Chromium VI test method updated

Explaining changes to this key assessment.

by Charlotte Lovell

Chromium sulphate salts are commonly used as tanning agents for leathers articles used in footwear uppers, handbags, belts and upholstery. These salts contain chromium in its trivalent state (chromium III). However, the chemical reaction from chromium III to chromium VI – Cr(VI) – is an oxidation process and can be facilitated by certain factors – for example, increases in pH, temperature, exposure to ultraviolet (UV) radiation and exposure to oxidising agents. This reaction could theoretically occur during leather processing after tanning, during storage or after the product reached the consumer, although as chromium-tanned leather is an acidic material, the likelihood of conversion to chromium VI in finished leather is reduced.

Since 1st May 2015, an EU regulation to restrict chromium VI in all leathers articles and articles containing leather parts in contact with the skin has been in force. Ensuring substances restricted under the REACH regulation (EC) No 1907/2006 are absent from products is essential for companies wishing to market products in Europe. This is a very important parameter for tanneries, brand owners and retailers – both from a general product safety perspective and in terms of legal compliance.

Chromium VI is also listed in California Proposition 65, so there are labelling implications if it is present in articles on sale in this US state. Due to the way leather is processed in batches (with each animal skin being slightly different) and the complex chemistry involved with chromium VI formation, testing is the only certain way of demonstrating compliance. The international test method to determine the chromium VI content in leathers is EN ISO 17075. This method has a limit of detection of 3mg/kg, which is the maximum allowable concentration specified in the REACH Annex XVII entry 47 (see box below).

At the time of writing, EN ISO 17075-1:2017 and EN ISO 17075-2:2017 were expected to be published imminently to supersede EN ISO 17075:2007. The new test methods were developed by CEN/TC 289 Working Group 01 and the current method has been split into two parts to allow for the use of alternative detection techniques.

It is likely that leather articles or articles containing leather may not contain chromium VI directly after production. The formation of chromium VI can be slow, which means that the reaction can occur during storage after the final article has been produced. As previously mentioned, organisations operating in the EU have legal obligations to ensure that all their products comply with the requirements in REACH Annex XVII. Under well-managed production conditions, chromium-tanned leather should pose a very low risk of containing chromium VI. Indeed, the International Union of Leather Technologists and Chemists Societies (IULTCS) produced a list of best practice back in 2013, with these nine key points listed in box 1.

The risks of chromium VI

In SATRA's experience, failures due to the presence of chromium VI are uncommon. However, the concerns with this dangerous substance sparked its inclusion on restricted substances lists and resulted in the European legislation. Nevertheless, not all forms of chromium are as hazardous as chromium VI. Trivalent chromium is a vital trace element within the human diet – specifically for the insulin hormone which requires chromium to work properly. Another less commonly known use for chromium is as the green pigment within tattoo inks. Chromium VI is a dermatological irritant, and exposure to a small amount of it can trigger an immune response, causing sensitisation of the skin. Allergic reactions to chromium VI can often be severe, leading to prolonged complications with the skin. The medical term for this type of skin irritation is 'chromium allergic dermatitis'. According to a scientific study carried out in Germany, the number of people hypersensitive to chromium VI is low.

The most dangerous hazard associated with chromium VI is its carcinogenicity (the ability to cause cancer). Chronic inhalation of chromium VI has been shown to increase the risk of lung cancer and may also damage the small capillaries in the kidneys and intestines. Another danger to health is through ingestion, but this is predominantly a risk in chromium-tanned footwear intended for children under three years of age, as it is an instinctive behaviour for infants to place items in their mouths.

Testing for chromium VI

Both parts of the international test method for quantifying the amount of chromium VI that can be extracted from leather involve the same extraction procedure. The principle of the testing is that any soluble chromium VI is extracted from the specimen of leather in a water-based solution containing a phosphate salt at a carefully controlled pH of 7.5 to 8. The extraction is carried out in this solution, as it contains the constituents of perspiration, and therefore is likely to be present on the surface of a person’s skin. In the test method, the pH of this extraction solution is crucial in order to avoid artificially promoting the conversion of chromium III to chromium VI, or instigating the formation of chromium VI precipitates.

These precipitates would, in effect, remove the chromium VI from the water-based solution and give an artificially low final result. The extraction solution is referred to as a 'buffer' solution, because it resists a change in pH when small quantities of acid or alkali are added. This means that for leather (bearing in mind that chromium-tanned leather will normally be acidic due to the processes involved in the tanning process), the pH of the extraction solution will generally remain between 7.5 and 8.

EN ISO 17075-1:2017

After the extraction, it is common for the extracted buffer solution to be coloured, as some dyes can easily be washed out of the leather test specimen. These dyes can interfere with the detection technique (UV-Visible spectrophotometer), so a clean-up stage follows where the solution is passed through cartridges to remove the colour from the extract. In EN ISO 17075-1:2017 – 'UV-visible spectrophotometer method', the extracted buffer solution is passed through glass or polypropylene cartridges to remove these coloured dyes from the extraction solution. The cartridges are packed with a sorbent which is activated by a pre-treatment with methanol and water. The sorbent in these cartridges consists of very small, solid particles, that adsorb the dye molecules onto the surfaces of the particles, physically adhering ions and molecules onto the surface of another phase.

In the context of EN ISO 17075, this means the liquid extraction solution travels around the sorbent particles and the dye molecules are removed from the liquid, and adsorbed onto the surface of the sorbent. This process is called ‘solid phase extraction’ (SPE), and is an efficient method of removing impurities or 'interferences' – substances in the extract which may be affected by the detection technique and give an erroneous result – without using large quantities of organic solvents or delicate speciality glassware.

Analysis

The extraction solution that has passed through the cartridges is then mixed with dilute phosphoric acid and a 1,5-diphenylcarbizide solution. This analytical procedure utilises the oxidative nature of chromium VI to react with an indicator (1,5-diphenylcarbizide) to form a strongly coloured red/violet complex of 1,5-diphenylcarbazone. The colour intensity of the solution containing 1,5-diphenylcarbazone has a direct relationship to the amount of chromium VI present in the solution. The intensity of this colour is measured using an ultraviolet visible light spectrophotometer (UV-Vis), which measures the absorbance of 1,5-diphenylcarbazone at a wavelength of 540nm.

EN ISO 17075-2:2017

The leather extraction is identical in both parts of the 2017 version of EN ISO 17075. However, part 2 ('HPLC detection') uses a different detection technique – high performance liquid chromatography with diode array detector (‘HPLC-DAD’). This is a separation technique where a liquid is passed through a column packed with a stationary phase and onto the detector – a direct detection technique for chromium VI at 372nm. After the liquid has passed through the packed column, the substances dissolved in the liquid reach the detector at different times. As a result, the mixture is separated into substances which appear as separate peaks on the HPLC chromatogram. It is this chromatography that reduces the possible interferences compared with the UV-Vis detection technique. It is considered the reference method if both parts of the method are tested and the results differ (although results from inter-laboratory trials did show a good correlation between the two methods).

In conclusion

As REACH Annex XVII entry 47 restricts chromium VI in leather articles and articles containing leather due to chromium allergic dermatitis and other health concerns, everyone involved in the supply chain has a responsibility to ensure that chromium VI is not present in their products.

The chemistry laboratory at SATRA can test to both revised methods available for the detection of chromium VI, although it is anticipated that EN ISO 17075-2:2017 will become the default testing procedure.

Publishing Data

This article was originally published on page 38 of the March 2017 issue of SATRA Bulletin.

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