Analysis of organic additives in plating baths

Production of metal goods often requires the surface to be modified according to functional, protective or aesthetic requirements.

This is much less expensive than producing items made solely out of metals which, despite their aesthetic appeal, are not suitable for the purpose of the item.

Typical functional specifications for the surface layer are air or chemical stability, low electrical resistance, catalytic effectiveness, smoothness, colour and shine.

In the plating industry, different metals can be electrochemically deposited onto the surface by galvanic, electroless, or immersion processes and the different types of plating processes depend on the operation of the plating bath and the electrolyte chemistry.

Most common are the electroplating and electroless plating baths although other types of surface treatment are available, such as anodising, passivating, black oxide/cold blacking, chromate conversion, pickling and colouring.

The electrolyte is extremely important in the plating process as the composition affects such deposition features as the rate, layer (especially in electroless applications), geometry and the metal to be deposited onto the surface (which must normally be present in an aqueous, conductive liquid).

Essentially, the electrolyte is an inorganic metal salt solution at a well-buffered pH.

The manner in which deposition occurs, and hence the surface characteristics, may be modified by using one or more organic additives in the electrolyte during the deposition.

Over time, these organic additives are degraded and consumed by the electronic charge being passed through them, competing side reactions or unwanted contamination and this reduces the efficiency of the plating bath or produces surfaces with different characteristics to the intended functional specification.

Re-balancing these electrolytes (returning them to the original composition via addition of bulk chemicals) enables process lines to continue plating to the required specification for longer, without the requirement to exchange the electrolyte.

Additives fall in to several categories depending upon their effect on the deposited layer.

Suppressors - inhibit the rate at which the metal is reduced from the electrolyte salt and deposited onto the substrate surface (the object being plated).

Brighteners - (also known as carriers, or accelerators) increase the rate at which the metal is deposited on the substrate surface.

Levellers - are used to create a smoother, more even surface than the substrate to be plated.

Analysis.

Analysis of the concentration of organic additives is achieved using a technique known as cyclic voltammetric stripping or CVS.

CVS applies a reversible potential sweep to first deposit the metal onto the surface of a suitable rotating disk electrode, and then strip the metal back into solution.

CVS can be used for most plating bath analysis both galvanic and electroless processes, however, a slight modification of the technique must be employed for electrolytes containing iron or in plating baths which use a 'pulse plating' technique - here a mode known as cyclic pulse voltammetric stripping analysis (CPVS) must be employed.

CPVS utilises a potential waveform rather than linear sweep, essentially the deposition and stripping are performed at fixed potentials, with cleaning potentials applied before and after the determination.

Using CVS/CPVS, the plating process and the efficiency of the electrolyte can be modelled on a small scale using just a few millilitres of sample.

The determination of these critical additives in a small amount of sample means that the electrolyte can be rebalanced and the plating continued without having to dispose of hundreds of litres of electrolyte, which, by definition has a high metal content and is subject to heavily regulated environmental legislation.

The analysis is done by comparing a sample of the electrolyte to virgin makeup solution (VMS) - the original composition of the electrolyte without any organic additives - in the case of copper plating baths for example, this is usually sulfuric acid and copper sulfate solution.

A potential sweep, encompassing the deposition potential for the conversion of Cu(2+) +2e(-) a Cu(0), is then applied to the rotating disk electrode and the metal plating occurs as would happen in the plating bath.

The potential sweep is then reversed, and the plated metal is oxidised back into solution: Cu(0) a Cu(2+) + 2e(-), the current is monitored and plotted against potential and the resulting peak can be evaluated.

Suppressors.

Suppressors act in a variety of ways to inhibit the reduction of the metal salts and hence the deposition onto the electrode or substrate surface.

Generally, they are electrostatically attracted to the substrate surface and have large organic groups to sterically hinder access of the metal ions to the electrode surface (mass transport inhibitors).

The suppressor determination starts with a establishing a calibration curve by 'dilution titration', the current is measured for the plating of the VMS.

Aliquots of suppressor standard are then added and the reduction in peak area is recorded, this continues to an evaluation point (usually to the point where amount of deposition has been inhibited by 50 percent).

The amount of suppressor used to reach the evaluation point is then used to calculate the concentration of suppressor in the sample, by replacing the VMS with sample and the suppressor standard again added in aliquots to the evaluation point, the reduction in amount of suppressor added to the sample to reach the evaluation point is equal to amount of suppressor in the sample originally.

Suppressor calibration curves can be recorded as little as once per week, and can be automated using an 800 series dosino, giving much greater precision and accuracy, and decreasing operator involvement to a minimum.

Brighteners.

Brighteners are a class of organic compounds that complex the metal salts and catalyse the deposition of the metal onto the electrode surface.

Typically they are sulphur containing organic molecules (R-S) and form layers on the substrate material.

The brightener content is determined by a standard addition calibration using a standard solution of brightener.

Initially an aliquot of VMS is taken, to which, suppressor is added to ensure the deposition is inhibited to the furthest possible extent (brightener intercept solution).

There will still be a small amount of deposition at this point and the respective peak provides the blank or 'intercept value' used in the 'linear approximation technique' (LAT) for the standard addition curve.

An aliquot of sample is added to the intercept solution and the increase in deposition caused by the brightener in the sample is recorded.

Further aliquots of brightener are added to the solution, and the increase in peak response plotted against concentration of brightener standard.

Effectively the minimum peak area of the intercept solution is used as a zero-point for the peak area of the standard addition curve.

Levellers.

Levellers are organic compounds that congregate at electrode 'hot-spots' or regions of high charge (regions of the electrode or substrate which protrude from the surface) and inhibit the rate of deposition - thus ensuring more deposition in concave regions of the substrate and less deposition in convex regions of the electrode or substrate.

Overall, levellers slightly inhibit the deposition rate but not to the same extent as suppressors.

The leveller determination is performed using a technique known as response curve calibration.

An aliquot of VMS is used that has an amount of suppressor necessary to inhibit the deposition to the maximum possible extent, along with a corresponding amount of brightener (leveller intercept solution).

This solution contains both suppressor and brightener in concentrations in excess of their maximum effect.

Aliquots of a leveller standard are added and a reduction in deposition is observed and recorded as a calibration plot.

The leveller intercept solution is then used in conjunction with a sample solution, the leveller intercept solution is measured first, then the analysis repeated with the addition of sample.

The reduction in deposition caused by the leveller can be interpolated from the nearest two points in the calibration graph, this improves sensitivity due to the low linear range of leveller calibrations.

The USB-controlled 797 VA Computrace from Metrohm ensures a reliable, fast, and extremely quiet method of analysis of the organic components in plating bath electrolytes.

The instrument is completely controlled by the freely distributable Computrace software and for CVS/CPVS requires only a computer and power supply to operate.

CVS and CPVS are two of the nine available modes of analysis available using the 797 Computrace and require only the electrolyte sample, the virgin makeup solution (or VMS) and standards for suppressors, brighteners and levellers - no external gases are required.

The footprint of the 797 is only slightly larger than a sheet of A4 paper and with the addition of Metrohm's uniquely accurate dispensing technology - the 800 Dosino, addition of brightener, suppressor and leveller standards can be automated, reducing the time and improving the accuracy of results.

Full automation can be achieved using either the 813 Compact VA Autosampler or the 838 Advanced VA Autosampler in combination with a 772 Relay Controller and appropriate pumps (either peristaltic or membrane versions are available).

The 797 is also rapidly convertible to analyse trace amounts of metals in waste waters by supplying a 1bar supply of white spot nitrogen and replacing the cell (reaction vessel), reference and working electrodes - so that trace determinations are not affected by contamination with concentrated metal species to form inter-metallic complexes.

All methods are pre-installed with 'dial-up and push go' functionality, but the software also has the ability to create custom methods or tailor existing methods, this gives users access to over 240 trace analysis methods, over 50 CVS and CPVS methods.

The new 'long-life' reference electrode is more robust and easily accessible than ever and maintenance (re-filling) takes seconds, whilst the working electrode can be easily removed and stored in ultra high purity water when not in use, ensuring that the surface of the RDE does not become passivated and that the instrument can be running analyses within minutes of start up.

The recently released version 1.2 of the Computrace control software has methods for CVS/CPVS analysis based on common plating electrolytes from several different suppliers and has several quality assurance procedures including the option to automatically condition electrodes to user defined repeatability limits.

The GLP wizard tests the electrodes prior to every analysis, and will not proceed if the reference, working, or auxiliary electrodes are defective.

The 797 has it's own electrode test board and diagnostic suite which can be used to ensure there is no problem with the background electronics, the instrument hardware (Rom/Ram settings), the software installation or the host operating system.

Methods can be optimised and validated using the GLP Validation feature which performs a validation of accuracy and precision using standard operating procedures.

CVS analysis for plating industries can offer significant savings in terms of extending the working life of plant electrolyte, reducing the cost of consumption and disposal of electrolytes.

The 797 VA Computrace is an incredibly flexible and powerful tool for a host of industrial applications, coupled with its low initial outlay costs and extremely small operating costs this makes it an ideal and reliable method for routine and non-routine applications, switching easily between both.

Metrohm's 60 plus years in electro-analytical chemistry coupled with the voltammetry competence centre provide in-depth technical knowledge and exceptional customer application support.