# The Evaluation of the Detection of Cr(VI) in Leather
## 3 Results
This supports the argument that variance in results is not because of Cr(VI) formation during the process but a consequence of the turbidity of Cr(III) olation products. The issues of turbidity caused by Cr(III) olation and protein interferences are significantly reduced with the chromatographic method. The method uses a cationic medium that is retentive of anions and therefore can differentiate between Cr(III), which exists in solution as the Cr\({}^{3+}\) cation, and Cr(VI), which exists as the CrO\({}_{4}\)\({}^{2-}\) anion [19]. Cr(VI) has two UV-visible absorbances: 290 and 372 nm. The absorbance at 290 nm is a consequence of a charge transfer between oxygen and Cr(VI) of the CrO\({}_{4}\)\({}^{2-}\) ion and is unaffected by pH; however, this absorption band has weaker absorptivity compared to the absorbance at 372 nm [20]. The maximum absorbance wavelength of Cr(VI) is pH dependent and shifts from 350 nm under acidic condition to 374 nm at pH 10 [21]. The absorbance is also an O-Cr(VI) charge transfer, but the absorbance affected by protonation and deprotonation of an O ligand [21]. Despite the pH dependence, the absorbance at 374 nm is used for the quantification of Cr(VI) in aqueous solution [21]. Figure 4 is a direct CrO\({}_{4}\)\({}^{2-}\) detection chromatogram, (arbitrary intensity, 372 nm) of a CN membrane-filtered solution containing Cr(III) and Cr(VI). Cr(VI) is hindered by the column medium and elutes at 8.61 min, whereas the Cr(III) is unhindered by the column medium and is eluted at the same time the sample injection front reaches the detector. Figure 3: (a) Apparent Cr(VI) concentration (mg kg\({}^{-1}\)) of a Cr(VI) and gelatine solution vs. extraction time (minutes), (b) UV spectra of the same solution after 115 min: with DPC (Blue), without DPC (Red). Although Cr(III) and protein interferences can be mitigated with the chromatographic method, dyes can still be a significant interference. Figure 5 is a chromatogram overlay (normalised arbitrary intensity) comparing a model Cr(VI) solution (blue), the extract from a Cr(VI)-free leather containing yellow dye (green) and the extract from a Cr(VI)-free leather containing red dye. Both dyed heathers showed significant dye leaching, which was present after a solid phase extraction step with a C18 silica medium (ACROS Chemicals, Cumbria, UK). The colourants used were examples of acid (anionic) dyes, which are impeded by the cationic exchange medium. Comparing peak 1 (Cr(VI), no dye) with peak 2 (no Cr(VI), yellow dye) shows how persistent dyes can be removed as an interference as the retention times are clearly different. However, comparing peak 1 and peak 3 (no Cr(VI), red dye) demonstrates there are situations, whereby a dye that can absorb at 372 nm and have a retention time matching Cr(VI), thus giving a false positive result. Figure 6 compares the visible spectra, represented in normalised absorbance, of the peaks designated in Figure 5: Cr(VI) (peak 1) (blue, absorbances at 272 and 372 nm), extracted yellow dye (peak 2) (green, absorbances at 232, 273 nm), and extracted red dye (peak 3) (red, absorbances at 236, 278 and 328 nm). The UV-visible spectra in Figure 6 provide a clear distinction between the red dye and Cr(VI) peaks, thus confirming that  
Figure 4: Chromatogram of a solution containing Cr(VI) (60 \(\upmu\)g dm\({}^{-3}\) and Cr(III) (280 mg dm\({}^{-3}\)). differentiation is possible. Given that false positives from dyes are a possibility, the UV-visible spectra of peaks suspected to be Cr(VI) should be interpreted to confirm species identity. The current edition of BS EN ISO 17075-2 does recommend scanning the 200-500 nm range to aid species identification; however, it is not a compulsory aspect of the methodology [12]. In addition, Annex C does not include a similar recommendation, nor is it a methodological requirement to use a diode array detector (DAD) [12].