Extended X-ray absorption fine structure studies of the role of chromium in leather tanning

Extended X-ray absorption fine structure (EXAFS) spectroscopy has been used to study basic chromium(III) sulphate salts as used for tanning collagen and samples of leathers tanned with such salts. The baths containing the leathers were adjusted to a solution pH of 4 by the addition of sodium carbonate at the end of the tanning reaction mimicking the way the process is conducted industrially. In the experimental leather samples, linear tetrameric chromium species appear to predominate with the collagen matrix. The chromium atoms are surrounded, on average by six nearest neighbour oxygen atoms, two chromium atoms are the next nearest neighbours and there are approximately six oxygen atoms in the next coordination sphere; the latter at the limits of experimental determination by EXAFS.