Cyanides and sulfides: acid contact, toxic gas and storage

One reaction defines this class

Most storage classes are defined by what the substance does on its own. This one is defined by what it does when something else reaches it.

A cyanide salt is stable and, in a sealed container, unremarkable. Lower the pH and it releases hydrogen cyanide. A sulfide is the same story with a different gas: acid contact releases hydrogen sulfide. Neither reaction needs heat, confinement or a catalyst — a leaking acid container and a floor with a slope will do it.

US federal regulation treats this as a textbook incompatibility. 40 CFR Part 264, Appendix V, the EPA’s examples of potentially incompatible waste, pairs Group 5-A — “spent cyanide and sulfide solutions” — with Group 5-B, the acids, and states the consequence in the table itself:

“Generation of toxic hydrogen cyanide or hydrogen sulfide gas.”

That single line is the reason this class exists as a separate storage category rather than being folded into acute toxics.

What the two gases actually do

The numbers matter here more than adjectives, because both gases are dangerous at concentrations that are unremarkable to look at.

	Hydrogen cyanide	Hydrogen sulfide
OSHA permissible exposure limit	10 ppm (11 mg/m³) 8-hour TWA, skin designation (Table Z-1)	No 8-hour TWA. Ceiling 20 ppm; peak 50 ppm permitted for 10 minutes once, “only if no other measurable exposure occurs” (Table Z-2)
NIOSH IDLH	50 ppm	100 ppm
NIOSH recommended limit	4.7 ppm (5 mg/m³) STEL, skin	10 ppm 10-minute ceiling

Sources: OSHA Table Z-1, Table Z-2, NIOSH IDLH documentation for hydrogen cyanide and hydrogen sulfide.

Two features of that table deserve to be read slowly.

Hydrogen cyanide carries a skin notation. The 10 ppm limit is an inhalation figure, but the “skin” designation means absorption through intact skin contributes materially to the dose. Respiratory protection alone is not a complete answer.

Hydrogen sulfide has no eight-hour average at all — only a ceiling and a short peak. OSHA wrote it that way because the hazard is acute rather than cumulative: what matters is the highest concentration reached, not the average across a shift.

The warning sense fails before the gas does

Hydrogen sulfide is famous for smelling of rotten eggs, and that reputation is dangerous. OSHA’s guidance gives the odour threshold as 0.01–1.5 ppm, notes that above 30 ppm the odour is “described as sweet or sickeningly sweet”, and records that at about 100 ppm the effects include:

“Coughing, eye irritation, loss of smell after 2-15 minutes (olfactory fatigue).”

So the concentration at which a worker stops smelling it is the same concentration NIOSH calls immediately dangerous to life or health. The absence of smell in a space where hydrogen sulfide has been detected is not evidence that it has cleared. It is not evidence of anything at all.

The label already carries this hazard

The EU supplemental hazard statements mark exactly this property:

Code	Statement
EUH029	Contact with water liberates toxic gas
EUH031	Contact with acids liberates toxic gas
EUH032	Contact with acids liberates very toxic gas

(ECHA sets out these supplemental properties and rules under Annex II to the CLP Regulation.) The step from EUH031 to EUH032 is the toxicity of the gas released, not the vigour of the reaction — so EUH032 is the marker you want when triaging a store.

Within this class, three substances carry EUH031 (ammonium polysulphides, potassium sulphide, di(benzothiazol-2-yl) disulphide), two carry EUH032 (calcium cyanide and nickel cyanide), and one carries EUH029 (diphosphorus pentasulphide).

How we classify this page’s substances — and where the list is short

Membership comes from three documented-reactivity groups, because GHS has no hazard class for “cyanide” or “sulfide”: CAMEO’s Cyanides, Inorganic (6 substances), Sulfides, Inorganic (8) and Sulfides, Organic (38). The organic sulfides dominate, and that shapes what you see in the list below.

Two honest qualifications follow, and both matter.

First: most of this class is organic sulfur chemistry, not acid-sensitive salts. Thirty-eight of the 52 are organic sulfides — thiophosphate insecticides such as phorate, terbufos, disulfoton and fenthion, thiocarbamates, mercaptans, thioureas. Their dominant hazard is acute toxicity, and several are among the most acutely toxic substances in the whole database: 17 of the 52 carry H300, H310 or H330 — the “fatal” band — and 40 carry the environmental pictogram GHS09. For those substances the class-level headline “keep away from acids” is true but secondary; the reason they are controlled is that they are poisons.

Second, and more useful: a chemical-family list is not the same as a hazard list. Across our whole database, 32 substances carry EUH031 or EUH032 — the statements that mark acid-sensitivity in law — and only five of them are in this class. The others are chemically unrelated and would be missed by anyone auditing by family name:

Not in this class, but liberates gas on acid contact	Gas
aluminium, magnesium and zinc phosphide	phosphine
sodium azide	hydrazoic acid
sodium fluoride	hydrogen fluoride
copper and nickel thiocyanate, thiocyanic acid	hydrogen cyanide
sodium and calcium hypochlorite, chloroisocyanurates	chlorine
sodium bisulphite, metabisulphite, dithionite	sulfur dioxide
barium, calcium, sodium and potassium (poly)sulphides	hydrogen sulfide

The last row is the awkward one: several plain inorganic sulfides carry EUH031 and are nonetheless outside this class, because the reactivity database did not link them. We are recording this as a known limitation rather than quietly presenting the list as complete. If you are surveying a store for acid-sensitivity, filter on EUH029, EUH031 and EUH032 first and use the class list second.

What is actually in this class

Family	Examples
Inorganic cyanides	hydrogen cyanide, calcium cyanide, nickel cyanide, mercury cyanide oxide
Inorganic sulfides	hydrogen sulfide, disodium sulfide, potassium sulfide, ammonium polysulphides, diphosphorus pentasulphide, tetraphosphorus trisulphide
Mercaptans (thiols)	methanethiol, ethanethiol — the odorants added to natural gas
Thiophosphate insecticides	phorate, terbufos, disulfoton, fenthion, demeton, fenamiphos, methidathion
Thiocarbamates and fungicides	captan, captafol, folpet, dazomet, disulfiram, thiabendazole, carboxin
Industrial sulfur chemistry	carbon disulfide, dimethyl disulfide, tetrahydrothiophene, thiourea, thioacetamide
Rubber and polymer additives	benzothiazole-2-thiol, di(benzothiazol-2-yl) disulphide

Forty carry the signal word Danger and twelve carry Warning. Twenty-eight carry GHS06, the skull and crossbones.

Segregation

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

Every prohibition on that list is an acid or an oxidizer, and both routes end in the same place. Acids liberate the gas directly. Oxidizers do it by a longer path — hypochlorite and a cyanide give cyanogen chloride, oxidizers and a sulfide give sulfur dioxide or worse — and oxidizer plus acid contamination in the same room reconstructs the direct route anyway.

Worth noting that organic acids are prohibited exactly as firmly as mineral acids. Acetic acid is a weak acid, and a weak acid is entirely capable of displacing hydrogen sulfide from a sulfide salt. “It’s only vinegar” is not a defence.

The two compatible classes have a chemical logic behind them. Bases keep cyanide as the non-volatile salt — alkaline conditions are how cyanide solutions are held deliberately in industry. Acute toxics are marked compatible because much of this class already is one: the controls that govern an acute toxic store — access control, inventory discipline, ventilation, containment — are the controls these substances need.

NIOSH’s own work practice for a neighbouring class states the same duty from the acid side: “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.”

What the law gives you

Less than the hazard deserves. There is no OSHA storage or segregation standard for cyanides or sulfides — not in Subpart H, not in the laboratory standard at 29 CFR 1910.1450, and not in OSHA’s own index of standards applying to hydrogen sulfide. The exposure limits in Tables Z-1 and Z-2 are enforceable, and 29 CFR 1910.119 Appendix A lists anhydrous hydrogen cyanide at a threshold quantity of 1,000 lb and hydrogen sulfide at 1,500 lb for process safety management — but neither of those tells you where to put the drum.

The duty is therefore built from the general duty clause, the incompatibility examples in the EPA waste rules, and sections 7 and 10 of each substance’s safety data sheet. In practice the design rules that follow are short and worth stating: cyanides and sulfides in a separate, bunded location; no acid — of any strength — in the same containment; no drain shared with an acid store; detection for hydrogen sulfide rather than reliance on smell; and rescue planning that assumes the gas is present, because both of these gases have killed the people who went in after the first casualty.

The skull-and-crossbones pictogram is GHS06. Class-level segregation is a starting point rather than a verdict on any individual substance: 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, organic acids, oxidizers and organic peroxides and self-reactives — and the acute-toxicity side of this class is covered on acute toxics.