An update on chromium (VI) in leather methodology

Europe
Published:  25 October, 2019
Credit: PrimeAsia China

The issue of chromium(VI) in leather has been debated for many years, but its significance has recently been raised due to newly proposed European legislation. The European Chemicals Agency (ECHA) has proposed (12/4/19 – Annex XV Restriction Report Proposal for a Restriction Substance Name(s): Skin Sensitising Substances) that the limit should be lowered from its current 3 ppm limit to 1 ppm, so that the leather industry is in line with the textile industry. While the textile industry uses little chromium, mainly in dyestuffs, the leather industry faces detecting 1 ppm of chromium(VI) in a background of 20,000-40,000 ppm of chromium(III).

A project to review the current status of chromium(VI) analysis in leather was jointly funded by the Worshipful Company of Leathersellers and Leather UK, conducted at the Institute for Creative Leather Technologies (ICLT) by Dr Stefan Davies and led by Dr Will Wise. The focus was narrowed when the project team identified possible sources of error in the standard method (BS EN ISO 17075) used for chromium(VI) determination. 

Review

The current limit for chromium(VI) in leather is ‘set’ at 3 mg/kg (3 ppm) having been derived from the graph (below) taken directly from Annex B of the BS EN ISO 17075 standard.

Figure 1.  Graphical determination of the quantification limit of the Cr(VI) test method. X axis is mean results of the interlaboratory tests. Y axis is uncertainties associated with the mean results.

In Figure 1, the solid line depicts 50% error and the broken line represents the error in the actual mean values.  It is widely acknowledged that greater than 50% error in the mean value is unacceptable, as it renders the results statistically unreliable – therefore the point at which these lines cross is the limit of detection for the method. Consequently, and perhaps most importantly, the standard method itself provides rigorous evidence that lowering the limit below 3 ppm would render the colorimetric method meaningless. A point supported by the ICLT’s recent findings.

Table 1, comparing analytical methods, further highlights the issue. In both cases, has the leather passed or failed? Just because the mean value is above/ below a threshold is not the final point because the error also must be considered. Imagine if the new limit of 1ppm is imposed – what then? The percentage error will only increase, leaving an unacceptable situation where one could have a value of 1 ppm ± 1 ppm! An analogous situation is taking a pregnancy test – it comes back as ‘yes’, meaning you are pregnant ± ‘yes’ and ‘no’!

Table 1:  Interlaboratory results from the colorimetric method (ISO 17075-1) and the ionic chromatography method (ISO 17075-2).

Colorimetric method (ISO 17075-1)

Ionic chromatography method (ISO 17075-2)

Mean

Standard Deviation

Mean

Standard Deviation

mg/kg or ppm

mg/kg or ppm

3.71

0.93

2.56

1.17

Table 1 not only highlights the issues with the colorimetric method, but also suggests that the supposedly more accurate chromatographic method is also not capable of testing to a limit of 1ppm - despite the analytical technique having greater sensitivity. The team at the ICLT have identified several ways that will improve the methodology and could reduce the error observed between samples and test houses. The proposed improvements include the following.

  • The extinction coefficient of the coloured complex from the calibration line can act as a check that equipment and calculations are correct.
  • The R value for the calibration line ensures consistency of calibration.
  • Tighter restrictions on timings are important in limiting the possibility of premature testing before a complete reaction has taken place.
  • Specifying a shelf life for analytical solutions minimises precipitation and oxidation in solutions that could alter results.
  • The addition of the term ‘inconclusive’ as a test result, when it’s not absolutely certain that a sample has passed or failed – this has precedent in other standard methods (e.g. BS EN 62321-7-1:2015).

Consequences

These changes will be put to the European Committee for Standardization (CEN) for inclusion in future updates of the Standard. If adopted, these changes will improve the reliability of results but are unlikely to allow the colorimetric method to remain as a viable method for measuring the lower 1 ppm limit.

The ICLT will be submitting a response to the consultation on the proposed legislation changes, explaining the fundamental inability of the current test methods to reach such low levels of detection reliably. This submission will be supported by Leather UK, which will separately examine the impact on the leather manufacturing industry. 

All affected parties are encouraged to submit a response before the consultation closes on December 19, 2019 (link here: https://bit.ly/31FE9lu)

 

Based on a presentation given at Lineapelle by Will Wise, October 3, 2019.

Dr Will Wise