Additive – Coagulant

Any chemical which can preserve the structure or chemical composition of animal or plant tissues is considered to be a fixing agent. In order to accomplish this preservation, fixing agents are primarily effective against the proteins in the tissue. In some way each agent must denature, or chemically alter, these proteins. In the process the individual agents may, or may not, combine with the proteins (or other tissue components). Likewise they may, or may not, precipitate (or coagulate) the proteins as a consequence of their fixing action.

Agents which combine with proteins are called additive, and those which do not are called non-additive. Agents which precipitate proteins are called coagulant, and those which do not are called non-coagulant.

By combining these characteristics it is possible to classify fixing agents into four groups:–

The terms coagulant and precipitant are synonymous in this context as they both refer to precipitation of proteins and coagulation of protein gels.

This classification is of limited usefulness as there is little to no effect on the handling of tissues, nor of the results obtained, whether a fixing agent is attached to the tissue or not or whether proteins are precipitated or not. The exception is chromium as it can be used as a mordant, so fixation with chromic acid or potassium dichromate may be useful for demonstrating some tissue components with a mordant dye.


Fixing agent Additive Coagulant
Acetic acid No No
Acetone No Yes
Chromium trioxide Yes Yes
Ethanol No Yes
Formaldehyde Yes No
Glutaraldehyde Yes No
Mercuric chloride Yes Yes
Methanol No Yes
Osmium tetroxide Yes No
Picric acid Yes Yes
Potassium dichromate Yes No
Trichloracetic acid Yes Yes


Baker, John R., (1958)
Principles of biological microtechnique
Methuen, London, UK.


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