Skip to main content

Acetic Acid

Fixing Agent

Acetic Acid



Chemical Formula:


State: Colorless fluid
Concentration: About 5%
Fixation Time: Overnight
Aftertreatment: None
Acid Dyes: Improves
Basic Dyes: Reduced
Additive: No
Coagulant: No
Hardens: No

Before You Begin

Please consult the following guide to safe working with this chemical fixing agent, including how to safely clean up spills.

Safety Note

Acetic acid is not a major safety concern in histology laboratories. Although consumed in foods, solutions of acetic acid that are much stronger than the 5% or less found in vinegar may cause burns. The concentrated acid must therefore always be treated with respect and handled carefully. Although it is a weak acid, that is a reference to how it ionizes and is not a comment on whether it can cause burns or not. It can burn when spilled on the skin and it can burn the mouth if a too-high concentration is ingested, as with faulty pipetting. Use standard laboratory practices when measuring glacial acetic acid in the same way as you would when measuring concentrated hydrochloric acid.

Spills may be flooded with water to dilute the acid, then mopped up with towels and discarded. There are no restrictions, usually, about disposal through municipal wastewater systems, although it is as well to check local regulations. Just ensure that plenty of water is used to flush it away.

It is often not realized that glacial acetic acid is flammable. It will burn with a pale blue flame if ignited. Unfortunately, this flame is difficult to see and it is easy to pass an arm over it should some acid be ignited. Always look carefully at the surface of an open container of the glacial acid to ensure it is safe to handle.


Vinegar has been used for centuries to preserve foods by pickling. The acetic acid in vinegar was produced by fermentation using acetobacter species, often of wine or malted barley and similar materials. Laboratory and industrial acetic acid is invariably chemically produced. The term “Glacial acetic acid” is the name of the concentrated acid which is water free. It is termed glacial because it freezes at about 16°C. Percentage solutions of acetic acid are invariably x mL of this acid diluted up to 100 mL with distilled water. Other forms of acetic acid have been used in the past, and the terms are sometimes encountered. “Pyroligneous acid” is one. This is produced by the destructive distillation of beechwood and contains approximately 5% acetic acid. A simple 5% acetic acid solution may often be substituted, but it should be kept in mind that pyroligneous acid may contain other chemicals from the beechwood which may be required. One of the myriad forms of vinegar may also be encountered. These vary from about 3% to 5% (sometimes more) of acetic acid. They may be replaced with simple acetic acid solutions.

How it Fixes


Acetic acid does not have much effect on proteins, other than to enable swelling by the absorption of water. Its major effect is to precipitate DNA, which is split off from nucleoprotein. For this reason, acetic acid is valuable for the preservation of nuclei, and is often added to fixatives specifically to do that.


Carbohydrates are not affected.


Lipids are largely unaffected at the concentrations normally used.


Collagen swells, and if acetic acid is used alone, it shrinks badly during subsequent dehydration. Other fixing agents can overcome this, so acetic acid is invariably used in conjunction with them. In mixtures, nuclei are very well preserved, but cytoplasmic inclusions may be destroyed.


Several hours to overnight.

Simple Solution

Acetic acid is not used alone for fixation but is incorporated into fixative mixtures, most commonly at a concentration of approximately 5%.


No specific aftertreatment, as it is miscible with both water and ethanol.


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