 4 ~ . The recoveries of lead are shown in Table I. TABLE I THE RECOVERIES OF LEAD AFTER A CATION-EXCHANGE SEPARATION FROM IRON(III) Lead taken, mg . . . . 1.00 4.11 8-19 20.3 Lead found, mg . . . . 1-00 4.08 8.22 20.3 ANALYSIS OF STEELS- mentioned previously. Two steels were analysed by the recommended method using the calibration graph The results are shown in Table 11.TABLE I1 RESULTS FOR THE DETERMINATION OF LEAD IN STEELS Lead found Lead content, polarographically, Alloy per cent. per cent. Mild steel, BCS 329 . . . . 0.050 0.050, 0.050 Lead steel, BCS 212jl . . 0.22 0.21, 0.22 DISCUSSION ,4 cation-exchange separation of lead prior to its polarographic determination is not actually necessary with the two steels analysed above, but the results, in conjunction with those for the synthetic solutions, are proof of the reliability of the separation scheme. Although the above results are satisfactory, the metallurgist is primarily interested in amounts of lead less than 100 p.p.m. Because the d.c. polarograph is incapable of producing precise results for lead determinations at concentrations below 100 p,p.m. in the alloy by the above method, we were unable to examine the full potentialities of the method.However, we see no reason why the lower limit of determination should not be lowered to 1 p.p.m. by using a more sensitive polarograph such as a differential cathode-ray or pulse polarograph. Parts per million of trace metals in alloys have already been determined using a square-wave polarograph. The method should be particularly suitable for the determination of lead in alloy steels containing titanium, vanadium, niobium, tantalum, molybdenum and tungsten, all of which are soluble in a mixture of hydrofluoric and nitric acids. The method could also be applied240 HAMZA AND HEADRIDGE [Analyst, Vol. 91 to the determination of lead in niobium- and tungsten-base alloys, etc. With all alloys precautions must, of course, be taken to ensure that the column is not overloaded with cobalt, copper, nickel and manganese, which are adsorbed with the lead. We are indebted to Riyadh University, Saudi Arabia, for providing one of us (A.G.H.) with a maintenance grant. 1. 2. 3. 4. 5. 6. 7. 8. 9. REFERENCES Ferrett, D. J., and Milner, G. W. C., Analyst, 1956, 81, 193. Scholes, P. H., Ibid., 1961, 86, 116. Meites, L., Editor, “Handbook of Analytical Chemistry,” McGraw-Hill Book Company Tnc., New Hamza, A. G., and Headridge, J. B., Talanta, 1965, 12, 1043. Headridge, J. B., and Dixon, E. J., Analyst, 1962, 87, 32. Nikitin, M. K., Dokl. Akad. Nauk SSSR, 1963, 148, 595. “Stability Constants,” The Chemical Society, London, 1964, pp. 263 and 266. Fritz, J. S., Garralda, B. B., and Karraker, S. K., Analyf. Chew., 1961, 33, 882. Faris, J. P., Ibid., 1960, 32, 520. York, 1963, pp. 5-127. Received September 3rd, 1965