White caprolactam-grade ammonium sulphate is the cleanest of the common by-product crystal grades: low free acid, low water-insoluble matter, low iron, and a colour close to white. Where a downstream process is sensitive to insolubles, to colour, or to trace organics, this is the grade to specify. For general broadacre spreading the off-white equivalent is normally sufficient.
Substance identity
Ammonium sulfate · (NH₄)₂SO₄ · CAS 7783-20-2 · PubChem CID 6097028 · 132.14 g/mol · white, well-formed crystals, 2–4 mm predominant.
Specification
— typical commercial ranges for the tighter white cut
| Parameter | Typical range | Note |
|---|---|---|
| Total nitrogen (N) | 21.0 % min | stoichiometric 21.2 % |
| Sulfur (S) | 24 % min | stoichiometric 24.3 % |
| Moisture (H₂O) | ≤ 0.1 – 0.2 % | |
| Free acid (as H₂SO₄) | ≤ 0.03 % | tightest of the by-product routes |
| Water-insoluble matter | ≤ 0.05 % | the parameter that matters most for solution use |
| Iron (Fe) | tens of ppm (commonly quoted ≤ 0.007 %) | |
| pH, 5 % solution | 5.0 – 6.0 | |
| Colour | white | assessed against a retained reference sample |
Properties
Solubility approximately 74–76 g per 100 g water at 20–25 °C, rising to roughly 104 g per 100 g at 100 °C — high enough that concentrated stock solutions are practical, but note that solubility is strongly temperature-dependent and a stock tank made up warm will drop crystals when it cools overnight. Dissolution is mildly endothermic and the solution cools slightly on make-up, which reduces effective solubility during dissolving; allow time and agitation rather than assuming instant dissolution. The solution is mildly acidic, pH 5–6.
Applications
Fertigation and soluble fertiliser blends, where low insolubles protect filters and drip emitters. Spray-water conditioning ahead of glyphosate and some other weak-acid herbicides, where dissolved ammonium sulphate sequesters calcium and magnesium in hard water and limits the formation of poorly active herbicide salts — this application needs a clean, fully soluble grade and rules out coke-oven material. Retail-bagged product and dyed granular product, where the base colour determines the finished shade. Technical and laboratory uses such as protein precipitation (“salting out”), where reproducible low-impurity material is the point.
Note that clean white crystal is agronomically identical to grey coke-oven crystal once it is in the soil. Buying white grade for straight field spreading buys handling and appearance, not yield.
Handling and storage
As for other crystal grades: dry, covered, away from strong alkalis. For solution use, keep bags sealed to prevent airborne dust contamination of otherwise clean material, and dissolve into agitated water rather than dumping into a still tank.
Notes
Do not mix ammonium sulphate with calcium nitrate or other soluble calcium sources in the same concentrated stock tank — calcium sulfate precipitates as gypsum and blocks filters and emitters. Use separate stock tanks and dilute injection.