Historical Articles

March, 1953 issue of Plating

 

Polarographic Determination of Lead
in Lead Plating Solutions

Rafael Diaz, Minneapolis-Honeywell Regulator Company, Minneapolis, MN

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A RAPID CONTROL method employing the use of the polarograph is described for the analysis of lead in lead plating solutions. The diffusion current of the lead ion in a medium containing potassium nitrate as indifferent electrolyte is very well defined and forms the basis for this determination. The time required for an analysis is just a fraction of that required when lead is analyzed by the common acid-sulfate gravimetric procedure because no weighing of the sample, precipitation or drying to constant weight are needed in the procedure.

According to Kolthoff and Linganet, the reduction of lead ion is reversible at the dropping mercury electrode and the diffusion current is very well defined in the presence of a small amount of gelatin. In 1N potassium nitrate the half-wave potential is found to be —0.703 volt when a stationary pool of mercury is used as the anode. This potential is quite reproducible under the experimental conditions. The concentration of gelatin, while not critical, should be kept below 0.01 per cent and the amount added provides for a final concentration of gelatin of about 0.004 per cent. This may be substituted by an even smaller concentration of the sodium salt of methyl red (about 2 x 10 per cent). Methyl red is itself reduced at the dropping electrode, but the concentration used is so small that it will have no appreciable effect on the diffusion current of the lead ion.

Fig. 1--Polarogram of lead in lead plating solution.	Fig.2--Calibration curve for lead in lead plating solutions. Data from Table 1.

The author knows of no interferences in the procedure with the exception of tin, whose chlorostannic complex gives a wave at—0.48 volt (vs. a stationary mercury pool). This interference would only be serious if tin were present with enough chloride to form a chlorostannic complex and in such high concentration that its wave would obscure that of the lead, which appears at a more negative potential. If such were the case, a method of separation would have to be used. Kolthoff and Linganet make use of sodium hydroxide for the simultaneous analysis of tin and lead and obtain two widely separated waves, thus eliminating any possibility of obtaining the two waves at approximately the same potential. This method seems very suitable for application in the case of the tin-lead alloys.
Since tin is not encountered in lead plating solutions except possibly as an impurity it would not interfere with the lead determination because its concentration would be so small that no danger of obtaining a coalescing wave with tin would be present.
A typical polarogram of lead in a plating solution is shown in Fig. 1.

REAGENTS AND APPARATUS
2M Potassium nitrate solution
Gelatin 1 per cent solution
Mercury, triple distilled, for the mercury pool and the dropping electrode
Sargent manual polarograph, Model 3

PROCEDURE
Calibration
A calibration chart was constructed by measuring the diffusion current of several lead plating solutions prepared in the laboratory from analytical reagents This calibration chart is, of course, valid only for the particular instrument and electrode with which it was prepared; if for any reason it becomes necessary to replace the capillary, then the curve must be redetermined. Even if no such replacement is necessary, it is advisable to make regular determinations on standard solutions to determine whether the characteristics of the instrument have varied with time. This is especially important as regards the sensitivity of the galvanometer. The data obtained from these polarograms, and the diffusion current constants calculated therefrom, are shown in Table I. The data from Table I are shown plotted in graph form in Fig. 2.

Lead Plating Solution
Pipette 1.0 ml of sample into a 500 ml volumetric flask. Add 4 ml of a 1 per cent solution of gelatin and dilute with water to the mark. Pipette 5.0 ml of this solution into the polarographic cell and add 5.0 ml of 2M potassium nitrate. Record the polarogram between—0.50 and —1.00 volts, applying a bridge potential of 1.0 volt.

RESULTS AND DISCUSSION
Table II shows the agreement obtained between the polarographic and gravimetric methods for the analysis of lead in some plating solutions. The figures show that the method gives results in agreement with the gravimetric values to within less than 2 per cent in the range of concentrations studied. Such accuracy is perfectly adequate for the purpose of routine control of the lead plating baths.

LITERATURE CITED
Kolthoff and Lingane, “Polarography”, Interscience Publishers, Inc.

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