Boric Acid: Properties, Uses & Lab Protocols

Boric acid is a mild, weakly acidic inorganic compound with distinctive physicochemical properties—including temperature-dependent solubility, characteristic thermal dehydration behavior, and a 0.1 mol/L aqueous pH of 5.1—that make it indispensable across biochemistry, molecular biology, and analytical chemistry laboratories.

Chemical Formula

H₃BO₃

Aqueous Solution pH (0.1 mol/L)

5.1

Solubility in Cold Water

1 g per 18 mL at room temperature

Solubility in Boiling Water

1 g per 4 mL

Dehydration Temperature

Loses one water molecule at 100–105 °C to form metaboric acid

Anhydride Formation

Converts to boron trioxide (B₂O₃) at 300 °C

SKU

B111599

Physicochemical Properties of Boric Acid

Boric acid (H₃BO₃) is a white, odorless solid with a slightly acidic-bitter taste that transitions to sweetness, and a characteristic slippery feel upon skin contact. It is stable when exposed to air under ambient conditions and undergoes no appreciable change during normal storage.

Its aqueous solutions are weakly acidic: a 0.1 mol/L solution exhibits a pH of 5.1. Boric acid is soluble in water, alcohols, glycerol, ethers, and essential oils. Notably, its solubility in mineral acids is lower than in water. The compound is also co-volatile with steam, a property exploited in certain analytical separations.

Solubility and Temperature Dependence

The solubility of boric acid in water increases significantly with temperature, a critical consideration when preparing concentrated stock solutions or performing gravimetric analyses. At room temperature, 1 g dissolves in 18 mL of cold water, whereas the same mass dissolves in only 4 mL of boiling water. A similar trend is observed in ethanol: 1 g requires 18 mL of cold alcohol but only 6 mL of boiling alcohol. In glycerol, 1 g dissolves in 4 mL.

Solubility is further enhanced by the presence of hydrochloric acid, citric acid, and certain other co-solutes, which should be taken into account when formulating multi-component buffers or reagent mixtures.

Thermal Behavior and Dehydration

Boric acid undergoes a well-defined sequence of thermal transformations upon heating:

• 100–105 °C: Loss of one water molecule yields metaboric acid (HBO₂).
• 104–160 °C (prolonged heating): Conversion to pyroboric acid (H₂B₄O₇).
• Higher temperatures: Formation of the anhydrous species.
• 300 °C: Complete conversion to boron trioxide (B₂O₃), the anhydride form.

These transitions are relevant when boric acid is used in high-temperature synthesis, ceramic applications, or when samples containing boric acid must be dried or ashed. Researchers should ensure drying temperatures remain well below 100 °C to preserve the intact H₃BO₃ form.

Laboratory Applications and Buffer Preparation

Boric acid is a cornerstone reagent in molecular biology and biochemistry. Its most prominent laboratory use is in the preparation of TBE (Tris-Borate-EDTA) and TAE (Tris-Acetate-EDTA) electrophoresis buffers for agarose and polyacrylamide gel electrophoresis of nucleic acids. TBE buffer typically contains 89 mM Tris, 89 mM boric acid, and 2 mM EDTA at pH 8.3.

Boric acid is also employed in:

• Borate buffers for protein electrophoresis and isoelectric focusing.
• Complexation reactions with diols and polyols for carbohydrate analysis.
• Microbiological culture media as a selective agent.
• Analytical chemistry as a primary standard for standardizing alkaline solutions, owing to its high purity and stability.

When preparing solutions, warming the solvent or using boiling water significantly reduces dissolution time for concentrations above the room-temperature solubility limit.

Safety, Handling, and Storage

Although boric acid has historically been considered relatively low in acute toxicity, it is classified as a reproductive toxicant under regulatory frameworks such as REACH (EU) and should be handled with appropriate precautions. Personal protective equipment including nitrile gloves, safety glasses, and a lab coat is recommended. Avoid inhalation of dust.

Store boric acid in a cool, dry, well-ventilated area in a tightly sealed container. Under these conditions the material is stable indefinitely at ambient temperature, consistent with its known air stability. Segregate from strong bases and reactive metals. Dispose of waste in accordance with local environmental regulations.

Consult the Safety Data Sheet (SDS) for Aladdin Scientific product B111599 before use for complete hazard information and first-aid measures.

Frequently asked questions

What is the pH of a 0.1 mol/L boric acid solution?

A 0.1 mol/L aqueous solution of boric acid has a pH of 5.1, reflecting its character as a weak acid. This makes it useful as a mild acidic buffer component in a range of biological and analytical applications.

How does temperature affect the solubility of boric acid in water?

Boric acid's solubility increases substantially with temperature. At room temperature, 1 g dissolves in 18 mL of water, while at 100 °C (boiling), 1 g dissolves in just 4 mL. Researchers preparing concentrated solutions should heat the solvent or use hot water to facilitate complete dissolution.

What happens when boric acid is heated above 100 °C?

Heating boric acid above 100–105 °C causes it to lose one water molecule, forming metaboric acid (HBO₂). Prolonged heating between 104–160 °C converts it to pyroboric acid. At 300 °C, complete dehydration yields boron trioxide (B₂O₃). These transformations are irreversible and must be considered when drying samples containing boric acid.

What is boric acid used for in molecular biology?

In molecular biology, boric acid is primarily used to prepare TBE (Tris-Borate-EDTA) buffer for agarose and polyacrylamide gel electrophoresis of DNA and RNA. It also serves as a component of borate buffers for protein analysis and in complexation chemistry for carbohydrate characterization.

Is boric acid soluble in solvents other than water?

Yes. Boric acid dissolves in ethanol (1 g per 18 mL cold alcohol, 1 g per 6 mL boiling alcohol), glycerol (1 g per 4 mL), ethers, and essential oils, in addition to water. Its solubility in mineral acids is lower than in water.

References

1. Aladdin Scientific. Boric Acid Product Page, SKU B111599. Product description and physicochemical data.
2. Sambrook J, Russell DW. Molecular Cloning: A Laboratory Manual, 3rd ed. Cold Spring Harbor Laboratory Press, 2001. TBE buffer formulation.
3. European Chemicals Agency (ECHA). Boric acid — Substance evaluation and classification under REACH. Helsinki: ECHA.

Products are supplied for research and development use only. Not for use in humans, animals, diagnosis, or therapy.