Ammonium sulfate is made by reacting ammonia with sulfuric acid. This straightforward neutralization reaction produces a white crystalline salt with the formula (NH₄)₂SO₄, widely used as a nitrogen and sulfur fertilizer. The process works at both industrial scale and in a simple lab or classroom setting, though the equipment and concentrations differ significantly.
The Core Chemical Reaction
Every method of making ammonium sulfate relies on the same basic chemistry: two molecules of ammonia react with one molecule of sulfuric acid to form ammonium sulfate. In shorthand, that’s 2 NH₃ + H₂SO₄ → (NH₄)₂SO₄. This is a neutralization reaction, meaning an acid and a base combine to form a salt. The reaction releases heat, which matters when you’re scaling up to industrial quantities but is barely noticeable in a small beaker.
The finished product contains 21% nitrogen and 24% sulfur, making it one of the most common fertilizers for crops that need both nutrients.
How It’s Made in a Lab
If you’re making ammonium sulfate for a chemistry class or small-scale experiment, the process involves three stages: neutralization, evaporation, and crystallization. The Royal Society of Chemistry outlines a reliable method.
Start by adding about 20 cm³ of dilute sulfuric acid (0.5 M) to a beaker. Slowly add ammonia solution (1 M) about 5 cm³ at a time, stirring after each addition. Keep adding ammonia until the solution smells faintly of ammonia even after stirring. This slight excess ensures all the acid has been neutralized. Wear eye protection throughout.
Transfer the solution to an evaporating basin and heat it gently over a medium flame so the water boils off steadily. When roughly half the water has evaporated, dip the end of a glass rod into the hot solution and let the drop cool. If it crystallizes on the rod, the solution is concentrated enough. If not, keep heating and test again. The key rule: do not boil it dry, or you’ll lose your product and potentially decompose it.
Pour the hot, concentrated solution into a shallow crystallizing dish and set it aside in a warm place. As the water slowly evaporates over the next day or two, ammonium sulfate crystals will form. If needed, filter the solution to collect the crystals, place them on a paper towel, and let them air dry.
How It’s Made Industrially
About 90% of the world’s ammonium sulfate comes from three industrial routes, according to the EPA. The most direct is synthetic manufacture: anhydrous (water-free) ammonia and concentrated sulfuric acid are combined in a large reactor. The reaction is fast, highly exothermic, and produces a concentrated slurry that gets dried and granulated into the product you’d buy in a bag.
The largest share of production actually comes from byproduct recovery rather than intentional synthesis. Caprolactam production, the process used to make nylon-6, generates ammonia-rich waste streams. Reacting those streams with sulfuric acid captures the ammonia as ammonium sulfate, turning a waste product into something commercially useful.
The third major route is coke oven byproduct recovery. When coal is heated to make coke for steelmaking, the off-gas contains ammonia. That ammonia is scrubbed from the gas and reacted with sulfuric acid, again producing ammonium sulfate. In all three cases, the underlying chemistry is the same neutralization reaction. The difference is where the ammonia comes from.
The Merseburg Process: An Alternative Route
There’s a lesser-known method called the Merseburg process that doesn’t start with ammonia and sulfuric acid at all. Instead, it reacts ammonia and carbon dioxide to first produce ammonium carbonate. That ammonium carbonate is then mixed with gypsum (calcium sulfate), and the two swap partners: you get ammonium sulfate and calcium carbonate (chalk) as a byproduct. This route is appealing in regions with abundant gypsum deposits or large supplies of waste gypsum from industrial processes like power plant flue gas scrubbing.
From Solution to Usable Product
Whether made in a factory or a classroom, the ammonium sulfate starts out dissolved in water. Getting it into dry, solid form requires concentrating the solution through evaporation, then allowing or encouraging crystals to form. In industrial settings, this happens in large forced-circulation evaporators and rotary dryers that produce uniform granules suitable for spreading with farm equipment. A high-purity grade is especially important for agricultural use, since impurities can clog spray nozzles when the salt is dissolved and applied as a liquid.
In a lab, the process is simpler but slower. You’re relying on gentle heating and patient cooling to grow crystals, which is why the crystallizing dish gets left out for a day or more. The crystals you get this way tend to be larger and more irregular than commercial granules, but chemically identical.
Storing Ammonium Sulfate
Ammonium sulfate absorbs moisture from the air, which causes it to clump and cake over time. Store it in a sealed container in a cool, dry, well-ventilated area. Keep it away from strong bases like lime, which would release ammonia gas, and away from oxidizing agents. If you’re storing it alongside other chemicals, the EPA recommends separating salts from acids, bases, and oxidizing powders to prevent dangerous reactions. For garden use, a sealed plastic bucket in a shed works fine as long as it stays dry.