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.