Samples Containing Boron

General Information

Occurrence — Boron is the first element in Group 13, and the only nonmetal of this group. The average abundance of boron is ≈9 ppm in earth's crust, ≈18-63 ppm in soils, ≈10 ppb in streams, and ≈0.01-10 ppm in groundwaters. Boron is essential for plant growth, but can be detrimental if it exceeds 2 ppm in irrigation water.

Borax, a general term denoting a number of sodium borate compounds, is found naturally in salt beds. Boric acid is made by combining borax with H₂SO₄ or HCl. When heated, boric acid will form metaboric acid (HBO₂) and then boron oxide (B₂O₃).

Uses — Borax is used for preparation of heat-resistant glass, detergents, porcelain and enamels, fertilizers, and fiberglass. It is also widely used as a flux, due to its ability to dissolve metallic oxides.

Boric acid is commonly used in antiseptics, eye lotions, electroplating solutions, as a fire retardant, and as a catalyst for chemical reactions.

Crystalline boron conducts electricity at high temperatures. Boron carbide (B₄C) is used as an abrasive or composite reinforcing agent. Borides (boron combined with various metals) are hard and heat-resistant, and often used for grinding and polishing. Boron oxide (B₂O₃) is used for the manufacture of heat-resistant glass.

Chemical Properties — Boron is atomic number 5, with atomic mass 10.81 g/mol. Boron in nature occurs as a mixture of boron-10 (19.9%) and boron-11 (80.1%). Typically, it is found in the +3 oxidation state. In compounds, boron is covalently bonded, and one of the 2s electrons is promoted to a p orbital, with an outer electron configuration of 2s¹2p².

At Inorganic Ventures — Inorganic Ventures uses a boric acid starting material that is recrystallized in-house. The purity is confirmed to be 99.9999% using ICP-MS and ICP-OES. The trace metallic impurities are reported on the Certificate of Analysis for each lot. The recrystallized boric acid is stabilized with high-purity ammonium hydroxide in our stock offerings, CGB1 and CGB10.

Sampling and Handling

Boron is moderately soluble in HCl, HNO₃, and HF aqueous matrices, and very soluble in NH₄OH. It is stable with all metals and inorganic anions at low to moderate ppm concentrations, but will "steal" HF from elements that need it for stabilization. If working solutions contain elements that need HF for stabilization, the amount of HF required will increase as boron concentration is increased.

We have observed that samples containing boron at 2 ppb-100 ppb are stable for months in 1% HNO₃. 1 ppm-1,000 ppm boron solutions are stable for years in 1% HNO₃. Higher concentrations of boron (1,000 ppm-10,000 ppm) can be stabilized for years using dilute NH₄OH. All the in-house stability data referenced here were collected using LDPE containers.

For general information on sampling and subsampling, see Part 3 of our Trace Analysis Guide.

Testing Methods — Boron standards are mainly used as primary standards or buffers. Inductively coupled plasma methods are preferred, but make note of the following interferences:

Atomic Spectroscopic Information: (red text indicates severe at ≈concentrations)

Contamination Risk — Boron can be leached from glass containers. Store samples in polyethylene bottles or alkali-resistant, boron-free glassware.

For more on sample contamination risks, see Chapters 8, 9 and 10 of the Inorganic Ventures Trace Analysis Guide.

The Metal and Alloys — The metal, in its crystalline form, is scarcely attacked by acids or alkaline solutions. The amorphous form is soluble in concentrated HNO₃ or H₂SO₄. A mixture of HF, H₂O, and HNO₃ (1:1:1 ratio) can also be used.

For more information on acid digestions of inorganic samples, please see Chapter 11 of the Inorganic Ventures Trace Analysis Guide.

Oxides, Minerals, and Ores — B(OH)₃ is soluble in water. Acid digestions should be avoided with the ores — use caustic fusions in Pt⁰.

Organic Matrices — Our recommendation for samples in organic matrices is dry ashing mixed with Na₂CO₃ in Pt⁰ at 450 °C, then increase the temperature to 1000 °C to fuse. An alternative would be to perform a Na₂O₂ fusion in a Ni⁰ crucible.

For more information on ashing procedures, please see Chapter 14 of the Inorganic Ventures Trace Analysis Guide.

For more information on sample preparation by fusion, please see Chapter 13 of the Inorganic Ventures Trace Analysis Guide.

Detailed handling information related to boron-containing solutions, as well as suggestions for ICP analyses of boron, may be found by clicking on the B element symbol on our Interactive Periodic Table.

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1. Bock, R. (1979). A Handbook of Decomposition Methods in Analytical Chemistry. Weinheim/Bergstr: Verlag Chemie GmbH.
2. Boron. (2021, February 21). Retrieved from Britannica: https://www.britannica.com/science/boron-chemical-element
3. Standard Methods for the Examination of Wastewater. (2012). Washington, DC: American Public Health Association.