Identity, natural occurrence, and analytical methods
Boron is a naturally occurring element that is found in the form of borates in the oceans, sedimentary rocks, coal, shale, and some soils. It is widely distributed in nature, with concentrations of about 10 mg/kg in the Earth's crust (range: 5 mg/kg in basalts to 100 mg/kg in shales) and about 4.5 mg/litre in the ocean.
The most important commercial borate products and minerals are borax pentahydrate, borax, sodium perborate, boric acid, colemanite, and ulexite. At the low concentrations and near-neutral pH found in most biological fluids, monomeric B(OH)3 will be the predominant species present (with some B(OH)4+), regardless of whether the boron source is boric acid or one of the borates. This is because boric acid is a very weak acid (p Ka 9.15). Sodium perborate hydrolyses to give hydrogen peroxide plus metaborate; consequently, it may exhibit chemical and toxicological properties that are somewhat different from those of the other borates.
Inductively coupled plasma (ICP) methods are preferred for the analysis of the low levels of boron found in biological and environmental samples; colorimetric methods must be used with caution.