A 99.5 % minimum assay defines a purity floor and, by subtraction, an impurity budget of half a percent. That budget is where the useful information is. Half a percent distributed as moisture behaves entirely differently from half a percent as insoluble matter, sulfate or iron, and a specification that states only the assay leaves the buyer with no information about which. This page sets out how to read and write the 99.5 % specification.
Substance identity
| Chemical name | Ammonium chloride |
| Synonyms | Sal ammoniac, ammonium muriate |
| Formula | NH₄Cl |
| CAS | 12125-02-9 |
| PubChem CID | 25517 |
| Molar mass | 53.49 g/mol |
| Appearance | White crystalline powder or granules, odourless when dry |
Specification
| Parameter | Value | Basis |
|---|---|---|
| NH₄Cl assay, dry basis | ≥ 99.5 % | Grade definition |
| Moisture | ≤ 0.3–0.5 % | Typical commercial range |
| Water-insoluble matter | ≤ 0.02–0.05 % | Typical commercial range |
| Sulfate (SO₄) | ≤ 0.02–0.05 % | Typical commercial range |
| Iron (Fe) | ≤ 5–10 mg/kg | Typical commercial range |
| Heavy metals, as Pb | ≤ 5–10 mg/kg | Typical commercial range |
| Residue on ignition | Low; specify a maximum | To be agreed |
| pH, 5 % solution at 25 °C | 4.5–6.0 | Typical range |
Note the basis. An assay quoted on a dry basis and a moisture limit quoted separately are consistent; an assay quoted on an as-received basis with moisture also counted is not, and the same lot will appear to meet or fail depending on which convention is used. Specify the basis explicitly, along with the analytical method for each parameter — a limit without a method is not enforceable, since different methods on the same sample legitimately give different numbers.
Properties
-
Solubility approximately 29 g per 100 g water at 0 °C, 37 g at 20 °C, 77 g at 100 °C.
-
Dissolution endothermic, about +14.8 kJ/mol.
- Solution pH 4.5–6.0 at 5 %, arising from hydrolysis of NH₄⁺ (pKa 9.25).
- Critical relative humidity near 77 % at 30 °C.
- Dissociates to NH₃ and HCl on heating; apparent sublimation near 337.6 °C; does not melt under ordinary conditions.
- True density 1.53 g/cm³.
Applications
This grade is specified where an impurity has a defined consequence in the process, and the specification should be written backwards from that consequence rather than copied from a template.
Water-insoluble matter matters wherever the salt is used in solution — plating baths, etching, oilfield fluids, fertigation — because insolubles accumulate, foul filters and settle in tanks.
Iron matters in plating and in any application where trace iron discolours a product or interferes with an electrochemical process.
Sulfate matters where sulfate participates in or interferes with downstream chemistry, or where its presence indicates a specific manufacturing route.
Residue on ignition matters in flux and high-temperature applications, because a salt that is supposed to volatilise cleanly must not leave non-volatile ash on the workpiece.
Moisture matters everywhere, since water is both a caking driver and, in fluxing, a source of spatter.
Handling and storage
Store dry and sealed. Moisture is the dominant storage risk for a high-purity grade — not because the salt degrades chemically, but because a moisture specification is easily exceeded in storage and moisture uptake initiates caking that a high-purity powder is otherwise well placed to avoid. Keep away from strong alkalis, strong oxidisers, silver salts, chlorates and nitrites. Control dust; the material is harmful if swallowed and causes serious eye irritation. Corrosive to copper alloys and unprotected steel in moist conditions.
Notes
A single number is a weak specification. Two lots of genuine 99.5 % material can behave very differently in a given process depending on how the remaining 0.5 % is composed. Identify the parameters your process is actually sensitive to, specify limits and methods for those, and treat the assay figure as necessary but not sufficient.