Organic acids: examples, hazards and safe storage

The acid class that is also fuel

Every other acid class on this site is corrosive and nothing else. Organic acids are corrosive and combustible, and almost every mistake made with them comes from filing them by the first property and forgetting the second.

CAMEO Chemicals, the NOAA/EPA reactivity database, states the pattern for the family: “many low molecular weight carboxylic acids (C1-C4) have flash points between 100 and 150 degrees F, and relatively wide flammability limits.” The two most common members bear that out:

Substance	Flash point	Consequence
Acetic acid, glacial	103 °F / 39 °C (closed cup, NIOSH)	Below OSHA’s 199.4 °F flammable-liquid threshold — it is a flammable liquid as well as a corrosive
Formic acid, pure	156 °F / 69 °C (closed cup)	Combustible
Formic acid, 90 % solution	122 °F / 50 °C (NIOSH)	NIOSH classes it a Class II combustible liquid

Two things follow. First, 39 °C is not a laboratory temperature — it is a sunny afternoon inside a metal-clad store, which means glacial acetic acid can sit above its flash point in ordinary European summer conditions. Second, the formic acid rows are a warning about naming: pure and 90 % differ by 34 °F, so the concentration on the drum decides the storage class, not the word on the label.

What organic acids react with

The reactivity is unusually broad, and CAMEO’s group description is worth reading as a list of storage rules rather than as chemistry:

• with bases, organic or inorganic — neutralisation producing “substantial amounts of heat”;
• with active metals — “gaseous hydrogen and a metal salt”;
• with cyanides — hydrogen cyanide;
• with sulfides, sulfites, nitrites, thiosulfates and dithionites — hydrogen sulfide, sulfur dioxide, “flammable and/or toxic gases and heat”;
• with carbonates and bicarbonates — carbon dioxide and heat;
• with strong oxidizing agents — oxidation of the acid itself.

The formic acid datasheet states the same set in one continuous sentence, which is the cleanest single quotation available for this class:

“FORMIC ACID reacts exothermically with all bases, both organic (for example, the amines) and inorganic. Reacts with active metals to form gaseous hydrogen and a metal salt. Reacts with cyanide salts to generate gaseous hydrogen cyanide. Reacts with diazo compounds, dithiocarbamates, isocyanates, mercaptans, nitrides, and sulfides to generate flammable or toxic gases. Reacts with sulfites, nitrites, thiosulfates (to give H2S and SO3), dithionites (SO2), to generate flammable and/or toxic gases and heat. Reacts with carbonates and bicarbonates to generate carbon dioxide but still heat. Can be oxidized by strong oxidizing agents and reduced by strong reducing agents.”

Note that strength is not the point. Acetic acid is a weak acid and it will still displace hydrogen sulfide from a sulfide salt. “It’s only vinegar” has no standing in a segregation argument.

How we classify this page’s substances

There is no GHS hazard class for “acid”. H314 and H290 catch corrosives and metal-corrosives without distinguishing an acid from a base, so this class cannot be anchored on a hazard code the way flammable liquids or acute toxics can. All 41 substances come from a single documented-reactivity group — CAMEO’s Acids, Carboxylic — which keys on the functional group.

That is chemically exact and produces one effect the page should name: the class collects herbicides. 2,4-D, 2,4-DB, 2,4,5-T, MCPA, dicamba, dichlorprop, fenoprop, glyphosate and endothall are all carboxylic acids, correctly grouped, and every one of them is here for a chemical property that is not the reason anyone controls it. Twelve of the 41 carry the environmental pictogram GHS09; six carry an acute-toxicity statement in the H301/H311/H331 band and one in the fatal band.

So the class splits, honestly, in two: bulk industrial acids where corrosivity and flammability govern, and agrochemicals where toxicity and environmental release govern. The segregation table below is written for the first group. For the second, the substance’s own data sheet is the authority.

Twenty-nine of the 41 carry the signal word Danger and twelve carry Warning; twenty-seven carry GHS05, the corrosion pictogram.

What is actually in this class

Where you meet it	Substance
Vinegar, solvents, acetate production	acetic acid
Leather, textiles, silage, descaling	formic acid
Food, cleaning products, descaling	citric acid, benzoic acid, fumaric acid, maleic acid
Rancid-butter and cheese odours	butyric acid, valeric acid, octanoic acid, decanoic acid
Rust removal, bleaching, laboratory	oxalic acid, edetic acid (EDTA)
Acrylic plastics and coatings	acrylic acid, methacrylic acid
Pharmaceutical and peptide chemistry	trifluoroacetic acid, glyoxylic acid, salicylic acid
Depilatories, permanent-wave solutions	thioglycolic acid
Chemical intermediates, alkylating agents	chloroacetic, dichloroacetic, bromoacetic, iodoacetic acid
Herbicides	2,4-D, 2,4-DB, 2,4,5-T, MCPA, dicamba, dichlorprop, fenoprop, glyphosate, endothall

Two of these deserve a flag of their own. Thioglycolic acid is also an organic sulfide and appears in the cyanides and sulfides class — acid and sulfur chemistry in the same molecule. Fluoroacetic acid is one of the most acutely toxic substances in the database and has nothing in common with the rest of the list except its functional group.

Segregation: six prohibitions and nothing marked compatible

Verdict	Classes
Never store with	bases · cyanides and sulfides · oxidizing acids · oxidizers · organic peroxides · water-reactives
Keep separate	flammable liquids · flammable solids · compressed gases · mineral acids · reactive metals · acute toxics
Generally compatible	none

This is one of four classes on the site with no compatible neighbour at all — the others being mineral acids, organic peroxides and compressed gases — and the reason here is the reactivity list above: an organic acid has a reaction route to almost every other class.

Bases are the obvious one, and the hazard is heat rather than the products. Neutralisation of a concentrated acid by a concentrated base in an uncontrolled volume — a bund, a drip tray, a drain — releases enough heat to boil and spatter both.

Oxidizers and oxidizing acids are prohibited because this class is fuel. An organic acid beside nitric acid is not two acids on a shelf; it is a combustible liquid beside an oxidizer, and nitration of organic acids is a deliberate industrial process.

Cyanides and sulfides are prohibited because acid contact liberates hydrogen cyanide or hydrogen sulfide — the mechanism EPA writes into its waste-incompatibility examples, and the subject of its own page.

Water-reactives are prohibited because most organic acids are supplied as aqueous solutions. The acid does not need to react with the water-reactive substance itself; the water it is dissolved in is sufficient.

The mineral acids row is the one most often got wrong in practice. It sits at keep separate rather than never, but “separate” is doing real work: an organic acid is combustible and a mineral acid is not, so the pair is a fuel and a non-fuel that people file together because both bottles say “acid”.

Practically, this class wants its own bunded location with acid-resistant containment, ventilation, no drain shared with a cyanide or sulfide store, and — for the glacial acetic acid drum specifically — the ignition-source discipline of a flammable-liquid store rather than of a corrosives cupboard.

The corrosion pictogram is GHS05 and the flame is GHS02. Class-level segregation is a starting point rather than a verdict on any individual substance, and for this class concentration changes the answer: check SDS sections 7 and 10, or look the substance up in the storage compatibility matrix. The prohibited classes have their own pages — oxidizing acids, oxidizers, organic peroxides and self-reactives, water-reactives and pyrophorics and cyanides and sulfides.