Silver Iodide
Properties
| State | Solid (crystalline) |
| Color | Bright yellow |
| Solubility | Extremely insoluble in water (3 × 10⁻⁷ g/L); insoluble in ammonia; soluble in KI, Na2S2O3 |
| Melting Point | 558°C |
| Boiling Point | 1506°C |
About Silver Iodide
Silver iodide is a bright yellow ionic solid with the formula AgI and a molar mass of 234.773 g/mol, sitting at the insoluble end of the silver-halide series with a Ksp of 8.52 × 10^-17 — about a million times less soluble than AgCl and a hundred thousand times less soluble than AgBr. The trend down the halide series follows Fajans' rules: as the halide ion gets larger and more polarizable, the Ag-X bond accumulates more covalent character, the lattice becomes less ionic and harder to break apart with water, and the salt becomes progressively more insoluble. AgI is best known for cloud seeding, the weather-modification practice that exploits a quiet structural coincidence: the beta-AgI polymorph crystallizes in the wurtzite hexagonal lattice with lattice constants (a = 4.59 angstroms, c = 7.51 angstroms) within 1 to 2 percent of those of hexagonal ice (a = 4.52 angstroms, c = 7.36 angstroms), and that lattice match makes AgI an unusually effective heterogeneous ice nucleator. Drop AgI smoke into a supercooled cloud and supercooled water droplets freeze on the AgI particles at temperatures around -5 °C instead of waiting until -15 °C or below for spontaneous nucleation. The frozen droplets grow by the Bergeron-Findeisen process and fall as rain or snow. Vincent Schaefer and Bernard Vonnegut at General Electric demonstrated the effect in 1946, and operational cloud seeding programs run today for hydroelectric watershed augmentation, drought mitigation, and hail suppression. AgI also has an unusual high-temperature alpha phase above 147 °C with silver-ion conductivity rivaling molten salts, making it a prototype superionic conductor.
Where you'll encounter it
If you've ever flown a small plane through cloud-seeding operations in the western United States or western China, the airborne generators below were burning a 2 percent solution of AgI in acetone with sodium iodide as a stabilizer, releasing roughly 10^14 AgI nucleation sites per gram into the cloud base. The Wyoming Weather Modification Pilot Program ran randomized trials from 2008 to 2014 and showed measurable snowpack increases of around 5 to 15 percent, which is meaningful for downstream river basins like the Colorado. In a solid-state physics lab, alpha-AgI above its 147 °C transition shows ionic conductivity above 1 S/cm — comparable to molten KCl — because the silver ions move freely through a body-centered cubic iodide sublattice as if dissolved in a fluid, and that discovery in the 1960s seeded the entire field of solid-state electrolytes including the lithium-ion battery-electrolyte research that came after. The third recurring use is as a teaching example of how Fajans' rules predict solubility trends quantitatively, with AgF (soluble) > AgCl > AgBr > AgI mapping cleanly onto increasing covalent character.
Common Uses
- Cloud seeding for snowpack augmentation, drought mitigation, and hail suppression
- Superionic conductor research and prototype solid-state electrolyte studies above 147 °C
- Antiseptic and topical antimicrobial preparations at low concentration
- Specialty photographic emulsions where higher contrast at low sensitivity is wanted
- Teaching demonstration of Fajans' rules and Ksp trends across the silver-halide series
Safety Information
GHS: Skin irritation Category 2 (H315), Eye irritation Category 2A (H319), Aquatic acute and chronic Category 1 (H400, H410). Acute oral toxicity of pure AgI is low because of its extreme insolubility, so very little Ag+ is bioavailable, but chronic silver exposure causes argyria — permanent slate-blue dermal discoloration. OSHA PEL for silver compounds (as Ag) is 0.01 mg/m3 (8-hr TWA); ACGIH TLV is 0.01 mg/m3 for soluble silver and 0.1 mg/m3 for metallic and insoluble silver. Cloud-seeding silver concentrations in downwind precipitation are typically below 1 ng/L, well below ecological concern thresholds, but cumulative deposition is monitored in long-running programs like the Wyoming and Idaho operations.
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.