Historical Articles

May, 1954 issue of Plating

 

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Radiometric Study of the Chromium-Sulfate Complex Formed in Chromium Plating Baths

Ronald L. Sass and Stanley L. Eisler
Chemist and Supervisor of Radiochemistry Section, respectively, Ordnance Corps, Rock Island Arsenal, Rock Island, III.
Presented at the 104th meeting of the Electrochemical Society, Wrightsville Beach, N. C., September 13-16, 1953.

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*The opinions or assertions contained herein are not to be construed as being official or reflecting the views of the Department

ABSTRACT
Tests conducted to determine the ionic nature of the coordination complex formed and the amount of sulfate so complexed are described. It was found that the chromium-sulfate complex formed is cationic in behavior and can be removed from the plating solution by employing the appropriate ion exchange media. Approximately seven per cent of the total sulfate is bound up in the complex after electrolysis. The formation of a large complex ion is indicated by the fact that the per cent of sulfate regained from the resin column is considerably less than and independent of the amount of trivalent chromium regained.

INTRODUCTION
Erich Müller,¹ a German investigator, has stated that current-voltage curves of the purest chromic acid solutions using gold electrodes show that the chromic acid present cannot be reduced electrolytically either to trivalent chromium or chromium metal. This condition is changed by the addition of sulfuric acid, when both trivalent chromium and chromium metal are formed at different potentials. Although methods of chromium electrodeposition are being developed using baths of trivalent chromium, chromic acid baths are still used almost exclusively by industry. Therefore, the sulfate ion remains an important part of chromium plating.

The recognition of the need of a complex forming radical in chromium plating is not new. Perhaps Robert Bunsen had some idea of the necessity of complex ions when he became the first person to plate chromium successfully in the year 1854. It was, however, in 1924, seventy years after Bunsen’s experiment, that a commercial method of chromium plating was developed. In that year Fink submitted to the United States Patent Office his process for converting laboratory plating practices into a commercially feasible process.²

At first the sulfate ion was added to the bath as Cr₂(SO₄)₃ and the trivalent chromium was mistakenly considered as the important constituent. Later, by the use of sulfuric acid, it was found that the sulfate ion was the important factor. By trial and error, platers have since found the proper sulfate concentration. They noticed that too small an amount of sulfate resulted in either no plate or brown stains, and too high an amount resulted in partial plating or none at all.³

Interest in the function of the sulfate ion was recently stimulated - at this laboratory when a large cationic resin exchange column was installed in the plating shop of this Arsenal. The purpose of this column was to regenerate the chromic acid plating solutions. It is well known that used plating solutions contain dissolved metals such as copper, iron and aluminum.

These metals in sufficient quantity render the solution inefficient by decreasing the conductivity of the bath. If they are not removed, the plating baths must be discarded. To prevent huge material waste and to minimize waste disposal problems, the impure chromic acid may be reclaimed by passing it through a cationic resin exchange column.

In the eleven months that such a column has been in use at this Arsenal, not only have the metal impurities successfully been removed, but also a considerable amount of sulfate ion has been lost. This fact indicates that some sulfate is in solution as a complex ion. The object of this investigation is to reveal the nature and amount of this complex and to apply the results obtained to explain the function of this ion in the plating process. Radioisotopic techniques were employed because the concentrations of sulfate encountered were considered too small to analyze accurately by ordinary gravimetric methods.

PREPARATION OF SOLUTIONS
The solutions used in this work were formulated to obtain a chromic acid-sulfate ratio of 100:1 and a chromic acid concentration of approximately 250 grams per liter. These concentrations resulted in solutions which were of the same strength as commercial plating baths. The volume used for each test was 1500 ml. Radiosulfur was added to each volume of solution in the form of sulfuric acid. This radioisotope was purchased for this investigation from the Oak Ridge National Laboratory by authorization of the Atomic Energy Commission, Isotope Division. It was received as sulfuric acid and was diluted to yield a stock radiosulfur solution having an activity of 200 microcuries per milliliter. The portion of the stock radiosulfur solution used in this investigation was of an amount calculated to give an activity of 0.1 microcurie per milliliter of chromic acid solution. A 250-ml portion of the chromic acid solution was then retained for testing purposes. This solution will hereafter be referred to as the starting solution.

The remaining 1250 ml of the starting solution was electrolyzed at room temperature for six hours at five amperes current. This current gave a cathode current density of 20 amperes per square foot and an anode current density of 150 amperes per square foot with the lead electrodes used. Seegmiller and Lamb⁴ stated that this high ratio of anode to cathode current density is conducive to the formation of trivalent chromium. To compensate for loss due to evaporation during electrolysis, deionized water was added to the solution to return it to the original volume. A portion of the electrolyzed solution equivalent to that taken from the starting solution was set aside for analysis. Results of analyses made, placed the concentration of trivalent chromium at 6 to 12 gram per liter as shown in Table II.

REMOVAL AND RECOVERY OF COMPLEX
The electrolyzed solution was considered to be equivalent to an actual plating bath which had been in use for a considerable length of time. It was then necessary to prove the presence of the chromium sulfate complex. In order to separate the complex from the remaining sulfate, a method had to be employed which would not cause its dissociation. Tests were conducted on the electrolyzed solution to determine if barium chloride would precipitate the complex-bound sulfate as well as the free sulfate radical. Results of these tests showed that complete precipitation of the total sulfate was effected. For this reason ordinary methods of precipitation were abandoned as a basis for separating the complex. The problem of separating the complex without destroying its ionic structure was solved by the use of a resin ion exchange column. Two forms of resin could have been employed, either a cationic or an anionic exchanger. If one were to assume the complex to be cationic in nature, a simple separation of the sulfate bound in the complex and the anionic free sulfate could be executed by using a cationic resin and employing radiosulfur (S³⁶) as a tracer. Since a cationic exchange resin was on hand, this material was tried first. The results obtained by using this method showed the formation of cationically bound sulfate. Therefore, the use of an anionic resin was abandoned. If the complex had proved to be anionic, another method would have been employed using radiochromium as a tracer.

The resin column used in this investigation consisted of a 500-milliliter glass delivery burette, one and one-half inches in diameter. The column was packed with 500 cubic centimeters of cationic resin. (Amberlite IR-120, Rohm & Haas Co., Philadelphia, Pa.) A fresh volume of resin was used for each of the six test runs. A flow valve was connected to the lower end of the column to permit a flow rate of 60-70 milliliters per minute.

Since the resin was received as the sodium salt, it had to be regenerated with hydrochloric acid. The manufacturer recommends for the purpose of regenerating a 10 per cent solution of sulfuric acid which is equivalent to 104 grams of hydrochloric acid for the volume of resin used for these tests. In this investigation, five different acid treatments were used for regeneration to avoid experimental error due to insufficient regeneration. All of these acid solutions contained amounts of acid greater than the theoretical amount necessary to charge the column. The amounts of acid used varied from 150 grams to 875 grams of HCl in either 10 per cent or 20 per cent deionized water solutions. The column after regeneration was flushed with deionized water until chloride free.

The electrolyzed solution was diluted 1:1 with deionized water prior to introduction into the resin column. This was done because the high concentration of chromic acid in the undiluted electrolyzed solution would decompose the resin molecules. The column was washed subsequently with deionized water until free of the color of the dichromate ion. The effluent and washings were combined and diluted to a volume in definite ratio to the electrolyzed solution.

Several different methods were used to recover those cations adsorbed in the resin column. These methods involved agitating the resin in a beaker with varying portions of hydrochloric acid, pouring the acid through the column of resin, and combinations of the two. The amounts of acid used for recovery purposes varied from 200 to 1000 grams of HCl in both 10 per cent and 20 per cent solutions. All methods proved that the cationic sulfate was bound more tightly to the resin than the free trivalent chromium. Although different methods of recovery resulted in varying amounts of sulfate and trivalent chromium recovery, no definite relationship could be set up between ion recovery and method of regeneration.

METHODS OF ANALYSIS
Trivalent chromium analyses were made on all solutions with the exception of those for test run number four. The method used for the determination of trivalent chromium was developed at this laboratory using a modification of the method of Willard and Young6 in order that the titrations could be made electrometrically on an automatic recording titrometer. (Manufactured by the Precision Scientific Co., Chicago 47, Ill.)

A radiometric method for determining small percentages of substances is given by Friedlander and Kennedy.⁶ This method has been adopted and modified by this laboratory for the determination of sulfate concentration in chromic acid baths and is described as the radioactive isotope dilution method.⁷ Briefly, this method involved adding to a solution containing an unknown quantity of a substance, a known amount of the same substance which is radioactively tagged. Then the specific activity (activity per unit weight) of the pure compound isolated from the solution is compared with that of the added substance The amount of unknown material present is given by the formula:

W = (Sa/Sb – 1) Wa

Where

W = Weight of the unknown
Wa = Weight of known added
Sa = -Specific activity of known added
Sb = Specific activity of mixture

Because of the extremely low concentration of the isolated complex in the regenerating acid solution, it was considered advantageous to use a revision of this method as given by Calvin.⁸ He stated that if only a small amount of substance is present in a sample, the ratio of non-active substance to active substance could be increased to better proportions if the active substance were placed directly in solution as the unknown instead of as the diluent. In this case the dilution is made with a known quantity of nonradioactive material. This modification, called the reverse isotope dilution method, was used for the sulfate determination of the various solutions. A brief outline of the method9 as formulated for this problem is as follows:

1. Three ml of the test solution were pipetted into a 250-milliliter beaker.
2. Then 7.5 ml of 0.1 Normal H₂SO₄ were added.
3. Then 50 ml H₂O, 10 ml concentrated HCl, 15 ml CH₃COOH, and 20 ml C₂H₅OH were added.
4. The sample was placed on a hot plate and boiled for 15 minutes.
5. While the solution is still hot 15 ml of 10 per cent BaCl₂ were added.
6. The sample was allowed to stand in a warm location for two hours.
7. Each sample was filtered using a 1-1/8 inch disc of S & S No. 597 filter paper mounted on a special suction apparatus.
8. The filter paper disc was mounted on a suitable aluminum disc support, dried under an infrared lamp and counted.
9. A control sample was prepared using 15 ml of the starting solution in a 250-ml beaker. Then steps 3 through 8 were followed.

The counting was done using a Model 163 Scaler employing a TGC-2 neon filled Geiger Müller tube with a mica window thickness of 1.7 mg/sq cm. The samples were centered on an aluminum plate placed on the first shelf of a Lucite mount so that the surface of the samples was ten millimeters from the tube window. The entire assembly was mounted in a vertical lead shield with 1-1/2 inches of lead shielding.

The formula for calculating the amount of unknown present by the reverse isotope dilution method

            Y
X =   ——
         R – 1

where

X = mg of SO₄—/3 ml of test solution
Y = mg of SO₄—added
R = the activity of the control (counts/minute) divided by the activity of the unknown solution (counts/minute).⁸

TEST RESULTS AND DISCUSSION
The results of the quantitative tests are summarized in Tables I, II and III. The trivalent chromium analyses were run in duplicate and the sulfate analyses in triplicate. The values given in the tables are average results for these tests. Examination of these tables has led to the following conclusions:

1. No significant change in the sulfate concentration was noted due to electrolysis. This fact shows that the sulfate ion does not affect chromium plating by entering into the actual oxidation reduction mechanism and also that it forms no insoluble products as a result of the plating process.
2. An average of approximately seven per cent of the total sulfate concentration was removed from the electrolyzed solution by a cationic resin column as shown in Table I. In order to determine whether this loss was due to actual cationic exchange instead of simple mechanical adsorption of free sulfate a control was run in the absence of trivalent chromium. A solution was prepared containing 250 grams per liter of CrO₃ and a CrO₃/SO₄ ratio of 100 to 1. All of the chemicals used were of reagent quality and free from trivalent chromium. A 750-ml portion of this solution was diluted to 1500 ml with distilled water and run through the cationic resin exchange column in a manner identical to that used in all of the tests described in this paper. Sulfate analyses then were made on both the starting solution and the effluent from the column. The starting solution and the column effluent were found to contain 2.400 and 2.415 grams per liter of sulfate respectively. This proved that sulfate is lost to the resin only in the presence of trivalent chromium and in the-form of a positive complex ion.
3. Of the amount of cationic complexed sulfate removed by the column, an average of 40 per cent was regained as shown in Table III. The differences between regeneration efficiencies noted in Table III are due largely to differences in methods of regeneration. However the difficulty in removing the sulfate containing complex was shown definitely.
4. Further proof of the presence of complex chromic sulfate ions was given by the fact that of the 92 per cent of trivalent chromium adsorbed by the column, an average of 79 per cent was recovered in the regenerating acid solution as shown in Table III. This was twice the regeneration efficiency of the complexed sulfate ion. Theoretically, the higher the positive valence of an ion and the larger its ionic size the harder it is to remove from the exchange resin. The fact that chromium has a valence of plus three means that it will be held relatively fast to the resin in comparison to an ion of equal size and lower valence. Since the valence of the complex would hardly be more than plus two or in part plus four (taking into consideration the minus two charge of the sulfate and a possible binuclear structure) the size of the ion must be considerably larger than the chromic ion. This accounts for the fact that the chromic ion was removed twice as easily as the chromium-sulfate complex ion.

DISCUSSION
Although the presence of trivalent chromium does produce a distinct decrease in the electrical conductance of chromic acid plating baths,¹⁰ plating has been lone directly from trivalent chromium solutions.¹¹ This fact leads to the conclusion that in a pure solution of chromic acid the formation of trivalent chromium itself-does not retard plating, but rather it forms certain compounds which polarize the cathode. Such a compound has been postulated by Erich Müller¹ and others who state that the pH near the cathode is high enough (between 2 and 3) to permit the formation of basic colloidal layer of CrOHCrO₄. This layer is permeable to the small, highly mobile hydrogen ions tint will not permit the hexavalent chromium ions to reach the cathode where they may be reduced. Hence in a bath of pure chromic acid only hydrogen is evolved. However when sulfate or some other ion such as chloride, fluoride and fluoborate is added to the bath the chromium will plate.

Although conclusive experimental data proving the function of these ions and the sulfate ion in particular are lacking, certain fundamental facts of chromium plating point to the importance of the formation of a positive chromium-sulfate complex ion such as that found in the work explained in this paper. When the sulfate ion is added to a chromium plating bath it combines with the available trivalent chromium to ,form complex species of the Werner-type according to the following equation:

mCr⁺⁺⁺ + nSO₄^-- + xH₂O [Cr_m(SO₄)_n · xH₂O]^3m-2n

This reaction is rather slow with low concentrations of Cr⁺⁺⁺ and SO₄^-- and would not take place immediately in a new plating bath. For this reason fresh commercial plating solutions must be run with ”dummy” cathodes for from a few hours to many days depending on the rate of formation of trivalent chromium. It is also to be expected that too small an amount of sulfate would reduce the rate of formation of any complex ion and thus reduce the efficiency of the plating bath. There is also an optimum concentration of sulfate which can be present. Above this value the efficiency of the plating reaction is greatly reduced and plating may even stop completely. With a higher concentration of sulfate present, complex ions containing a greater number of sulfate groups are formed. These complexes would be anionic in nature and thus would not migrate to the cathode film layer.

In order to explain the above facts, the following theory is offered. At the cathode in a pure chromic acid bath the insoluble basic compound CrOHCrO₄ is formed. When the sulfate ion is added to the bath, it combines with the available Cr⁺⁺⁺ to form a positive complex ion such as Cr(OH₂)SO₄+ which shifts the equilibrium of the insoluble CrOHCrO₄ and causes it to ionize.

CrOHCrO₄ + SO₄^-- + XH₂O [Cr(OH₂)SO₄]+ + OH^– + CrO₄^--

The freed hydroxyl ion then moves toward the anode and the compound dissolves thus permitting the hexavalent chromium ion to compete with the hydrogen ion in the reduction process.

Based on the results of the several tests conducted and the discussion evolved therefrom, it is concluded that a chromium sulfate complex is formed in the chromic acid plating bath during electrolysis, and that the function of this complex is to dissolve the basic colloidal compound around the cathode, thus permitting the free migration of the hexavalent chromium ion to the cathode where it may be reduced to the

ACKNOWLEDGMENT
The authors wish to express their appreciation to their co-workers at the Rock Island Arsenal Laboratory for their assistance and to the Ordnance Corps, Research and Development Division of the Department of the Army and the supervisory staff of the laboratory for permission to publish the information in this paper.

References
1. Erich Müller (Tech. Hochschule Dresden), Reichsamt Wirtschaftsausbau, Chem. Ber. Pru-Nr. 1S (PB52010), 61-69 (1942)
2. C. G. Fink, U. S. Patent 1,581,188 (April 20, 1926).
3. George Dubpernell, Modern Electroplating, p. 120, published by The Electrochemical Society, N. Y. (1942).
4. R. Seegmiller and V. A. Lamb, Proc. Am. Electroplaters’ Soc. 35, 125-132 (1948).
5. Hobart H. Willard’ and Philena Young, Trans. Electrochem
Soc. 67, 347-356 (1935).
6. G. Friedlander and J. W. Kennedy, ”Introduction to Radiochemistry,” John Wiley & Sons, Inc., N. Y. (1949).
7. Stanley L. Eisler, TING 39, 1019-1023 (Sept. 1952).
8. Melvin Calvin, ”Isotopic Carbon,” John Wiley & Sons, Inc., N. Y. (1940).
9. H. H. Willard and R. Schneidewind, Trans. Electrochem. Soc., 56, 333-349 (1929).
10. Gunter Dehmel, ”Metall-und Schmuckwaren,” Fabrikat Verchrom, 24, 16 (1943).
11. R. R. Lloyd, W. T. Rawles and R. G. Feeney, Trans. Electrochem. Soc. 89, 443-454 (1946).

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