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Ammonium Sulfate

(NH4)2SO4 salt
Molar Mass
132.140 g/mol
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Properties

StateSolid at room temperature
ColorWhite crystalline solid
SolubilitySoluble in water (76.4 g/100 mL at 20 °C)
Melting Point235 °C (decomposes)

About Ammonium Sulfate

Ammonium sulfate is the protein biochemist's most-used salt and one of the most-used solid fertilizers in agriculture, and the two communities encounter it for entirely different reasons. In a biochemistry lab, it's the classical salting-out reagent — drop ammonium sulfate into a protein extract at progressively higher percent saturation (typically 30%, 45%, 60%, 75% steps), centrifuge the precipitate at each step, and you get a crude fractionation based on protein hydrophobicity. The chemistry is straightforward: sulfate sits at the kosmotrope end of the Hofmeister series, ordering water around itself and effectively pulling solvent away from protein hydrophobic patches, which forces the patches to associate and precipitate. The technique remains the first step in countless purification protocols because it concentrates dilute extracts cheaply and tolerates a wide range of starting buffers. In agriculture, ammonium sulfate provides 21% nitrogen plus 24% sulfur — the dual-nutrient profile is what distinguishes it from urea or ammonium nitrate, and the modest acidifying effect on soil makes it the preferred fertilizer for blueberries, rhododendrons, and other acid-loving crops. The world produces around 15 million tonnes per year, much of it captured as a byproduct of caprolactam manufacture (where the Beckmann rearrangement of cyclohexanone oxime generates ammonium sulfate stoichiometrically) and steel-mill flue-gas scrubbing. Food-additive use as E517 is small in volume but real: it stabilizes commercial yeast and acts as a dough conditioner in industrial bread.

Where you'll encounter it

If you've ever purified an enzyme from a tissue extract or a bacterial lysate, the first step almost certainly involved ammonium sulfate precipitation — the cuts at 30/45/60% saturation are standard enough that biochemists keep the percent-saturation tables on the wall above the cold room. The reagent comes as a fine white powder, and the typical protocol calls for slow addition with stirring at 4 °C until the appropriate cut is reached, followed by centrifugation. In an agricultural context, ammonium sulfate is what gets recommended for blueberry beds when the soil pH is borderline alkaline — the slow proton release as the ammonium hydrolyzes drops pH into the 4.5–5.5 range that the plants prefer. In meteorological data, ammonium sulfate is the dominant fine-particulate aerosol over much of the eastern US and Europe, formed from atmospheric reaction of sulfate (from coal-fired-power-plant SO2) with ammonia (from agriculture).

Common Uses

  • Salting-out reagent for protein purification by fractional precipitation
  • Ammonium-sulfate-based nitrogen fertilizer with sulfur micronutrient
  • Soil acidifier for blueberries, rhododendrons, and acid-loving crops
  • Yeast stabilizer and dough conditioner (food additive E517)
  • Flame retardant for cellulose-based textiles and paper

Safety Information

Very low toxicity. Mild dust irritant to skin, eyes, and airways at high airborne concentrations; no significant systemic hazard from ordinary handling. Prolonged contact can mildly defat skin. Generally safe for food use at the levels permitted by E517. The main practical caution is that bulk storage in damp conditions can cake the granules into blocks, which is a logistics annoyance rather than a safety issue.

This safety summary is for educational reference only and may not be complete. It is not a substitute for Safety Data Sheets (SDS), medical advice, or professional chemical safety guidance. Always consult appropriate SDS and qualified professionals before handling chemicals.

Constituent Elements

N — Nitrogen H — Hydrogen S — Sulfur O — Oxygen

Frequently Asked Questions

What is the molar mass of ammonium sulfate?
132.14 g/mol. Sum 2(14.007) for the two nitrogens, 8(1.008) for the eight hydrogens, 32.06 for the sulfur, and 4(15.999) for the four oxygens, giving 132.14. Worth knowing for biochemistry: 100% saturation at 4 °C is 3.93 M (about 519 g/L), and the percent-saturation tables that biochemists use as ammonium-sulfate-precipitation references work in fractions of that 3.93 M endpoint.
How does ammonium sulfate precipitation work?
Sulfate ions sit at the kosmotrope (water-ordering) end of the Hofmeister series. As ammonium sulfate concentration rises in the protein extract, sulfate ions pull water away from the protein hydration shell, exposing hydrophobic patches on the protein surface; those patches then aggregate with each other to minimize their contact with water, and the aggregated proteins precipitate out as the suspension's solubility limit is exceeded. Different proteins precipitate at different ammonium-sulfate concentrations because they have different surface hydrophobicities, which is what makes the technique useful for fractional separation.
Why is ammonium sulfate preferred as a fertilizer for some crops?
Three reasons. First, it provides both nitrogen (21%) and sulfur (24%); sulfur deficiency is increasingly common in modern agriculture as cleaner air laws have reduced atmospheric sulfur deposition. Second, the slow ammonium hydrolysis releases protons that gently lower soil pH — useful for acid-loving crops like blueberries. Third, the ammonium form of nitrogen is less prone to leaching and denitrification than nitrate, which gives it a longer effective application window in cooler or wetter conditions where nitrate-based fertilizers would lose nitrogen to runoff.