Potassium Bromide
Properties
| State | Solid (white crystalline) |
| Color | Colorless to white |
| Solubility | Freely soluble in water (678 g/L at 25 °C) |
| Melting Point | 734 °C |
| Boiling Point | 1435 °C |
About Potassium Bromide
Potassium bromide (KBr, 119.002 g/mol) is the colorless cubic alkali halide that for most chemists means one specific thing: the IR pellet. Because KBr has no fundamental absorption from about 400 to 40,000 cm-1, you can mix a few milligrams of solid sample with about 200 mg of dry KBr in an agate mortar, press the powder under 10 tonnes in a Specac die, and pull out a translucent disc that runs cleanly in any FT-IR. That technique has been the default for solid-state organic and inorganic IR characterization since the 1950s — the pellet is so transparent you can read text through it. Outside of spectroscopy, KBr was historically a giant of two industries. In silver-halide photography it was the primary restrainer in developer formulations like Kodak D-76, where ~1 g/L bromide concentration suppressed fog by raising the activation energy for development of unexposed grains. In medicine it was the first effective anticonvulsant, introduced by Sir Charles Locock in 1857 for catamenial epilepsy and used heavily into the early 20th century before phenobarbital displaced it; it persists today as a veterinary anticonvulsant for canine epilepsy, typically dosed at 30-50 mg/kg/day to a target serum bromide of 1-3 mg/mL. Industrially KBr is made by neutralizing K2CO3 or KOH with HBr, often generated in situ from Fe + Br2 in water.
Where you'll encounter it
If you've worked in any organic chemistry lab with an FT-IR, you've ground a sample with KBr, pressed a pellet, and cursed the disc that fractured because the powder was even slightly damp — KBr is hygroscopic enough that a humid morning in the lab will cloud your spectrum with a broad O-H stretch you have to subtract out. The fix is keeping a 200 mL bottle of KBr in a 110 °C oven and only weighing aliquots immediately before use. KBr also shows up as the entrance and exit window material in mid-IR liquid cells and in IR microscope salt plates, where its low cost relative to CsI or ZnSe makes it the default for routine work. If you have a dog with epilepsy, the bottle of bromide solution your veterinarian prescribes is the same compound that 19th-century neurologists used on human patients — a rare case of a 170-year-old drug still in active clinical use, just on a different species.
Common Uses
- FT-IR pellet matrix and salt-plate window material (transparent 400-40000 cm-1)
- Veterinary anticonvulsant for canine epilepsy at 30-50 mg/kg/day to 1-3 mg/mL serum target
- Restrainer in classic silver-halide photographic developers (Kodak D-76 at ~1 g/L)
- Feedstock for AgBr emulsion synthesis in photographic film and paper manufacturing
- Densifying salt in oilfield completion fluids (alternative to NaCl/CaCl2 brines)
- Bromide source for ionic-liquid synthesis and in situ bromination reagents
- Optical-grade single-crystal substrate for far-IR spectroscopy
- Standard reagent for ion-selective electrode calibration
Safety Information
Low acute toxicity (oral LD50 rat 3070 mg/kg). Chronic excess causes bromism — drowsiness, ataxia, acneiform rash, mood changes — because bromide accumulates with a serum half-life of 12-15 days and competes with chloride for renal reabsorption. OSHA: no PEL; ACGIH classifies bromide salts as nuisance dust at 10 mg/m3 inhalable. Not classified as flammable, carcinogenic, or environmentally hazardous in routine use. Standard lab PPE adequate; veterinary patients on KBr need periodic serum bromide monitoring.
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.