Historical Articles

July, 1953 issue of Plating

 

Polarographic Determination of Zinc in Alkaline Zinc Plating Solutions

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Rafael Diaz and E.H. Lindemann, Chemical Laboratory Minneapolis Honeywell Regulator Company, Minneapolis, MN.

The following paper is the third in its particular field published in the pages of this journal within recent months. The accuracy obtained by the authors with the use of the technique compares favorably with that obtained through use of volumetric methods.

INTRODUCTION
An analytical control method employing the polarographic is described for the analysis of zinc in zinc plating solutions. It is based on the fact that zinc produces a well defined wave, proportional to its concentration, in solutions containing an excess of alkali chloride. The interferences are few and not commonly encountered in zinc plating solutions but, if present, could easily be eliminated by the proper choice of supporting electrolyte or by a simple chemical separation. The polarographic determination also has the advantage that, if desired, certain metallic impurities present such as copper and lead may also be estimated provided they are reducible at the dropping mercury electrode in the medium used as supporting electrolyte and provided they give a well-defined wave suitable for measurement and proportional to the ion concentration.

According to Kolthoff and Linganel, zinc produces a well-defined wave in solutions of alkali chlorides as supporting electrolyte. The wave shows a small maximum which is easily suppressed by the common maximum suppressors. In 1N potassium chloride E1/2 = 1.022 volts vs. the S. C. E. (Saturated Calomel Electrode). The reduction is reversible in this medium and the diffusion current is directly proportional to the concentration over a wide concentration range. In the authors’ procedure, hydrochloric acid is added to destroy the cyanide and carbonate present suppress the maximum, gelatin is used in a final concentration of 0.005 percent. Whether a smaller concentration of gelatin is equally effective in suppress the maximum was not investigated.

Common impurities of zinc plating baths like copper and lead do not interfere with the determination because they are reduced at much more positive potentials than zinc. It should be possible, by running a complete polarogram (over the range of –0.0 to –1.0 volts of applied potential), to estimate these impurities. The polarographic detection and estimation metal impurities in zinc and other plating baths will be investigated in the future.

Chromium, nickel and cobalt are the only elements which might interfere directly with the development of the zinc wave in the medium used, because of their reductions at potentials close to that of the zinc. Chromium gives a wave at 0.88 volts (S. C. E.) and may interfere if present in a relatively large amount; nickel and cobalt will interfere seriously because their waves are very close to that of zinc. The elimination of any interferences due to chromium, nickel or cobalt may possibly be dealt with by the proper choice of supporting electrolyte or by a preliminary chemical separation. In the case of zinc plating solutions the amount of the above metals which might be present as impurities would be so small as-to cause no effects on the development of the zinc wave.

A typical polarogram of an alkaline zinc plating solution is shown in Fig. 1.

Fig. 1. Typical Polarograph of an Alkaline Zinc Plating solution.

REAGENTS AND APPARATUS

Potassium chloride solution, 2 molar
Gelatin solution, 1.0 percent
Mercury, purified, for use in the dropping electrode
Electrolysis cell, H type, external electrode
Sargent, Model III manual polarograph

Zinc plating solutions for the calibration of the capillary were prepared in the laboratory from standard analytical reagents. It was desirable to establish a comparison between the ferrocyanide method and the polarographic method of analysis for alkaline-zinc plating solutions. If a synthetic plating solution is used for calibration by weighing the appropriate amounts of all the components, the calculated value may be taken as a ”certified” value so that both methods (ferrocyanide and polarographic) may be compared to it. It should be realized that the main errors involved in using this synthetic zinc solution would be those of weighing the zinc salt in an analytical balance and dissolving all the components of the plating solution to a definite volume in a calibrated volumetric vessel. They are, no doubt, much less than those involved in using a regular zinc plating solution and analyzing it by some other method (for zinc content) so as to give it a ”certified” or assigned value.

Fig. 2. Calibration curve for Zinc in alkaline zinc plating solutions. Data from Table 1.

PROCEDURE

Calibration
A calibration curve was prepared by polarographing the synthetic zinc plating solutions of known concentration according to the procedure described for the analysis of the samples. Table I shows the data regarding the calibration and these are plotted in the form of a calibration curve in Fig. 2.

Zinc Plating Solution
Pipette 1.0 ml of the sample and transfer it to a tall 100 ml beaker. Add 1 ml of concentrated hydrochloric acid (HOOD) and evaporate to incipient dryness. Dissolve the residue in distilled water and transfer to a 500 ml volumetric flask, adding 5 ml of a 1 per cent gelatin solution. Cool and dilute with water to the mark.

Transfer 25.0 ml of the solution from the volumetric into a 50 ml volumetric flask and add 2N potassium chloride solution to the mark. After mixing, record the polarogram of an aliquot portion between –0.6 and –1.3 volts of applied potential against the saturated calomel electrode.

The concentration of zinc, usually expressed in ounces per gallon, is found by comparison of the height of the wave obtained with the previously prepared calibration curve. All wave heights were measured by the extrapolation method described by Kolthoff and Lingane2. The, direct method of polarographing the supporting electrolyte alone and then correcting for its residual current would, of course, be the best, but since this extrapolation method proves to be satisfactory and less time-consuming (an important factor in routine analysis), it was used in all of the authors’ measurements.

RESULTS AND DISCUSSIONS
The results of some synthetic mixtures are shown in Table II, in comparison with the ordinary volumetric method using diphenylamine as internal indicator. They indicate that the concentration of zinc can be determined by this method to within ±5 per cent which is suitable for routine control. This is about the same accuracy obtained in the authors’ laboratory by using volumetric methods.

The temperature of the electrolysis cell was controlled by a water bath at 25° ± 0.2° C.

The diffusion current was measured at a potential of –1.040 volts (S. C. E.). The reason for measuring the diffusion current at –1.040 volts is that the authors’ values of E1/2 came out to be –1.040 (vs. S.C.E.) instead of –1.022 volts as given in the literature. This discrepancy may be caused by such factors as a high cell resistance in the calomel electrode used for reference or less probably by using a potassium chloride solution of higher concentration than was intended. It should be mentioned, however, that the measurement of the diffusion current does not have to be made at the half-wave potential; on the contrary, measurements of the diffusion current step at a potential when the diffusion current is fully developed are usually more reliable.

Precision
As regards the precision of the measurements, Table III shows the values of the diffusion current step obtained with a synthetic alkaline zinc plating solution of 2.67 ounces per gallon of metallic zinc. From the results in Table III and the calibration chart prepared for the method, it can be calculated that the error in the precision is never over 2 percent for the three runs analyzed.

LITERATURE CITED
1. Kolthoff and Lingane, ”Polarography”, Interscience Publishers, New York, 1st Edition (1941).
2. Ibid, p. 57.