Boric Acid
Formula: H₃BO₃ — Orthoboric acid, sassolite
Appearance: White crystalline powder or pearly flakes
Hazard: Harmful · Reproductive toxin · Irritant
Properties
Weak monobasic acid (pKa = 9.24), mildly acidic in solution. White crystalline solid with a slightly slippery feel due to its layered structure (layers slide on each other like graphite). Readily converts to borax (sodium tetraborate) in alkaline solution. Like borax, the borate ion cross-links polyvinyl alcohol to form slime. Burns with a distinctive green flame — one of the most reliable green flame test agents. Mildly antiseptic; used in eye drops and contact lens solution. Dissolves in hot water more readily than cold.
Historical Context
Boric acid occurs naturally as the mineral sassolite in volcanic regions, particularly at the Larderello fumaroles in Tuscany. This Tuscan source supplied European chemistry and medicine from the 15th century onward. The compound was described by Wilhelm Homberg in 1702, and the site remained commercially significant until synthetic production from borax became standard.
The boron compounds became industrially important through the 20th century with the discovery of borosilicate glass (Pyrex, developed by Corning in 1908) and boron neutron capture in nuclear reactors. Boric acid’s role as a neutron absorber was critical in the Chernobyl accident response — helicopters dropped thousands of tons of boric acid (along with sand and lead) onto the burning reactor to moderate the chain reaction.
The green color boric acid gives to flames was used historically as a test for the presence of boron — it remains one of the most reliably vivid colors in the flame test palette.
Experiments
Green Flame Test: Dissolve boric acid in a small amount of methanol (or ethanol), then ignite carefully in a ceramic dish in a dark room. The flame burns bright green — one of the most vivid colors achievable in a flame test. The color comes from excited BO₂ and B(OH)₂ radicals emitting in the green region. For a safer demonstration, dissolve boric acid in water, soak a cotton wick, and hold the wick in a Bunsen or candle flame.
Slime: Boric acid can substitute for borax in slime-making recipes. Make a dilute boric acid solution (5 g in 250 mL water), adjust to near-neutral pH with a little sodium carbonate, then mix with PVA glue solution. The borate ions cross-link the PVA polymer chains into a viscoelastic slime.
Experiments using this chemical:
- Flame Tests — Distinctive bright green boron emission
- Slime — Alternative boron source for PVA cross-linking
Safety
Moderate concern — reproductive toxin; avoid prolonged skin contact and ingestion.
Boric acid carries the same reproductive toxin classification as borax — avoid repeated or prolonged skin contact, and keep away from children. Acutely, large doses are harmful; small amounts (as in eye drops) are safe. Wash hands after handling.
Incompatible with: Strong bases (converts to borate salts); potassium (violent reaction); acetic anhydride