Borax — Frequently Asked Questions

Because in solution it establishes an equilibrium between boric acid and borate that resists pH change across a wide dilution range, holding near pH 9.2. That stability is unusual and is why borate buffer is a laboratory reference standard, and why borax appears in cleaning products and adhesives that must not drift in pH during use or storage.

At brazing temperature it melts and dissolves the metal oxides on the joint surfaces, forming a low-melting glassy layer that floats them away and simultaneously shields the clean metal from air. Without that, the oxide film reforms instantly at temperature and the filler metal will not wet the joint.

It is classified in the EU as toxic for reproduction category 1B, which has removed it from most consumer applications including many traditional household uses. Industrial use under controlled conditions continues, but permitted-use status must be confirmed for the destination market and the specific application before it is designed into a product.

Borate ions form reversible ester bonds with pairs of hydroxyl groups on adjacent polymer chains, tying them into a network. Because the bonds are reversible the resulting gel is viscoelastic rather than permanently set, which is exploited in adhesives, in oilfield fracturing fluids that must gel then break, and in the familiar slime demonstration.