bouin solution, or bouin's solution, is a compound fixative used in histology. it was invented by french biologist pol bouin and is composed of picric acid, acetic acid and formaldehyde in an aqueous solution. bouin's fluid is especially useful for fixation of gastrointestinal tract biopsies because this fixative allows crisper and better nuclear staining than 10% neutral-buffered formalin. it is not a good fixative when tissue ultrastructure must be preserved for electron microscopy. however, it is a good fixative when tissue structure with a soft and delicate texture must be preserved. the acetic acid in this fixative lyses red blood cells and dissolves small iron and calcium deposits in tissue. a variant in which the acetic acid is replaced with formic acid can be used for both fixation of tissue and decalcification. the effects of the three chemicals in bouin solution balance each other. formalin causes cytoplasm to become basophilic but this effect is balanced by the effect of the picric acid. this results in excellent nuclear and cytoplasmic h&e staining. the tissue hardening effect of formalin is balanced by the soft tissue fixation of picric and acetic acids. the tissue swelling effect of acetic acid is balanced by the tissue shrinking effect of picric acid.