Redox Reactions in Chromium-Containing Fixatives for Biological Materials

—A study of redox reaction kinetics in chromium-containing fixatives (mixtures of chromic acid, bichromate, formaldehyde, and acetic acid) showed that these fixatives were unstable chemical mixtures characterized by rapid kinetics of redox processes. Therefore, biological material fixation occurs in a non-stationary solution of a variable composition, including chromic acid, bichromates (chromium(VI)), formaldehyde, acetic acid, formic acid, and chromium(III) acetate. We propose the division of chemical fixation in these fixatives into two successive stages (“two-step fixation protocol”): the tissue is fixed in an acetic acid–formaldehyde mixture (a rapidly penetrating fixing mixture) at the first stage and washed thoroughly, and treated by the mordant for the staining at the second stage. Such a protocol will at least preserve the original composition of chromium-containing fixatives.