Historical Articles

March, 1954 issue of Plating

 

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The Polarographic Analysis of Nickel Plating Solutions

J.V. Petrocelli and G. Tatoian, Director of Research and Chemist, respectively, The Patent Button Company, Waterbury, CT

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ABSTRACT
A brief review is given of polarographic methods and their potentialities in the study and control of plating solutions. The general principles and techniques are outlined and a new manual polarograph is described which has been specifically designed and developed for the application to electroplating solution analysis. Experimental data are presented showing how the polarographic method may be applied to the determination of the principal constituents of the Watts’ type nickel plating solution. Standard curves and detailed procedures are given for the determination of nickel sulfate, nickel chloride and boric acid. The effect of pH and brighteners is briefly discussed.

INTRODUCTION
Polarographic methods of analysis have been used successfully and with great advantage in many fields of applied analytical chemistry. They generally require less chemical manipulation and expenditure of time than the regular chemical methods and are especially useful for the determination of small quantities of substances.

A consideration of the basic principles and techniques involved in polarography indicates that it should lend itself readily and with great advantage to the analysis and the control of plating solutions and electrodeposits. It also should be a very useful tool in plating research. Among the many possible uses of the techniques in this latter field, the following may be mentioned: the mechanism of electrode reactions, adsorption, the study of the potentials at which metals may be reduced from various solutions, the formation of complex ions and their effect on electroplating, and the reaction of brighteners on electrode potentials.

Since the polarographic method is based upon electrochemical phenomena such as electrolysis, polarization, and the measurement of current-voltage curves, it should appeal to the electrochemist and the plater as an elegant and interesting tool for the control and study of his processes.

In light of these attractive features, it is not surprising that interest is increasing in the practical application of polarography to plating control.

Gordon and Roberts¹ have successfully applied the polarograph to the analysis of brass plating solutions and deposits. Sazanova and Korshunov² have developed polarographic methods for the analysis of the acid-zinc solution. Vyakhiirev³ used the polarographic method in developing a procedure for the amperometric titration of sulfates in plating solutions. Recently, Diaz and Lindemann^4,5 have reported methods for the polarographic determination of tin, lead and zinc in their respective plating solutions.

The work reported in this communication is a part of a general study undertaken in the authors’ laboratory to develop polarographic methods for the control of some of the more common plating solutions and to design and develop a simplified polarographic instrument which can be used readily by the control chemist. These procedures have been used in the control laboratory of the writers’ company for several years with great success. It has been found that the polarographic methods save a considerable amount of time; not only are they much more rapid but the time consumed in making and maintaining the standard solutions usually necessary for the more conventional chemical methods is reduced considerably.

Procedures have been developed for determining the principal constituents of nickel plating solutions, silver, copper, zinc, brass and cadmium-cyanide plating solutions, and chromium plating solutions, and for the detection and determination of some of the impurities usually found in these solutions. The data presented here are concerned with the determination of the principal constituents of a Watts’ type and a cold nickel plating solution; it is planned to present data on other plating solutions in subsequent papers.

GENERAL PRINCIPLES
As previously indicated, the principles of polarographic analysis are based upon subjects already familiar to the electroplater, namely, electrolysis and polarization. Since these fundamentals are treated in great detail in Kolthoff and Lingane’s excellent monograph6 only a very brief outline of the fundamentals will be presented here.

The method is based on the polarization characteristics of a micro-electrode when a solution containing an electro-oxidizable or an electro-reducible substance is electrolyzed. The system consists of a micro-electrode and a relatively large nonpolarizable electrode in a solution containing a small concentration (about 10-3 M/l) of the substance to be determined, a relatively large concentration (0.1 M/l) of a supporting electrolyte and a small amount of a maximum suppressor such as gelatin.

The most versatile micro-electrode is the dropping mercury electrode (D.M.E.) which consists of a capillary glass tube connected to a leveling bulb filled with mercury. The diameter of the capillary and the height of the leveling bulb are adjusted so that the mercury drops from the capillary in small uniform drops. Usually, each drop has a lifetime of about 3 seconds. The nonpolarizable electrode may consist of a large pool of mercury or a calomel half-cell; the saturated calomel half-cell makes a very satisfactory electrode for this purpose. A schematic cell arrangement is shown in Fig. 1.

Fig. 1—Schematic polarographic circuit and electrode assembly.

The substance which is to be determined should be in solution form and a suitable portion added to the supporting electrolyte, gelatin is added and the resulting solution is transferred to the polarographic cell. Since oxygen reacts at the D.M.E. it is desirable to remove dissolved air from the solution by passing nitrogen through it before electrolysis. After shutting off the nitrogen the solution is electrolyzed by slowly applying an increasing voltage. The current is recorded at various values of the applied voltage and the results plotted. This gives the current-voltage curve known as a polarogram. It is then possible to determine the nature and concentration of the reacting substance from the polarogram. The following typical example is given in order to show the technique involved.

As a typical polarographic cell reaction, one may consider the electrolysis of a deaerated solution containing 0.001 M NiCl₂, 0.10 N KNO₃ and a small amount of gelatin. The cell and the basic polarizing circuit are shown schematically in Fig. 1.

The basic circuit consists of a uniform slide wire R₂ through which a steady current passes. The total potential drop, V, across the slide wire is adjusted by means of the resistance, R₁, usually at 1.00 v or 2.00 v.

The slide wire contact is moved so as to vary the voltage, E, applied to the cell. Generally, the slide wire has some indicating mechanism such as a graduated dial so that the applied voltage, E, may be read directly. The switch, S, is used for setting the polarity of the dropping mercury electrode. A sensitive galvanometer, G, with its appropriate shunt. R₃, is generally calibrated as a micro-ammeter.

If the solution is electrolyzed by making the dropping mercury electrode the cathode and the applied voltage is increased slowly from –0.3 v to –1.5 v at first only a very small current flows, which increases approximately linearly with the voltage and is known as the residual current. When the voltage is reached that is high enough for the discharge of nickel ions at the dropping electrode, the current rises very rapidly and finally reaches a

limiting diffusion current. If the values of the current are plotted versus the corresponding values of the applied voltage, the curve, C, in Fig. 2 will be obtained. This curve is the polarization curve for the deposition of nickel on mercury and is known as a ”polarogram” or polarographic wave. Curve D represents the polarogram for a greater concentration of nickel while curve B represents the wave of lesser concentration.

Fig. 2—Schematic polarographic curves. A—residual; B, C & D—various concentrations of reacting ion.

If the solution did not contain nickel, but only 0.1 M KNO₃, the curve A, in Fig. 2, would be obtained. This consists only of the residual current and the difference between curves C and A, at any voltage along the top of the plateau of the wave, is the current increase due to the nickel ion and is called the diffusion current, I_d.

The residual current is due predominantly to the flow of current required to maintain the surface charge on the continually renewed surface of the mercury drop.

As each mercury drop grows and falls, the are changes from practically zero to some maximin value. The current’ therefore, oscillates between some minimum and maximum value so that the galvanometer spotlight performs oscillations at each current value. For purposes of measurement, maximin values are satisfactory.

The gelatin at a concentration of about, 0.01 per cent is necessary in order to suppress the usual tendency of the current-voltage curve to develop an abnormal rise in current before the limiting current is reached.

The diffusion current is due to the extreme state of concentration polarization which is reached at the dropping electrode with respect to the deposition of nickel. Generally there is a linear relationship between the concentration of nickel in the solution and the diffusion current. Ilkovic⁷ derived the following equation for the diffusion current at the dropping mercury electrode at constant temperature:

I_d = 607 n D^1/2 m^2/3 t^1/6 C

I_d is the limiting diffusion current expressed in microamperes, n is the number of electrons involved in the electrode reaction, D is the diffusion coefficient of the electro-active material in the units cm² sec^-1, C is the concentration of the electro-active material expressed in millimoles per liter, m is the mass of mercury per drop expressed in mg sec^-1 and t is the lifetime of a drop in seconds.

This linear relationship is the basis for the application of polarography to quantitative analysis. The qualitative aspects, the identification of the ion is based on the measurement of the half-wave potential, E. This is the voltage at which the current has reached one-half the value of the diffusion current. The half-wave potential is characteristic of each ion in ally given supporting electrolyte.

If several substances, which undergo reaction at the dropping mercury electrode, are present in the solution⁷ each one will produce its own characteristic polarographic wave. It is thus possible to obtain qualitative and quantitative analysis for several substances in one solution. It is necessary, however, that the half-wave potentials of the substances are not too close together.

Fig. 3—A schematic calibration curve.

Calibration Curves
As shown above, the diffusion current of a substance reacting at the dropping mercury electrode is proportional to the concentration of the substance in any given supporting electrolyte, and given capillary characteristics and temperature. There may be at times some deviations from a linear relationship; it is therefore highly desirable to prepare calibration curves.

A calibration curve should be prepared by running polarograms on a series of test solutions of varying and known concentrations of the substance to be determined. It is absolutely necessary that all other factors remain constant and exactly as those encountered when the unknown is run. These factors are: drop time at a given voltage, supporting electrolyte, concentration of maximum suppressor and temperature. A typical calibration curve is shown in Fig. 3.

INSTRUMENTATION
The essential parts of an instrument for obtaining polarographic curves are a polarizing unit by means of which a known and variable voltage may be applied to the cell and a sensitive current measuring device such as a galvanometer.
It soon became apparent during the course of the authors’ investigations that a relatively simple manual instrument is all that would be required for the applications to plating solution work. Since most of the commercial instruments were more elaborate and hence, much more costly than required, a division of the authors’ company developed (Electropolarizer Model C1, manufactured by Patwin Instrument Division, The Patent Button Company, Waterbury, Conn.) a unit to overcome such objections. It has been especially designed to meet the requirements at hand; it is simple in its construction and by the elimination of features which are not important to this application its cost was held down to a vicinity that would be attractive to the control chemist and plater. The result was achieved by simplification and not by sacrificing quality of parts; precision components have been used throughout.

The polarizing unit of the instrument contains a 10 turn helical potentiometer with a linearity tolerance of 0.1 per cent and is operated by an integrating dial graduated in one-thousand units. The span voltage may be varied from 1 to 3 volts. With the 3-volt span the accuracy obtained is more than sufficient for polarographic work of this nature. The current is provided by dry cells and regulated by a variable resistor. A voltmeter of 1 per cent accuracy designates the span voltage. A sensitive galvanometer (about 0.005 µa/ mm) with a low internal resistance and a long period of swing is used as a micro-ammeter. This is shunted by an 11 point multiplying shunt providing suitable current ranges. A photograph of the instrument is shown in Fig. 4.

Fig. 4—The Patwin Electro-Polarizer Model C1.

EXPERIMENT
Apparatus and Materials
The exploratory work and the determination of the calibration curves were performed on a precision manual instrument assembled in the authors’ laboratory.(Currently manufactured as Model R1 by Patwin Instruments Division, The Patent Button Company, Waterbury, Conn.) The experiments and actual analyses of plating solutions were carried out with the simplified model which as previously stated had been designed especially for this application. A polarographic cell, also made by the company, in combination with a saturated calomel half-cell was used. The side arm of the calomel cell which serves as a liquid junction was filled with agar gel containing a solution of saturated KCI. The agar plug together with the sintered glass disc in the polarographic cell prevent contamination of the solution by chloride. The cell was immersed in a constant temperature water bath when calibration curves were obtained.
The capillary was made from glass barometer tubing.(Supplied by The Corning Glass Works, Corning, N. Y.) It was about 8.0 cm in length with a bore diameter of about 0.06 cm. The constant m^2/3 t^1/6 was 1.830 mg^2/3sec^-1/2. The drop time, t, was about 4.5 sec at -1.375 v.

Standard solutions were made up with distilled water and chemically pure salts. The concentrations were determined by standard chemical methods.
The gelatin solution was made by dissolving 0.200 gram of powdered gelatin in 100 ml of distilled water which had been boiled previously for about 10 minutes, and then cooled to 60° C. A few drops of toluene were added and the flask firmly fitted with a glass stopper. All the operations were performed in the flask in which the solution was stored. A solution usually kept for several days. Bacterial action produces a cloudiness which is readily visible and whenever bacterial action was suspected a fresh solution was prepared.

Fig. 5—Polarographic waves for chloride, nickel and boric acid as obtained on a nickel plating solution.

Tank nitrogen was used without further purification; this was bubbled through a wash bottle containing distilled water before entering the solution to be polarographed in order to saturate the gas with water.

Supporting Electrolyte
A study of the half-wave potentials of nickel, chloride ion and boric acid indicated that it might be feasible to use one supporting electrolyte for all three constituents. Exploratory experiments showed that 0.1 N KNO₃ (10 g/l) containing 150 g/l of mannitol would be a suitable electrolyte. In this medium a polarogram extending from +0.40 v to –1.90 v yields three waves in succession, the chloride, the nickel and the boric acid wave. The chloride wave is anodic with E +0.26 v while the nickel and boric acid waves are cathodic with E of about –1.07 v and –1.65 v, respectively. A typical polarogram is shown in Fig. 5.

It was found that boric acid does not show a wave in KNO₃ without the presence of mannitol. This is probably due to the fact that boric acid is a very weak acid, and as is well known8 mannitol forms a relatively strong acid with boric acid.

The concentration of KNO₃ has a definite effect upon the boric acid wave. If the concentration of KNO₃ is greater than about 0.25 N the wave is obscured by the beginning of the much larger potassium wave. The effect is shown in Fig. 6. As a result of these experiments a solution of 0.1 N KNO₃ containing 150 g/l of mannitol was chosen as a supporting electrolyte.

Fig. 6—Effect of KNO3 concentration on the boric acid wave. A—1.0 N, B—0.50 N, C—0.25 N and D—0.10 N KNO3.

Deaerator
It is essential to deaerate the solution when nickel and boric acid are being determined. The second wave for the reduction of oxygen occurs in the same region as the nickel wave so that the diffusion current for nickel will be about 1.5 to 2.0 pa too high. The effect on the boric acid is of even greater importance. The hydroxide ion produced by the reduction of oxygen combines with an equivalent amount of hydrogen ion at the electrode surface and low results will be obtained for boric acid.6
Since the calomel half-cell contains a high concentration of chloride ion, there is the possibility that some chloride may diffuse into the polarographic cell if sufficient time elapses. It is, therefore, desirable to determine the chloride ion as soon as possible after the half-cell is connected to the polarographic cell. This determination may be performed before the solution is deaerated. The solution may then be deaerated and the nickel and boric acid determined.

Procedures and Calibration Curves
The methods developed in this study are based upon the use of a 0.25-ml sample of plating solution diluted to a final volume of 50 ml for the polarographic solution. This dilution yields a satisfactory concentration (about 1.0 to 10.0 millimoles/l) of reacting substance for polarographic analysis. The concentration of supporting electrolyte is high enough so that either of the two following procedures may he used. A 0.25-ml sample of plating solution may be used directly or a 5.0 ml sample may be diluted to 100 ml with distilled water and a 5.0-ml portion of the resulting solution used.

Calibration curves were obtained as follows: a series of standard solutions were made containing known and varying concentrations of NiSO₄ · 7H₂O, NiCl₂· 6H₂O and H₃BO₃. A 5.0-ml sample was diluted to 100 ml with distilled water and a 5.0-ml sample of the resulting solution was pipetted into a 50-ml volumetric flask, 1.0 ml of 0.2 per cent gelatin solution was added and the whole diluted to the mark with supporting electrolyte. About 25 ml of this final solution was transferred to the polarographic cell and nitrogen gas was slowly bubbled through the solution for about 15 minutes and then disconnected. The D.M.E. was then polarized anodically from +0.10 to +0.50 v in order to obtain the chloride wave; and cathodically from –0.70 v to –1.90 v to obtain the nickel and boric acid waves.

Fig. 7—Calibration curves for NiSO4 · 7H2O

Since the waves are well defined and reproducible, the diffusion currents were obtained by pointer readings. The currents were read at +0.40 v for chloride, –1.38 v for nickel and –1.82 v for boric acid. A correction was not made for the residual current, therefore this current appears as the intercept of the calibration curves on the current axis. The calibration curves were obtained at three temperatures, 68°, 77° and 86°F.

All concentrations are reported as those in the sample, e.g., the plating solution; where the total nickel content as obtained from the polarogram is expressed as NiSO₄ · 7H₂O, the total chloride content as NiCl₂· 6H₂O and the boric acid as H3BO3. It should be noted that in order to obtain the actual concentrations of NiSO₄ · 7H₂O a correction must be applied tor that portion of the nickel present as NiCl₂· 6H₂O. This is the usual correction which is applied when standard chemical methods are used.

Calibration curves are shown in Figs. 7, 8 and 9. The numerical values are given in Table I; where the diffusion current has been corrected for the residual current. It will be seen that a good linear relationship between current and concentration is shown for all three constituents.

Fig. 8—Calibration curves for NiCl2· 6H2O	Fig. 9—Calibration curves for H3BO3

Effect of pH
It was found that if the pH of the plating solution is lower than about 3.0, the results for boric acid are too high. Polarograms ran on solutions which did Tot contain boric acid showed that a ”hydrogen” wave is obtained when the pH is lower than 3.0.

Although it is possible to make a correction for the ”hydrogen” wave, it appeared more desirable to neutralize the strong acid prior to running the boric acid polarogram. The following modification was therefore introduced into the procedure when the pH of the plating solution is below 3.0. After the sample of plating solution is pipetted into the 50-ml volumetric flask, the flask is only partially filled with the supporting electrolyte; a few drops of bromphenol blue are added and then a solution of NaOH (about 01 N) is slowly added dropwise until the indicator just changes color (from yellow to blue). Then 1 ml of 0.2 per cent gelatin is added and the solution is diluted to the mark with more supporting electrolyte. The polarograms then are run as usual.

Effect of Brightening and Other Addition Agents
Although it was not possible to examine the effect of all the commercial brighteners, the effect of several of the more common ones was determined. It was found that only one of those examined had any appreciable effect: it caused high results for the boric acid. The result, however, is also too high when conventional chemical methods are used and is due to the presence of boric acid in the brightener.

The procedure was used on a bright cobalt-nickel bath⁹ and yielded satisfactory results.

No significant effects on the results were found when wetting agents, hydrogen peroxide or both were added to the bath.

DISCUSSION
As previously indicated, the above methods have been used successfully in the control laboratory of the authors for several years to control both a Watts’ type solution and a plain cold nickel solution containing ammonium chloride. Generally, it has been found that the precision for the nickel and chloride determination is about 2 per cent. Since the diffusion current for boric acid is about one-third that for nickel and occurs after the nickel wave, the precision in this case is somewhat lower. A comparison of the polarographic results with those obtained by the conventional chemical methods shows that they generally agree to within about 0.25 oz/gal. The values shown in Table II were chosen at random and are representative.

It should be pointed out that it is not necessary to make special solutions in order to obtain calibration cures. If one wishes to set up the polarographic methods, a very convenient and satisfactory procedure is to use the plating bath with its known composition as a primary standard. Suitable samples of the plating bath may be diluted in such a manner that the resulting solutions will represent various concentrations of the constituents. Polarograms may be run on each dilution and a calibration curve plotted.

ACKNOWLEDGMENT
The authors wish to express their appreciation to Miss E. Lawler and Mr. D. Lake who assisted performing the experimental work reported in this study.

BIBLIOGRAPHY
1. H. E. Zentler Gordon and Eric R. Roberts, Trans. Electrochem. Soc. 90, 27 (1946).
2. I. A. Korshunov and L. N. Sazanova, C. A. 42, No. 22, 8701 (1948).
3. D. A. Vyakhirev, C. A. 40, No. 10, 2759 (1946).
4. R. Diaz, PLATING 40, 45, 261 (1953).
5. R. Diaz and E. H. Lindemann, PLATING 40, 762 (1953).
6 I. M. Kolthoff and J. J. Lingane, ”Polarography” 2nd Edition, Interscience Publishers, N. Y. (1952).
7. D. Ilkovic, C. A. 29, No. 9, 2858 (1935).
8. M. Ho]lander and W. Rieman, III, Ind. Eng. Chem. (Anal. Ed.) 17, 602 (1945).
9. W. Blum and G. B. Hogaboom, ”Principles of Electroplating and Electroforming” 3rd Edition, McGraw-Hill Book Co., Inc., New York (1949).