borax and boric acid
/ BOR-aks; BOR-ik AS-id /
Boron is rare in the universe but it crops up in two homely substances people have used for centuries: borax, the white powder in old laundry boosters and slime recipes, and boric acid, the mild antiseptic and ant-killer in the bathroom cabinet. These are the everyday face of boron-oxygen chemistry, mined from dried lake beds long before anyone understood their structure.
Boric acid, B(OH)3 (also written H3BO3), is a very weak, unusual acid. It does not donate its own proton like a normal acid; instead its electron-deficient boron grabs a hydroxide from water, B(OH)3 + H2O giving [B(OH)4]- + H+, so it acts as a Lewis acid toward water. The flat B(OH)3 molecule has trigonal-planar boron, three OH groups around it, hydrogen-bonded into sheets, which is why boric acid feels soapy and slippery and flakes off in layers. Borax is sodium tetraborate, usually written Na2B4O7 times 10 H2O, but the borate anion it really contains is a small ring-and-chain cluster [B4O5(OH)4]^2- that mixes three-coordinate (planar BO3) and four-coordinate (tetrahedral BO4) boron, a first glimpse of the endless borate framework chemistry that parallels the silicates.
These compounds matter as the gateway to boron chemistry and as workhorses in their own right. Borate buffers hold pH steady; borax flux dissolves metal oxides in soldering and glassmaking; borosilicate glass (Pyrex) owes its low thermal expansion to boron oxide; and boron compounds are neutron absorbers and flame retardants. The borate equilibrium also underlies boric acid's gentle action and its use as a buffer that does not protonate or deprotonate sharply.
Add borax to a white-glue solution and it crosslinks the polymer into slime: the [B(OH)4]- ions bridge between the hydroxyl groups of the polymer chains, a tidy classroom demonstration of borate's appetite for OH groups.
The slimy crosslinking is the same Lewis-acid grabbing of OH groups that makes boric acid feel slippery.
Boric acid is not acidic in the ordinary proton-donor sense; the H+ it releases comes from water, not from its own O-H bonds. It is a Lewis acid acting on water, which is why it is so weak. Also note boron compounds are toxic and not as harmless as their household uses suggest.