The tanning process in the manufacture of leather relies upon the interaction of $\mathrm{Cr}^{3+}$ with the fibrous protein collagen. Although glycine and L-proline are the most important amino acids (see Table 29.2 ) in collagen, glutamic acid $\left(\mathrm{p} K_{\mathrm{a}}=3.8\right)$ and aspartic acid $\left(\mathrm{p} K_{\mathrm{a}}=4.2\right)$ are also present. During tanning, the $\mathrm{pH}$ of an aqueous solution of $\mathrm{Cr}(\mathrm{OH}) \mathrm{SO}_{4}$ is lowered from $\approx 2.8$ to $3.8 .$ (a) What chromium(III) ion is present in aqueous solution at very low pH and why is $\mathrm{H}^{+}$ needed to stabilize this species? (b) In the absence of collagen, aqueous solutions of $\mathrm{Cr}^{3+}$ at $\mathrm{pH} 3.8$ contain linear, tri- and tetranuclear
(FIGURE CAN'T COPY)
species. Suggest structures for these species. (c) A study [A.D. Covington et al. (2001) Polyhedron, vol. $20, \text { p. } 461\rfloor$ of the interaction of chromium( (11) with collagen states that, at pH $3.8,$ carboxylate groups should compete with hydroxido ligands for chromium. The study concludes that the
predominant chromium-containing species bound to leather is a linear oligomer with nuclearity 2 or 3 Using this information, suggest how $\mathrm{Cr}^{3+}$ interacts with collagen and indicate how $\mathrm{Cr}^{3+}$ may be involved in cross-linking of collagen fibres. (d) $\mathrm{A}$ major concern in the tanning industry is to prevent toxic waste arising from the oxidation of $\mathrm{Cr}(\mathrm{III})$ to Cr(VI). Chromium(VI) may be present as $\left[\mathrm{Cr}_{2} \mathrm{O}_{7}\right]^{2-}$ and two other anions, depending upon the pH. Write equilibria to show how the three $\mathrm{Cr}(\mathrm{VI})$ species are interrelated, and suggest a method to remove them from waste water.