Is Ammonium Sulfate Flammable?

Is Ammonium Sulfate Flammable?

Ammonium sulfate is classified with a flammability degree of 1 according to the NFPA 704. Materials with that flammability classification require significant preheating to catch on fire. Ammonium sulfate has no measured flash point or autoignition temperature.

If ammonium sulfate is subjected to fire there can be the formation of toxic and possibly explosive gases such as ammonia (NH3). If mixed with strong oxidizing agents and strong acids.

As is the case for other ammonium salts, ammonium sulfate can be used to produce explosives.

Ammonium sulfate has a series of different applications including in: food additive, water treatment, fire retardant and even in some vaccines. But the main applications are in laboratories of chemistry and biology.

Ammonium Sulfate General Properties

Ammonium sulfate is a white solid at normal temperatures and pressure. Chemically, it is a salt composed of two ammonium cátions (NH4+) per one sulfate ânion (SO4), it is highly soluble in water.

Ammonium sulfate has an acid character provided by the sulfate ânion, the acidity of ammonium sulfate is relatively low (pKa = -3). In the presence of very strong acids however ammonium sulfate can act as a base in an acid-base chemical reaction.

Below some are presented some ammonium sulfate physical properties.

Chemical formula(NH4)2SO4
Molar mass132.1 g/mol
Density1.8 g/cm3
Melting pointat 235 ºC (455 ºF) it starts to decompose
Water solubility767 g/l at 25 ºC (77 ºF)
Vapor pressure> 0,1 hPa at 25 ºC (77 ºF)
Flash pointNo data
Autoignition temperatureNo data
Crystal structureOrthorhombic

Remarkably there is no known data for the flash point (the lowest temperature in which a material produces vapors that will ignite in the presence of air and an ignition source).

Also, there is no known autoignition temperature given that at around 235 ºC (455 ºF) this chemical starts to decompose into different substances.

Ammonium Sulfate Can Become Ferroelectric

When at temperatures below – 49.5 ºC (57 ºF) ammonium sulfate can become ferroelectric (has its electric polarization capable of being reversed when submitted to an external electric field).

What Happens If Ammonium Sulfate Touches Fire?

If ammonium sulfate goes in contact with fire toxic fumes will likely be formed. It is known that heating ammonium sulfate eventually leads to the formation of ammonia and sulfur dioxide along with nitrogen gas. Both ammonia and sulfur dioxide gases are toxic.

At the same time, the formation of gases leads to an increase in pressure if there is no ventilation for the gases to be released. In a situation in which ammonium sulfate is extensively heated there is a risk for an increment in the fire or to cause an explosion.

What Chemicals Are Dangerous To Mix With Ammonium Sulfate?

Ammonium sulfate reacts violently if mixed with other chemicals such as chlorates, potassium nitrate (K2NO3),  sodium hypochlorite (NaClO).

A solution of 0.1 mol of ammonium sulfate in one liter of water has a pH of 5.5 (meaning it is mildly acidic. 

If barium chloride (BaCl2) is added to an aqueous solution of ammonium sulfate, barium sulfate (BaSO4) is formed as a precipitate along with ammonium chloride (NH4Cl).

Ammonium Sulfate Associated Health Hazards

Overall ammonium sulfate is not highly toxic. Below some relevant data on the known data regarding the toxicity of ammonium sulfate as provided in its safety data sheets.

Acute toxicity LD50 Oral – Rat – male and female4,250 mg/kg (OECD Test Guideline 401)
LD50 Dermal – Rat – male and female> 2,000 mg/kg (OECD Test Guideline 434)
Skin corrosion/irritation Skin – RabbitNo skin irritation – 20 h Remarks: (ECHA)
Serious eye damage/eye irritation Eyes – RabbitNo eye irritation Remarks: (ECHA)
Respiratory or skin sensitization Maximization Test – Guinea pig Result: negative (US-EPA)

OECD: Organization for Economic Co-operation and Development. ECHA: The European Chemical Agency. US-EPA: United States Environmental Protection Agency.

Ammonium Sulfate Production

There are at least two well established methods to produce ammonium sulfate at large scales. The main chemical reaction by which ammonium sulfate is produced is the reaction between ammonia (NH3) and sulfuric acid (H2SO4):

2 NH3 + H2SO4 → (NH4)2SO4 

The industrial process relies on the introduction of a mixture of ammonia and water in gaseous form into an aqueous solution of saturated ammonium sulfate and 2 to 4% sulfuric acid at 60 ºC. As the reaction proceeds more sulfuric acid is gradually added to continue the production.

This chemical reaction is significant exothermic – that is it releases a significant amount of heat.

Another method for the production of ammonium sulfates utilizes gypsum  (CaSO4·2H2O) and ammonium carbonate ((NH4)2CO3) as the raw materials. From the reaction of these two raw materials it is obtained ammonium sulfate and calcium carbonate (CaCO3).

(NH4)2CO3 + CaSO4 → (NH4)2SO4 + CaCO3

Ammonium Sulfate Natural Occurrence

The mineral mascagnite is made of ammonium sulfate. This mineral is found in fume vents (fumaroles) in regions with volcanic activity. Coal fires can also lead to ammonium sulfate formation.

Ammonium Sulfate Applications

Ammonium sulfate has a diverse array of applications, they include:

  • As fertilizer.
  • As a precipitating agent in biology and chemistry laboratories.
  • As food additive.

Ammonium Sulfate Fertilizer

The vast majority of the produced ammonium sulfate is used to fertilize alkaline soils.

The ammonium cation (NH4+) acts as a weak acid in the soil reacting with alkaline substances. This way, the pH of the soil is lowered.

Ammonium sulfate has a lower nitrogen content than ammonium nitrate. However it is considerably more safe to transport and store than ammonium nitrate, which is known to have considerable risks for explosions.

Application As Precipitation Agent

In aqueous solutions, proteins that are otherwise soluble in water can turn insoluble once ammonium sulfate is added to the solution. The main contributing factor for that is the higher affinity of water with ammonium sulfate than with the proteins.

Methods that employ ammonium sulfate as a precipitating agent are called ‘’salting out’’.

Is Ammonium Sulfate Used As A Food Additive?

It has application as a food additive as an pH regulator in breads and flours.

Is Ammonium Sulfate a Fire Retardant?

Ammonium sulfate is used as an additive in ABC fire retardants. Helping out, slowing down or stopping fires. Ammonium sulfate is effective in this application because it is capable of easily absorbing heat. As it does absorb the heat the fire loses its potency.

Additionally, it increases the combustion temperature some materials

Other Applications of Ammonium Sulfate

In the treatment of drinking water, ammonium sulfate can be employed alongside aqueous chlorine sources to generate monochloramine (NH2Cl), which in turn acts on the elimination of microorganisms in water.

Ammonium Sulfate Pollution

Ammonium sulfate is often produced during coal burning, as a consequence there is a significant amount of it suspended in high population centers. Ammonium sulfate, finely divided particles of evaporated ammonium sulfate comprise approximately 30% of fine particulate pollution worldwide.


Ammonium sulfate has relatively low immediate hazards associated with fire and explosions. It is possible for ammonium sulfate to be ignited by heat, but normally, extensive heat would need to be applied.

There are significant risks regarding mixing ammonium sulfate with other chemicals. No handling and mixing of any chemical should be performed without the proper safety measures.

Frequently Asked Questions (FQ): Is Ammonium Sulfate Flammable?

Is ammonium sulfate an explosive?

Ammonium sulfate should not burn or explode under ordinary fires. It is possible for this chemical to cause an explosion if extensive heat is applied to it inside a very confined space. That is because at temperature above 235 ºC (455 ºF) at 1 atm ammonium sulfate starts to decompose into ammonium bisulfite, which in turn decomposes into the gases nitrogen, ammonia and sulfur dioxide if heat is continuously applied.

Can ammonium sulfate be used to make a bomb?

It is technically possible but not feasible. Ammonium nitrate on the other hand can and is in fact used in explosives alongside specific fuels. It has been shown that ammonium sulfate adds stability to ammonium nitrate making it safer to handle and transport.

Is sodium sulfate flammable?

No, sodium sulfate is not flammable and it is not combustible. In a fire it can lead to formation of toxic fumes.

Can urea be used as an explosive?

Urea, (NH2)2CO, itself is not used as an explosive. A derivative of urea, urea nitrate (NH2NH3CO)NO3 has been used in the past as an improvised and powerful explosive.

Are fertilizers flammable?

It depends on the fertilizer, but most commonly used fertilizer are stable in the presence of heat and fire. Ammonia, which is widely used as a raw material for the production of many fertilizers is also not flammable but it does poses significant risks for explosions if not properly handled or stored.

Ammonium nitrate, which can be used as a fertilizer or in the production of other fertilizers, poses significant hazards associated with fire. This chemical can cause explosions under heat and/or mechanical shock if there is a combustible material also present.

Is sodium sulfite hazardous?

This chemical can decompose into toxic gases including sulfur dioxide.

Are Sulfates Flammable?

Technically, sulfate is the name given to the anion with the molecular formula SO4. Sulfates ions are almost always accompanied by either a cation or bound to another atom. Therefore, it can be said if sulfates are flammable or not because that will depend on the entire substance. For example, sodium sulfate (Na2SO4) and calcium sulfate (CaSO4) are inorganic salts and are not considered flammable. However there are many organic substances which contain the sulfate group, and they can be flammable.


Safety data sheet for ammonium sulfate:

Study in the ferroelectric properties of ammonium sulfate:

Y. Okaya, K. Vedam, R. Pepinsky. Acta Cryst. 1958. 11, 307 

Ammonium Compounds Karl-Heinz Zapp, Karl-Heinz Wostbrock, Manfred Schäfer, Kimihiko Sato, Herbert Seiter, Werner Zwick, Ruthild Creutziger, Herbert Leiter First published: 15 June 2000 

Study on the thermal decomposition of ammonium sulfate:

Liu Ke-wei, Chen Tian-lang. Chemical Research and Application. 2002. 14, 6. doi:10.3969/j.issn.1004-1656.2002.06.038

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