# The Evaluation of the Detection of Cr(VI) in Leather
## 1 Introduction  
Tanning is a process in which a hide or skin is made resistant to biological attack by the reaction of collagen with stabilising (tanning) agents [1]. A range of tanning agents are commercially available, including synthetic materials and plant extracts. Chromium(III) salts have been used in the production of leather as tanning agents since the early twentieth century [2]. Trivalent Chromium (Cr(III)) tanning rose to prominence due to a comparatively quick reaction and relative ease of manufacture compared to plant-based tanning agents [2]. The denaturation temperature of chromium named leather is greater than 100 \({}^{\circ}\)C and cannot be reached by plant tanning, which allows for superior material properties [1]. Currently 33% basic chromium(III) sulphate (Cr(OH)SO\({}_{4}\)) accounts for approximately 80% of the global leather production [3]. Modern chromium tanning utilises chromium(III) salts; however, procedures in leather production have the potential to oxidise Cr(III) to Cr(VI). Industry research has shown this occurrence is related to post-tanning treatment with polyunsaturated oils and high final leather pH [4]. Secondary tanning (retanning) with polyphenic plant extracts and mildly acidic final leather pH can prevent or reverse the oxidation of Cr(III) during production and ageing [5]. Cr (VI) is a carcinogen, mutagen, allergen, skin irritant, and sensitier [6]. Leather goods come into repeated contact with the skin, and the sensitising effects are of current concern [7]. Cr(VI) has been shown to easily penetrate the skin, which is then reduced to Cr(III) by intracellular anti-oxidants or proteins [7]. The in-situ Cr(III) then inserts into DNA or proteins, causing damage [8]. Although Cr(III) is the agent for sensitisation, the transport mechanism through the skin requires Cr(VI), which is considered to be an agent of concern [7]. Due to the impermeability of Cr(III), it is considered to be significantly less hazardous than Cr(VI) [9]. Because of the possibility of Cr(III) oxidising Cr(VI) in leather articles, an international standard test to quantify Cr(VI) in leather is required and is currently provided by BS EN ISO 17075-1&2:2017. Analysis of Cr(VI) in leather presents challenges not encountered by other industries, such as complete extraction from a porous matrix and reliable differentiation and detection in the presence of ca. 10,000 mg kg\({}^{-1}\) of Cr(III) [10]. In situ analysis would limit issues with complete extraction and limit changes in species concentration, but commercially accessible techniques are not available. Currently, Cr(VI) ions must be removed from the substrate, which is achieved physically by leaching into a pH 8 phosphate buffer [11]. There are currently two separate official methods for the analysis of Cr(VI) content within leather: BS EN ISO:17075-1:2017 and BS EN ISO:17075-2:2017. BS EN ISO 17075-1:2017 is a colorimetric method, where extracted Cr(VI) is reacted with 1,5-diphenylcarbazide (DPC) to produce a Cr(III)-1,5-diphenylcarbazone complex, which can be analysed photometrically at 540 nm [11]. BS EN ISO 17075-2:2017 is a chromatographic method where the Cr(VI) is extracted from a leather sample in a technique identical to colorimetric method [12]. The extraction solution undergoes ion chromatography with an anion exchange column followed by UV-visible spectrophotometric detection [12]. The eluted Cr(VI) can be detected by two separate techniques: direct photometric analysis of the chromate ion at 372 nm (annex B) or a post-column reaction with DPC, and the Cr(III)-1,5-diphenylcarbazone complex can be analysed at 540 nm [12]. The European Chemicals Agency (ECHA) has proposed, under the April 2019 Annex XV restriction report proposal for skin-sensitising substances, that the compliance limit for Cr(VI) in dry leather should be lowered from 3 mg kg\({}^{-1}\) limit to 1 mg kg\({}^{-1}\) in line with the textile industry [13, 14].