Boric Acid
Properties
| State | Solid at room temperature |
| Color | White crystalline powder or flakes |
| Solubility | Soluble in water (4.7 g/100 mL at 20°C, much more at higher temperatures) |
| Melting Point | 171°C |
| Boiling Point | 300°C (decomposes) |
About Boric Acid
Boric acid is the textbook example of a Lewis acid that masquerades as a Brønsted acid. With three -OH groups and a central boron, you might expect it to ionize the way phosphoric or carbonic acid does — but it doesn't. Instead the empty p-orbital on boron grabs a hydroxide from water to make tetrahedral B(OH)4-, and the proton released is what drops the pH: B(OH)3 + H2O ⇌ B(OH)4- + H+. That mechanism makes it remarkably weak (Ka = 5.8 × 10^-10, comparable to HCN) and explains why it never quite saturates a solution — even at high concentration the equilibrium leaves plenty of neutral B(OH)3 around. The naturally occurring mineral is sassolite, found in volcanic fumaroles in Tuscany where superheated steam carries it up from depth, and the white waxy flakes have been mined there since the early 19th century. Boric acid sits in a strange position between a benign household pesticide and a critical industrial reagent. In the pantry it's a cockroach killer that works as both stomach poison and abrasive — beetles groom themselves clean and ingest the powder, and it dehydrates them by stripping the cuticular wax. In a nuclear power plant, it's dissolved in primary coolant water at 1000-2500 ppm boron, where the 10B isotope (20 percent natural abundance, with a 3837 barn neutron-capture cross-section) absorbs thermal neutrons via the 10B(n,alpha)7Li reaction to control reactor power. And in the glass furnace, it's the precursor that turns ordinary silicate glass into thermal-shock-resistant Pyrex.
Where you'll encounter it
If you've ever sprinkled roach powder behind a fridge, swabbed a wound with antiseptic eye drops, or held a Pyrex baking dish straight from oven to wet counter without it shattering, you've used boric acid in some form. In a glass furnace it goes in as a calcined frit that helps lower the melting point and reduces the coefficient of thermal expansion of the final glass from about 9×10^-6/K (soda-lime) to 3.3×10^-6/K (Pyrex). In a PWR control room, operators dose boric acid into the chemical-and-volume-control system every fuel cycle, gradually diluting the boron concentration as the fuel burns up.
Common Uses
- Cockroach and ant powder that acts as stomach poison and cuticular abrasive
- Soluble boron source for borosilicate glass, fiberglass, and ceramic frit production
- Antiseptic eyewash at 0.5-3 percent in saline (sodium borate buffered)
- Neutron-absorbing chemical shim in PWR primary coolant at 1000-2500 ppm boron
- Flame retardant treatment for cellulose insulation and treated lumber
Safety Information
Reproductive toxin Category 1B under EU CLP — chronic exposure causes testicular atrophy and developmental effects, which is why it has been progressively restricted in consumer products in the EU since 2010. Acute oral toxicity is moderate (LD50 about 2660 mg/kg in rats), but small children can ingest dangerous amounts from improperly stored insecticide bait. OSHA PEL: 1 mg/m3 (TWA) for boron compounds as boron. Not classified carcinogenic. Standard household use is safe with normal precautions; keep powder out of reach of children and pets, and don't use it near pet food.
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.