analysis systems. The authors believe that the sensitivity could be increased by the use of a multiburner system, and in the above cases a multipass arrangement along with a more sophisticated slit and lens system. Interferences. The concentrations of the interfering agents in Table IV are 500 ppm of cation and 5000 ppm of anion. The amount of interference was as expected but is difficult to compare with that of the hollow cathode. At first analysis,
interferences are expected to be somewhat higher than those of a sharp line source because a broader band of energies is available for absorption.
RECEIVED for review November 4, 1965. Accepted January 27,1966. Work performed in partial fulfillment of the requirements for the Ph.D. Degree (C.W.F.) in Chemistry,
Simultaneous Determination of Tin and Lead Using Cyclic Stationary Electrode Polarography Sidney L. Phillips IBM Corp., Systems Development Division, Poughkeepsie, N . Y TIN AND LEAD may be determined simultaneously at a dropping mercury electrode in supporting electrolytes containing a high concentration of chloride ( 1 ) or bromide (2) ions. In the former method, the tin concentration is determined from the first wave, while the sum of tin and lead is estimated from the second wave. This sum is obtained from the second wave because the second tin reduction step takes place at the same potential as lead, and is superposed on the lead wave. In the latter method, only one tin wave is observed, and this is shifted to potentials more cathodic than that of lead. Under these conditions, it is possible to determine the tin and lead contents from the separated waves. In the present work, the simultaneous determination of tin and lead from a solution which does not require a large excess of halide ions is reported. When the supporting electrolyte is a formate buffer solution containing added pyrogallol, the lead wave is interposed between the two tin waves so that simultaneous determination is feasible. In addition, the analytical procedure utilizes an amalgam stripping technique so that the oxidation state of tin is unimportant and better resolution is obtained between the tin and lead peaks. EXPERIMENTAL
Apparatus. All data were obtained using a Wenking Model 61R potentiostat (Brinkmann Instruments). The initial potential was obtained from the mercury battery low voltage power supply built into the instrument, while the triangular wave was obtained from an Erwin Holstrup Motor Potentiometer (Brinkmann Instruments). Experimental data were recorded as a voltage drop across a decade box load resistor by means of a Leeds and Northrup Type G Speedomax recorder, This instrument had a full scale pen response time of 1 sec and a chart speed of 20.2 inches per min. The hanging mercury drop electrode, cell, and associated equipment were similar to those shown by Alberts and Shain (3), except that the counterelectrode was a platinum sheet of about 2 cm2 area, and the electrode shield was not used. The cell was immersed in a water bath maintained at 25" C. Chemicals. All chemicals were of reagent grade and were used as received. Stock 0.0100M tin(1V) solutions were (1) D. E. Sellers and D. J. Roth, ANAL.CHEM., 38, 516 (1966). (2) T. Kitagawa and K. Nakano, Rev. of Polurog. (Jupun) 9 (3) (1961). (3) G. S. Alberts and I. Shain, ANAL.CHEM., 35, 1859 (1963).
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ANALYTICAL CHEMISTRY
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prepared by dissolving the required amount of sodium stannate dihydrate in 0.1Msodium hydroxide, or by dissolving metallic tin in a heated mixture of 20 ml concentrated hydrochloric acid and 5 ml concentrated nitric acid. This latter solution was then cooled, transferred to a 200-ml volumetric flask and diluted to mark. Standard 0.010M Iead(I1) solutions were prepared by dissolution of lead acetate in 0.1M perchloric acid. A stock solution of 0.5M pyrogallol was prepared by dissolving a weighed amount of the solid in 0.1M perchloric. This last solution remained colorless for several weeks under ambient conditions, indicating negligible oxidation. The supporting electrolyte was a buffer solution composed of 2 M formic acid and 2M ammonium formate (4M in total formate), which gave a final pH of 3.7. All solutions were prepared from twice-distilled water, with the second distillation being made from an alkaline permanganate solution. Procedure. A 50-ml aliquot of the supporting electrolyte was deaerated by nitrogen bubbling for 8 to 10 min. Following this step, 2 ml of the pyrogallol solution and an appropriate aliquot of the tin and lead solutions were added to the electrolysis cell. The solution was mixed by bubbling with nitrogen for 5 min, after which the fritted glass inlet was raised above the liquid level so that nitrogen passed constantly through the upper portion of the cell. A mercury drop (0.067-cm radius) was then attached to the working electrode while applying the initial potential (0.0 volt us. SCE). The solution was allowed to remain quiet for 30 to 60 sec after attaching the mercury drop to minimize stirring effects caused by movement of the solution during this step. Cyclic stationary electrode polarograms were then recorded by application of the signal voltage and replicate results were obtained readily by attaching a fresh mercury drop and repeating the procedure. Data obtained in this manner for the variation in anodic peak currents as a function of bulk concentration are recorded in Table I. As shown in Figure 1, the stripping peak potentials for the first tin wave and the lead wave are about 100 mV apart. RESULTS AND DISCUSSION
The electrochemical behavior of lead appears to be relatively uncomplicated in both the absence and presence of pyrogallol. Addition of pyrogallol to a concentration as high as 0.1M did not cause a shift in the lead peak, indicating negligible complexation. On the other hand, the behavior of tin(1V) is not as straightforward and appears to be complicated by a
Table I. Variation in Anodic Stripping Peak Currents as a Function of Concentration Scan rate: 41.7 mV/sec; switching potential: -0.75 volt; supporting electrolyte: 2M formic acid, 2M ammonium formate, 0.0050M pyrogallol
2.00 4.00 6.00
8.00 10.0 14.0 14.0a 14.W 16.Prc
0.60
1.20 1.85 2.30 2.77 3.95 3.95 3.95 4.40
0.65 1.25 1.95 2.60 3.20 4.53 4.45 4.42
0.300 0.300 0.301 0.287 0.277 0.282 0.282 0.282 0.275
5.00
a
0.010M pyrogallol.
c
Spike observed on second tin anodic stripping peak.
0.325 0.312 0,325 0.325 0.320 0.323 0.318 0.316 0.313
* 0.015M pyrogallol. 100
350 ..E,mv
600
850
VS. SC E
Figure 1. Cyclic staticinary electrode polarogram of tin(IV) and lead(I1) in 2M fornnic acid, 2M ammonium formate containing 0.015M chloride at three pyrogallol concentrations Both metal ion concentrations, 1.40 X lo-' M . Pyrogallol concentration: A , 0.0050 M; B , 0.010 M ; C , 0.015M. Initial potential, -0.100 volt. Scan rate, 41.7 mV/sec number of factors. Fclr example, sufficient pyrogallol must be present or tin(1V) ail1 not be quantitatively reduced (4, but if too large an exce:is is present, the cathodic and anodic portions of the second wave exhibit sharply defined spikes. At intermediate conceni.rations, the first tin reduction wave shows the usual shift of the half-wave potential toward more negative values charactlxistic of ligand complexation. The effects of intermediate and high concentrations of pyrogallol on tin(1V) are shown in Figure 1. As seen in Figure 1, with increasing pyrogallol concentration the negative shift of the first tin wave is such that the limiting current region of the first tin wave coincides with the foot of the lead wave causing a mutual interference. That is, this overlap complicates determination of both the peak current of the first tin Nave and extablishment of the base line required to calcu1a:e the lead peak current. Primarily for this reason, and partly because the peak currents are higher, the anodic stripping method used here was selected for analytical applications. In addition, the oxidation state of tin is unimportant for the stripping method, because both tin(11) and tin(1V) are rcduced to form tin amalgam in the supporting electrolyte uscd. (4) S. L. Phillips, Ph.D. Thesis, Univ. of Wisconsin, Madison, 1964.
As noted, the analytical procedure described measures the anodic stripping peak heights obtained during this portion of the cyclic scan, so that the finite volume of the mercury drop electrode must be considered at the relatively slow scan rate used here. Thus, the switching potential must be maintained constant because of the continual deposition of amalgamforming metal into the mercury drop during the cathodic scan. CONCLUSION
The method described here is useful for the simultaneous determination of tin and lead at concentration levels as high as 1.6 X 10-4M. If the solution is stirred during the preelectrolysis plating step, then the lower limit of analytical applicability should be extendable to the usual 10-7 to 10-8M levels attainable by conventional anodic stripping analysis. Because peak separation rather than sensitivity was of importance in the present work, the solution was not stirred. In addition, the supporting electrolyte used here permits potentials as positive as +0.2 to f0.3 volt us. SCE whereas the anodic limit is about -0.2 volt in the halide supporting electrolytes used in the dropping mercury polarographic methods. This additional voltage range should permit the simultaneous determination of metal ions, such as bismuth and copper, provided intermetallic compound formation does not occur. In this respect, no indication of intermetallic compound formation between tin and lead was observed in the present work.
RECEIVED for review December 2, 1966. Accepted January 30,1967.
VOL. 39,
NO. 4, APRIL 1967
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