Oxidizing acids: nitric, perchloric and concentrated sulfuric

Two hazards in one bottle

An oxidizing acid does both jobs. As an acid it destroys tissue and attacks metal, releasing hydrogen as it goes. As an oxidizer it supplies oxygen chemically, so material that would merely char in air can ignite instead.

Neither hazard is unusual on its own. What makes the class difficult is that the two hazards ask for different neighbours. Ordinary acid storage says: keep acids together, away from bases. Oxidizer storage says: keep this away from every fuel, including the organic acids that are themselves fuel. Put the two rules on the same shelf and the stricter one has to win — which is why this class ends up with the most restrictive profile on the site, level with the organic peroxides: nine of the twelve other storage classes are prohibited outright, and the single class it can share space with is the oxidizers.

They are the yardsticks of the classification system

The oxidizing-liquid categories are set by mixing the candidate 1:1 by mass with cellulose and timing the pressure rise. The reference mixtures, in OSHA’s classification criteria (Table B.13.1) and in the UN GHS criteria they transpose, are:

Category	The candidate must beat, mixed 1:1 with cellulose
1	spontaneous ignition, or a faster pressure rise than 50% perchloric acid
2	40% aqueous sodium chlorate (and not Category 1)
3	65% aqueous nitric acid (and not Category 1 or 2)

ECHA’s own committee opinion on nitric acid confirms the third line for the EU: “a 1:1 mixture of 65 % aqueous nitric acid with cellulose is given as a reference mixture in the CLP criteria for Category 3 for oxidising liquids.”

Two of the four substances on this page are therefore not simply members of the class — they are the instruments it is calibrated with. That is a useful thing to carry into a walk-round: when you look at a bottle of concentrated nitric acid, you are looking at the definition of a Category 3 oxidizer.

The four substances

Nitric acid — the classification is concentration, not chemistry

The EU harmonised entry treats nitric acid as a sliding scale. As adopted in the seventh adaptation to technical progress (Regulation (EU) 2015/1221, index number 007-004-00-1), the entry is written as “nitric acid … %” with specific concentration limits:

Concentration	Classification
C ≥ 99 %	Ox. Liq. 2 — H272
99 % > C ≥ 65 %	Ox. Liq. 3 — H272
C ≥ 20 %	Skin Corr. 1A — H314
5 % ≤ C < 20 %	Skin Corr. 1B — H314

with the supplemental statement EUH071, corrosive to the respiratory tract.

That table is the whole storage argument in four rows. A bottle labelled “nitric acid” tells you almost nothing until you know the strength: at 5 % it is a corrosive, at 70 % it is a corrosive and an oxidizer, and the segregation duty changes with it. ⚠ These are the values as adopted in 2015; the harmonised list is amended regularly, so confirm the current entry against ECHA before you rely on a specific limit.

Perchloric acid — the hazard is in the ductwork, not the bottle

Perchloric acid carries H271 (may cause fire or explosion; strong oxidizer) and H314. But the reason it has its own body of engineering guidance is what happens after it evaporates.

Perchloric acid vapour condenses in extract ducts and dries to perchlorate salts. The National Research Council’s Prudent Practices in the Laboratory — the document OSHA reproduces as the non-mandatory Appendix A to its laboratory standard, 29 CFR 1910.1450 — describes hoods built for materials that “deposit shock-sensitive crystalline materials”, and notes that these deposits “become pyrophoric when they dry or dehydrate”. The engineering answer is a wash-down hood: ductwork of plastic, glass or stainless steel, spray heads roughly every 10 feet on vertical runs and at each change of direction, horizontal runs avoided. And on heated work the instruction is unconditional:

“The water spray should be turned on whenever perchloric acid is being heated in the chemical fume hood.”

The EPA describes the same equipment from the water-use side: “to prevent corrosion and reduce explosive perchlorate buildup, perchloric acid fume hoods use a system of nozzles to wash down the fume hood and exhaust system surfaces after each period of use”. NIOSH’s analytical method 7300 puts it in one line: “All perchloric acid digestions are required to be done in a perchloric acid hood.”

⚠ Note what this is and is not. OSHA has no perchloric-acid hood requirement — the laboratory standard, OSHA’s laboratory safety guidance and its fume-hood QuickFacts do not mention perchloric acid at all. This is consensus engineering practice with federal endorsement by reference, not a citable legal duty.

Sulfuric acid — in this class on experience, not on its label

The EU harmonised entry for sulphuric acid is a corrosive one: H314, and no oxidizing hazard statement. Strictly by the label, it does not belong in a class called “oxidizing acids”.

It is here because concentrated and hot sulfuric acid does behave as an oxidizing agent, and because the federal handling guidance treats its incompatibilities accordingly. NIOSH’s criteria document for occupational exposure to sulfuric acid states the work practice directly:

“Sulfuric acid shall be isolated from organic materials, nitrates, carbides, chlorates, chromates, cyanides, metallic sulfides, and metal powders or other noncompatible materials because contact with these materials may cause evolution of toxic gases and/or ignition.”

That list is an oxidizer’s incompatibility list, not a plain corrosive’s.

⚠ Be precise about the disagreement. OSHA’s chemical database, the NIOSH Pocket Guide and EPA’s assessments describe sulfuric acid’s headline reactivity as violent reaction with water — none of them calls it an oxidizer in those words. Institutional laboratory guidance frequently does. We place it here because the segregation consequences follow the behaviour, and because getting this wrong is asymmetric: treating sulfuric acid as an oxidizer costs a shelf, treating it as an ordinary acid costs more.

Concentration governs, as it does for nitric acid. Dilute battery acid and 98 % oleum-adjacent material are not the same storage problem.

Chlorosulfonic acid — the odd one out

Chlorosulphonic acid carries H314 and H335. Its defining behaviour is hydrolysis, not oxidation. EPA’s acute exposure guideline documentation records that it “decomposes violently and exothermically in water instantaneously to form equimolar quantities of HCl and H2SO4”, that it “fumes upon contact with air, and the fumes can react with air moisture to form dense mists of HCl and H2SO4”, and that it is “a strong sulfating and sulfonating agent”.

The same EPA document does also call it “a strong oxidant” that “evolves hydrogen on contact with moist metals”, so its membership here is not baseless. But a store planning around it should plan around water first: this is a substance where a wet floor, a damp pallet or a sprinkler discharge is the initiating event.

How we classify this page’s substances

There is no GHS hazard class for “oxidizing acid” — the classification system records corrosivity and oxidizing capacity separately and never combines them. So unlike most classes on this site, membership here cannot be anchored on a hazard code. All four substances come from a single documented-reactivity group, CAMEO’s Acids, Strong Oxidizing.

That is a deliberate choice with a known cost. It is why sulfuric acid, which carries no oxidizing statement, is included — and it follows the same principle OSHA states in its own classification criteria, that where test results and documented handling experience disagree, judgements based on known experience take precedence (Appendix B, B.13.3.3 and B.14.3.3). It is also why the class is small: four substances, where a broader reading of “acid that can oxidise” would pull in chromic acid solutions, hypochlorous acid and others that have no harmonised entry under that name.

All four carry the signal word Danger and the GHS05 corrosion pictogram; nitric and perchloric acid additionally carry GHS03, the flame over a circle.

Segregation: nine prohibitions

Verdict	Classes
Never store with	bases · cyanides and sulfides · flammable liquids · flammable solids · mineral acids · organic acids · organic peroxides · reactive metals · water-reactives
Keep separate	compressed gases · acute toxics
Generally compatible	oxidizers

Three of those prohibitions are the ones most often broken in practice:

Other acids. Filing all acids together is the single most common storage error this class produces. An oxidizing acid mixed with an organic acid is an oxidizer mixed with a fuel; nitration chemistry is an industrial process, not an accident waiting to be discovered. Non-oxidizing mineral acids are prohibited too — mixing nitric with hydrochloric produces aqua regia, and mixing concentrated sulfuric with almost anything aqueous produces a great deal of heat very quickly.

Cyanides and sulfides. Acid contact liberates hydrogen cyanide or hydrogen sulfide. This is not a subtle mechanism — it is written into US federal waste regulation as an incompatibility example, and it is covered in detail on the cyanides and sulfides page.

Reactive metals. Acid plus finely divided metal produces hydrogen at a rate set by surface area, and with nitric acid also produces nitrogen oxides. See reactive metals.

The oxidizer pictogram — the flame over a circle — is GHS03, and the corrosion pictogram is GHS05. 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. Related pages: oxidizers, organic acids, organic peroxides and self-reactives, cyanides and sulfides.