Ferrous Ammonium Sulfate
Properties
| State | Solid (light blue-green crystals; hexahydrate most common) |
| Color | Light blue-green (hexahydrate); white to pale green (anhydrous) |
| Solubility | Soluble in water (269 g/L at 20°C, hexahydrate); insoluble in ethanol |
| Melting Point | 100-110°C (hexahydrate dehydrates); anhydrous decomposes >300°C |
| Boiling Point | Decomposes before boiling |
About Ferrous Ammonium Sulfate
Ferrous ammonium sulfate, universally called Mohr's salt after the 19th-century German pharmacist-turned-analytical-chemist Karl Friedrich Mohr who introduced it as a primary standard, is the answer to a real analytical problem: simple ferrous sulfate (FeSO4·7H2O) oxidizes in air on the time scale of weeks, drifting from green crystals to brown crusts as Fe²⁺ becomes Fe(OH)3 and Fe³⁺ basic sulfates. The hexahydrate (NH4)2Fe(SO4)2·6H2O — what's actually on every reagent shelf — is a Tutton salt: an isostructural double sulfate where the [Fe(H2O)6]²⁺ octahedron sits in a stable lattice with NH4⁺ and SO4²⁻, and that lattice protects Fe²⁺ from atmospheric oxygen well enough that a sealed bottle stays usable for years. The crystals are pale blue-green, well-formed monoclinic blocks that recrystallize beautifully from slightly acidified water. The defining application is redox titrimetry: Mohr's salt is the gold-standard primary standard for KMnO4 and K2Cr2O7 because (1) it's available at >99.5% purity, (2) it has a high enough MW (392.14 g/mol for the hexahydrate) to make weighing errors small, (3) it's stable enough for accurate gravimetric standardization, and (4) the Fe²⁺/Fe³⁺ → MnO4⁻ → Mn²⁺ stoichiometry is exact and well-behaved at 1.51 V driving force. The classic permanganate titration — 5 Fe²⁺ + MnO4⁻ + 8 H⁺ → 5 Fe³⁺ + Mn²⁺ + 4 H2O, self-indicating by the disappearance of the MnO4⁻ purple — is the reference experiment in every undergraduate quantitative analysis course. Mohr's salt is also the active reagent in the Fricke dosimeter (Fe²⁺ → Fe³⁺ by ionizing radiation, quantified by 304 nm absorbance) used as a chemical reference standard for radiation therapy dosimetry.
Where you'll encounter it
If you've ever titrated a green Mohr's salt solution against bright purple KMnO4 in a quant analysis lab, you remember the moment of the endpoint — a single drop turns the whole flask from clear pale yellow to persistent faint pink and you stop, read the burette, and breathe. The titration self-indicates because MnO4⁻ is intensely colored and Mn²⁺ is essentially colorless; no external indicator needed. In radiation oncology dosimetry the Fricke solution is exactly the same Fe²⁺ chemistry put to a different use: a dilute aqueous Mohr's salt + H2SO4 + NaCl mixture sealed in a quartz cell is irradiated, and OH• radicals from radiolysis oxidize Fe²⁺ to Fe³⁺ with a known G-value of about 15.6 ions per 100 eV deposited. Reading absorbance at 304 nm gives an absorbed dose traceable to a national standard, which is why Fricke dosimetry remains a primary reference even in the age of electronic dosimeters.
Common Uses
- Primary standard for KMnO4 and K2Cr2O7 in redox titrimetry
- Fricke dosimeter solution for ionizing radiation reference dosimetry
- Calibration standard for spectrophotometric Fe(II)/Fe(III) determination (1,10-phenanthroline method)
- Fenton reagent precursor (Fe²⁺ + H2O2 → OH• for advanced oxidation processes)
- Iron(II) source in slow-release fertilizer and analytical-grade plant nutrient studies
- Demonstration of Tutton-salt double-sulfate isomorphism in undergraduate inorganic teaching
Safety Information
GHS: H315 skin irritation, H319 eye irritation, H335 may cause respiratory irritation. Low acute toxicity (rat oral LD50 ~3250 mg/kg). The main practical concern is that Fe²⁺ generates Fenton chemistry in the presence of H2O2 — never combine Mohr's salt with peroxides outside controlled conditions. Iron salts irritate the gut mucosa at high doses; pediatric ingestion of iron-containing supplements is a real ER scenario, though Mohr's salt itself isn't sold OTC. Standard lab PPE — nitrile gloves, eye protection, lab coat — is sufficient. Store in a tightly capped bottle to slow oxidation; a slight rust-brown skin on the surface of crystals indicates oxidation has begun and the bottle should be replaced for high-precision work.
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. We deliberately do not publish occupational exposure limits or other regulatory thresholds: those values are revised over time and differ between jurisdictions, so the only correct source is the current SDS and the regulations that apply where you work.