Characterization of Chromium Complexes in Chrome Tannins, Leather, and Gelatin Using Extended X-ray Absorption Fine Structure (EXAFS) Spectroscopy

Chrome tannage is the most intensively investigated tanning process. Nevertheless, the nature (size) of the chromium - collagen complexes has not been proofed experimentally yet.
We investigated the structure of the chromium complexes in powder and dilute solutions of chromium alum, a basic chrome(III) tannin (Chromosal B), an acetone dehydrated, Chromosal tanned Wet Blue leather, an industrial manufactured leather before and after artificial shrinkage, and gelatin by Extended X-ray Absorption Fine Structure (EXAFS) spectroscopy.
It is shown that in solid Chromosal B every Cr atom is surrounded by approximately two other Cr atoms. In 0.2 M Chromosal B solution binuclear, in solid chromium alum and its 0.2 M solution mononuclear chromium complexes are present. In leather and gelatin the bound chromium complexes are approximately binuclear. Partial area shrinkage caused by repeated thermal treatment of the chrome leather did not lead to a significant change of the Cr complex inside the leather. These results support the current theoretical concept of chrome tanning in a direct way and demonstrate the great potential of EXAFS investigations in the leather field.