An Alternative to Bromide-Titration Method
by Will Ward(wward@pavco.com) and Mike Kavran (mkavran@pavco.com)

Sodium allyl sulfonate (SAS) is a common additive used in Watts-nickel plating solutions¹. SAS contributes to the overall leveling of the deposit in plating solutions. In some manufacturing facilities, analysis of the SAS concentration provides a rough estimate as to the amount of leveling additive in a plating solution. A determination of SAS concentration is performed by generating excess bromine in situ in an acidic medium using bromate-bromide, while also using mercuric sulfate or stannic chloride to catalyze the addition of bromine to the allyl double bond. The excess bromine then is oxidized by iodide, and the iodine formed is titrated with sodium thiosulfate².

This method works well when SAS is the only reactive material present. However, most proprietary nickel additives contain, in addition to SAS, ethoxylated and propoxylated derivatives of propargyl alcohol and butyne diol, which aid in leveling and provide brightness to plated parts. The propargyl alcohol derivatives react with bromine and give erroneous high results because they react with two moles of bromine, whereas the allyl sulfonate compounds only react with one mole. The unsaturated bond in butyne diol compounds is slow to react, even with a catalyst present, and does not pose as great of a reaction problem as the propargyl alcohol compounds.

Determining the correct SAS concentration is further complicated if aryl sulfinic acids are present. These materials are added to some proprietary nickel additives to overcome deleterious effects that are caused by high levels of copper and zinc in plating solutions. Aryl sulfinic acids react with bromine in acidic media and yield high false measurements for the allyl sulfonate concentration³.

To determine the accurate concentration of SAS in a nickel plating solution, and, thereby, the accurate concentration of leveling agent, a different means of analysis was needed. Ion chromatography (IC) was thought to be the best method to explore.

Table I: Makeup of Standard Watts Nickel Plating Solution

Table II: Hydroxide Eluent Gradient Program

Experimental Procedure
Reagents and solutions: The makeup of a standard Watts-nickel plating solution is shown in Table I. Technical-grade nickel sulfate hexahydrate, nickel chloride hexahydrate, and boric acid were used in solution makeup. Powdered sodium allyl sulfonate with >99% purity was used. The standards, test samples, and instrument eluent were prepared with deionized water (18 MW).

Sample preparation and chromatographic conditions: A 500-µl sample of nickel-plating solution was pipetted into a 100-ml volumetric flask. Before bringing the solution up to volume, 2 ml of 1-M sodium hydroxide was added to the flask to precipitate nickel hydroxide out of the solution. Prior to injection, the sample was filtered through a 0.2-µm nylon filter.

The ion chromatography analysis was performed using an ion chromatograph (Dionex 500, Dionex Corp., Sunnyvale, Calif.) equipped with an AS 50 auto-sampler and a 25-µl sample injection loop. A metal trap column (Dionex MFC-2) was used to prevent metal contamination of the analytical column(s). Following the trap column, there were a guard column (Dionex AG17) (4 x 50 mm) and analytical column(s) (Dionex AS17) (4 x 250 mm). A self-regenerating anion suppresser (Dionex ASRS-Ultra II) was used to reduce background conductivity. All data were acquired using an ED50 electrochemical detector programmed in conductivity mode and processed using proprietary software (Dionex Peaknet 6) provided by the instrument manufacturer.

Figure 1: A typical ion chromatogram of a laboratory-prepared Watts-nickel plating solution.

Table III: Effect of Sodium Hydroxide Treatment on SAS Determination

Hydroxide used for the eluent was generated electrolytically using an EG40 eluent generator. The eluent gradient program is detailed in Table II.

Results
Anion separation and interference: A typical ion chromatogram of a laboratory-prepared Watts-nickel plating solution is shown in Figure 1. The SAS peak is well resolved from the chloride and sulfate peaks.

Since Watts-nickel plating solutions do not contain any chelators, formation of nickel hydroxide precipitates was a concern in the hydroxide eluent, resulting in blockage of the analytical column or instrument tubing⁴. To prevent this from occurring, samples were treated with 2 ml of 1-M sodium hydroxide to precipitate nickel hydroxide while still in the flask. Comparison of precipitated and unprecipitated samples showed no difference in the peak area of SAS (Table III). It was, therefore, decided upon to make the addition of sodium hydroxide part of the procedure to prevent possible future problems with instrument performance and, thereby, increase long-term operation.

Initial analyses of working nickel plating solutions showed the presence of a large peak not found in the Watts-nickel plating solutions that coeluted with SAS (Figure 2). This was suspected to be sodium propane sulfonate, which is formed from the hydrogenation of the allyl compound. Solutions containing both sodium allyl sulfonate and sodium propane sulfonate were prepared and found to give chromatograms similar to those obtained from the working nickel plating solutions where the two peaks would coelute.

Figure 2: Initial analyses of working nickel plating solutions showed the presence of a large peak not found in the Watts-nickel plating solutions that coeluted with SAS.

Figure 3: A good seperation technique, including good baseline resolution, between SAS and its breakdown product was produced by increasing column length.

Table IV: SAS Calibration Curve

Table V: Comparison of SAS Determination Methods in Working Plating Solutions

Closely spaced peaks can be separated by either varying the hydroxide eluent concentration or increasing the column length. Altering the hydroxide eluent concentration yielded a partial separation of the two components but did not separate them far enough apart to achieve baseline resolution. Instead, column length was increased by adding a second analytical column (4 x 250 mm) in series with the first column. This produced a good separation technique, including good baseline resolution, between SAS and its breakdown product (Figure 3).

Calibration and linearity: A series of sodium allyl sulfonate standards with varying concentrations was prepared and analyzed. The concentration range of the standards exceeded the normal operating parameters of the recommended Watts-nickel plating solution to account for the variability found in different manufacturing operations. A calibration curve was drawn, and the correlation coefficient was calculated to be 0.99968 (Table IV).

Working plating solutions: A series of several solutions were obtained from various nickel plating operations that varied in SAS concentration, age, and work throughput. All samples were analyzed for SAS concentration both by the bromine titration and by ion chromatography. The results, shown in Table V, show significant differences between the two methods.

Conclusion
Determination of sodium allyl sulfonate by the bromide titration worked well when SAS is the sole component. But when it is found in a complex system, such as a working-nickel plating solution, then the ion chromatography method is much more desirable.

The data from samples 3 and 4 of the working-nickel solutions showed similar levels of SAS when analyzed by chromatrography, but very different levels of SAS when analyzed by the bromine titration. This difference is theorized to be due to the varying levels of acetylinic compounds present in the working nickel plating solutions. Examining the data from sample 1, where an SAS-free sample still yielded a value for SAS by the titration method, also supported this. In addition, by looking at the data from all five samples, no simple correction factor was observed that would assume a proportional difference between the two methods.

The data supported that analyzing sodium allyl sulfonate by ion chromatography using a dual analytical column setup provided an accurate way to clearly measure SAS in working nickel plating solutions regardless of whatever additives or breakdown products are present. The bromide titration method should only be used for gauging the maximum amount of SAS possible in a working nickel plating solution and, when ion chromatography is not readily available, a very rough estimate of the SAS concentration in a working nickel plating solution.

References

1. Dennis, J.K. & Such, T.E., Nickel and Chromium Plating, Butterworth & Co., London; 1972.
2. Critchfield, F.E., Organic Functional Group Analysis, Pergamon Press, Oxford, New York; 1963.
3. Kolthoff, I.M. & Elving, P.J., ed., Treatise on Analytical Chemistry Part II–Analytical Chemistry of Inorganic and Organic Compounds (Vol. 15), Wiley & Sons, Inc., New York; 1976.
4. Bolger, P.T. & Szlag, D.C., Plating & Surface Finishing, 89(3):52–55; 2002.

Will Ward has been involved in analytical method development since 1977. He joined Pavco as a research chemist in 1995 and manages the Instrumentation Laboratory at Pavco. He can be reached at (e-mail) wward@pavco.com.
Mike Kavran joined Pavco in 1997. He works in the Instrumentation Laboratory as a research chemist. He can be reached at (e-mail) mkavran@pavco.com.