Putting the chrome in chromatography

When it comes to the delicate business of chrome electroplating, a fine balance between additives and impurities is key – and ion chromatography can help you get it just right says Peter Jones.

Inorganic additives are added to plating baths for their catalytic and functional effects on the plating process, but can also enter as impurities that build up over time and reduce the plating efficiency. Chrome electroplating is achieved by use of relatively simple formulations, utilising the reduction of chrome(VI) in chromic acid (CrO3-) to Cr(III), that is, in turn reduced to the unstable Cr(II) and eventually plates as Cr(0). Trivalent chrome (Cr(III)) is usually present in chrome plating baths – in its absence no plating will occur – but above 2-3% w/v and the cathodic efficiency drops considerably and causes a variety of plating problems.

Chrome is commonly used for plating metals due to its hardness and the fact that it can be highly polished to give a lustrous surface. It does not tarnish in air, because it is unstable in the presence of oxygen and forms a thin chromic oxide layer which protects the surface beneath. It has been used in the tanning industries and amongst refineries to impart this corrosion resistance to alloys such as stainless steel. Environmental restrictions on heavy metals waste must also be considered. The most common ionic forms of Chrome are the trivalent Cr(III) and the hexavalent chrome Cr(VI). 

Trivalent chrome is an essential nutrient and can be obtained from many different vegetables and meats and shortages can cause heart conditions, metabolic dysfunction, and diabetes. Exposure to hexavalent chrome, however, causes kidney and liver damage, lung cancer and death in cases of severe exposure. Even mild symptoms include respiratory problems and skin rashes. As chrome is used extensively in metal production and leather tanning industries, workers in this sector are most at risk. It is therefore preferable to reduce the amount discharged to the environment by re-using the chrome containing electrolytes as much as possible, and monitoring the effluent that may be contaminated with Cr(VI). Re-using electrolytes and restoring the chemical composition to the original specification can be achieved by chemically “re-balancing” the electrolyte either by reaction with additives to precipitate the impurities, adjust the pH or increase the levels of catalysts in solution. Impurities can also be removed by a process known as dummy plating – running the solution through a bath with a either a low current density to plate out the impurities or a high current density to oxidise the contaminant. Dummy plating can be a batch or continuous process.

Inorganic impurities in plating bath solutions can cause a variety of problems by interfering with the deposition processes. Similarly should the level of certain additives, e.g. sulphate in chrome(VI) baths, drop below their optimum level the plating process can produce rough, burned deposits or dull spots. It is therefore beneficial to periodically “re-balance” the electrolyte in the baths to restore it to its optimum characteristics, reducing the amount of heavy metal waste to be treated, saving on raw material costs and increasing the availability of the bath for plating.

The common anions found in plating baths (such as nitrates, silicates and phosphates) generally do not adversely affect the plating efficiency of the bath unless present in high concentrations. Hydroxides and carbonates, similarly, do not usually cause problems unless the pH rises to a value sufficient to precipitate the metal hydroxides causing a rough surface to form on the item being plated. Anions of interest in plating baths such as chrome baths include chloride, sulphate, fluoride and chromate - as chrome (VI).

Chloride is used in the chrome plating industry primarily as a catalyst but at higher concentrations it can have detrimental effects on the finished surface – resulting in duller, thinner plating with hazy deposits. It can also etch the cathode surfaces not being plated and erode the anodes at an accelerated rate. Chloride can be removed from the bath either by a precipitation reaction with silver oxide or silver carbonate to form insoluble silver chloride (which is then filtered off) or by dummy plating the electrolyte.

The concentration of sulphate in the bath is critical to maintain a consistent plate appearance as it, also, acts a catalyst for the plating process. However, after a certain tipping-point it behaves as an impurity and causes defects in the plating surface. It can be removed by addition of barium chloride to precipitate the insoluble barium sulphate.

Fluoride is used in some baths as a catalyst, to improve the lustre and surface characteristics such as improving hardness. In non-fluorinated baths the concentration can be kept below 100ppm by dummy plating.
These ions can be quickly and easily analysed in chromic acid bath electrolytes by Ion-Exchange Chromatography (IC). The sample pre-treatment is usually a case of dilution (1:1000 or 1:500) with 50mMol.dm-3 and injecting a 20uL of sample into the ion chromatograph. Even at this dilution factor, high level parts-per-billion of analytes should be detectable with relative ease. A mid-sized (150mm) column of high capacity anion-exchange resin retains the anions to differing extents (based on their charge distribution and shape) under a continuous flow of carbonate based “mobile phase”. The effluent from the column is then monitored by an extremely sensitive flow-through conductivity meter that is thermally stabilised to better than 0.01ËšC. As anions elute from the column they give rise to peaks, whose area can be digitally integrated and is proportional to the concentration of the ions after calibration with external standards. Selection of the appropriate exchange resin, flow rate, and mobile phase is important to keep the analysis time down, whilst adequately separating the anions of interest. Using the Metrosep A Supp 5-150mm column a run time of 25 minutes can be achieved with the fluoride eluting 3 minutes after injection of sample, chloride at 5 minutes, sulphate at 13 minutes and the chromate as Cr(VI) between 21 and 23 minutes and all peaks are baseline resolved. Anion exchange chromatography can also be used to measure the amount of chrome(VI) in undiluted waste waters to extremely low levels, typically single figure parts per billion. 

The A Supp 5 column resin is based on a rugged and versatile poly vinyl alcohol (PVA) polymer backbone, which, as opposed to the more traditional poly styrene divinyl benzene (PS/DVB) columns, shows excellent separation between fluoride and the injection peak – the peak resulting from all the ions that are not retained by the column (i.e. cations). The sample can be analysed without neutralisation, however, this causes a baseline disturbance around the system peak (7-8 minutes). The disturbance is improved by either adjusting the pH, or using a carbonate suppression system to remove all evidence of this system peak. Thus, the inorganic anionic components of the bath can be analysed from g.dm-3 to parts-per-billion level with minimal sample preparation.

Figure 1.

Figure 1 shows a chromatogram showing the analysis of fluoride, chloride and sulphate in a chrome bath sample. Analysis of alkaline, alkaline earth and some transition metals can be performed by using a column containing a different form of ion exchange resin that is selective to cations. The silica based strong cation exchange C2 column provides excellent separation between the most common cations, alternatively the brand new polyvinyl alcohol C3 column can be used, which has the advantage of a larger working pH range and improvements in both capacity and peak symmetry. Cations, however, these tend not to cause problems with the bath unless they are present at levels of about 0.01% w/v or above. Sample neutralisation is not required because the mobile phase in cation analyses is generally a weak acid solution (e.g. 3mMol.dm-3 nitric acid).  A chromatogram of the separation of the six most common cations on the C2-250mm column is shown in Figure 2. 

Figure 2.

The most common metal contaminants though are transition metals such as copper, zinc, nickel and iron, but other metals such as tin, and aluminium may also be present and these all serve to increase the resistance of the bath electrolyte. Higher electrolyte resistivity means that the deposition voltage has to be increased to maintain optimum current density during the plating (according to Ohm’s Law: V=IR). Usually these metals will not ‘plate out’ or deposit during the electrolysis, but over time, increase in concentration unless they are removed. They can also affect the redox equilibrium between chrome(III) and chrome(VI) potentially causing the cathodic plating efficiency to drop. Transition metals like these can be detected with direct conductivity, but the detection limits are significantly increased when the column effluent is reacted with an organic reagent to form a chromophore (an organic bond that absorbs radiation). Using an ultraviolet or visible detector these organo-metallic complexes can be detected with ease at a single detector wavelength.

In summary, ion exchange chromatography with conductivity or ultraviolet detection provides an inexpensive and simple method of analysing the state of health of bath electrolytes in a relatively short time using either the laboratory based compact range of ion chromatographs or with the process based on-line analysers – for continuous monitoring and automation of key plant processes. The compact instruments are the 861 Advanced Compact IC and the 844 UV/Vis Compact IC, each unit contains all the hardware modules necessary for the analysis in a single cabinet with a foot print roughly approximate to a single sheet of A4 paper. The 844 is also capable of monitoring up to 3 wavelengths at a time for more complex analysis.  Automation can be achieved with either system by adding the 838 Advanced Sample Processor to the system. The advanced sample processor can manually prepare up to 148 samples with three rinse stations – using Metrohm’s unique Dosino technology which uses incredibly accurate motor driven burettes to measure volumes with better than grade ‘A’ accuracy, and a solvent-compatible stream selection valve offers up to 3 liquid lines. The 838 Autosampler can also be configured for in-line dilution, or dialysis. This reduces the time spent manually preparing sample and improves the accuracy and reliability of the analysis.

Continuous process monitoring and control is also achievable using the Metrohm range of on-line analysers. The 811 and 821 range of analysers can be fully automated to sample from several different streams or baths and automatically dilute the sample – if necessary adjusting the pH of the sample accordingly. The 821 is the entry level process analyser with a durable and secure case, complete separation of wet parts and electronics, and a chemically inert PEEK flow path ensures no corrosion of the component modules. The 811 analyser offers a significant increase in performance over the 821, up to 10 times better detection limits, it is also available in a dual channel version allowing the simultaneous (twin track) determination of anions and cations.

By Peter Jones, an application specialist at Metrohm UK