# Boric acid

Orthoboric acid identity, commercial grade distinctions, property reporting bases, aqueous chemistry, and thermal transformations.

Reviewed: 2026-10-30

Tags: chemicals, borates, substances

## Identity and related substances

U.S. Borax reports Optibor Technical Granular at **56.25–56.80% B₂O₃** and **99.9–100.9% equivalent H₃BO₃**. Those columns express different chemical bases. Its November 2022 SDS identifies boric acid (orthoboric acid, H₃BO₃, CAS 10043-35-3) above 99.9% for the named granular and powder grades. [U.S. Borax SDS, sections 1 and 3](https://s3-us-west-1.amazonaws.com/agrian-cg-fs1-production/pdfs/Optibor_TG_MSDS.pdf)

Neutral aqueous B(OH)₃ and the borate ion B(OH)₄⁻ are different species. The Canadian draft assessment gives about 17.5% elemental boron by boric acid mass. [ECCC and Health Canada, sections 2–3](https://www.canada.ca/en/environment-climate-change/services/evaluating-existing-substances/updated-draft-assessment-boric-acid-salts-precursors.html)

Sodium tetraborate hydrates, often called borax, contain sodium as well as boron. The Optibor product page distinguishes boric acid from sodium borate sources used in glass manufacture. Boric acid supplies boric oxide without the sodium contribution of those salts. This is a compositional distinction, not a claim of interchangeability in a formulation. [U.S. Borax Optibor product page](https://www.borax.com/products/optibor)

## Commercial grades and assay language

The following are two specific supplier specifications, not generic definitions of technical grade. Each sheet states that U.S. Borax analytical methods determine its specification data. Neither sheet prints a calendar issue date. [Optibor TG, USB30-v.1](https://www.borax.com/media/b3c7526a82/optibor-technical-granular.pdf), [Optibor TP, USB31-v.1](https://www.borax.com/media/95c6b3406d/optibor-technical-powder.pdf)

| Reported item | Optibor TG, Technical Granular | Optibor TP, Technical Powder |
| --- | --- | --- |
| B₂O₃ | 56.25–56.80% | 56.25–57.00% |
| Equivalent H₃BO₃ | 99.9–100.9% | 99.9–101.3% |
| Sulfate, reported as SO₄ | ≤250 ppm | ≤1000 ppm |
| Chloride, reported as Cl | ≤10 ppm | ≤18 ppm |
| Iron, reported as Fe | ≤4 ppm | ≤14 ppm |
| Sieve fraction | ≤2.0% retained on U.S. No. 20, 0.850 mm | ≤10.0% retained on U.S. No. 200, 0.075 mm |

The impurity limits and sieve fractions differ despite the common chemical identity. A retained fraction is not a mean particle diameter, and these two sieve entries do not describe complete size distributions. The sheets label impurity values in ppm without separately defining the denominator. They do not provide full analytical procedures or measurement uncertainty. [TG specification and method note](https://www.borax.com/media/b3c7526a82/optibor-technical-granular.pdf), [TP specification and method note](https://www.borax.com/media/95c6b3406d/optibor-technical-powder.pdf)

Derived chemistry: the balanced composition relation 2 H₃BO₃ → B₂O₃ + 3 H₂O gives approximately 56.3% B₂O₃ equivalent for pure H₃BO₃. This agrees with the manufacturer's theoretical composition. Dividing a B₂O₃ mass percentage by about 0.563 expresses an H₃BO₃ equivalent. An equivalent assay above 100% is therefore not a literal mass fraction of intact H₃BO₃ molecules. The cited sheets do not establish why an individual result exceeds 100%, or identify a particular impurity or dehydration state. [Optibor product data sheet, August 2026, page 3](https://a.storyblok.com/f/143307/142d613abd/optibor-boric-acids.pdf)

A specification describes a supplier's declared limits. It is distinct from measured results on a current lot's certificate of analysis. No current lot certificate was examined here. Grade names and a shared CAS number do not establish identical impurity profiles, particle distributions, or suitability for a finished product. This is a synthesis of the two grade specifications, not a lot release conclusion.

## Physical properties and concentration bases

| Property | Source value | Applicability |
| --- | --- | --- |
| Molar mass | 61.83 g/mol | Optibor August 2026 product data sheet, page 3 |
| Bulk density | 881 kg/m³ | Same sheet; packing and test method unspecified |
| Relative density | 1.49 at 23 °C | November 2022 SDS, section 9; not bulk density |
| Water solubility at 20 °C, volume basis | 49.2 g/L | Same SDS; denominator described only as litres |
| Water solubility at 20 °C | 4.72% by mass of saturated solution | August 2026 product data sheet, page 3 |
| Water solubility at 25 °C | 5.46% by mass of saturated solution | Same table |
| Water solubility at 100 °C | 27.53% by mass of saturated solution | Same table; not an operating temperature recommendation |

Sources: [Optibor product data sheet, page 3](https://a.storyblok.com/f/143307/142d613abd/optibor-boric-acids.pdf), [Optibor SDS, section 9](https://s3-us-west-1.amazonaws.com/agrian-cg-fs1-production/pdfs/Optibor_TG_MSDS.pdf).

The product sheet labels 1.50 g/cm³ as specific gravity, although specific gravity is dimensionless. The SDS instead labels 1.49 at 23 °C as relative density. These entries concern the substance, whereas bulk density includes spaces between particles. The product sheet gives no reference temperature for its 1.50 entry. [Product sheet, page 3](https://a.storyblok.com/f/143307/142d613abd/optibor-boric-acids.pdf), [SDS, section 9](https://s3-us-west-1.amazonaws.com/agrian-cg-fs1-production/pdfs/Optibor_TG_MSDS.pdf)

Derived basis conversion: 5.46% by mass means 5.46 g solute per 100 g saturated solution, leaving 94.54 g water. It corresponds to approximately 5.78 g per 100 g water. Conversion to grams per litre of solution additionally requires solution density at the same temperature. Crystal density and bulk density cannot supply that value. The SDS's 49.2 g/L entry and the product sheet's mass percentages retain their different bases; the cited entries do not provide enough information for an exact reconciliation. Neither establishes dissolution rate or solubility in mixed electrolytes.

## Aqueous acidity and species

Boric acid acts as a Lewis acid in water. The dilute aqueous equilibrium can be written B(OH)₃ + H₂O ⇌ B(OH)₄⁻ + H⁺. Canada's updated draft describes the neutral acid and borate anion for dilute solutions up to 0.025 mol/L. Its figure uses pKₐ 9.0 at 25 °C, while its property table gives representative pKₐ 9.2 and a wider literature range. Those rounded assessment values are not a new precision measurement. [ECCC and Health Canada, section 3, figure 3-1 and table 3-1](https://www.canada.ca/en/environment-climate-change/services/evaluating-existing-substances/updated-draft-assessment-boric-acid-salts-precursors.html)

Manov, DeLollis, and Acree's original NBS study used cells without liquid junctions over 0–60 °C. It extrapolated apparent ionization constants to zero ionic strength and reported pK about 9.234 at 25 °C. The authors used dilute borax–sodium chloride solutions to reduce polyborate formation. Their equations use the historical BO₂⁻ notation and account for activity coefficients and hydrolysis. The paper distinguishes thermodynamic pK from an apparent constant in a salt solution. [NBS RP1609, October 1944, pages 288–291 and 299–301](https://nvlpubs.nist.gov/nistpubs/jres/33/jresv33n4p287_A1b.pdf)

Synthesis: pKₐ is an equilibrium property with a temperature and activity basis, not the pH of every boric acid solution. A two-species dilute-water description does not establish speciation in concentrated commercial formulations, added salts, or complexing organic ingredients. No quantitative model for those mixtures is established here.

## Heating and phase changes

The August 2026 product sheet lists 171 °C under melting point. Its next page describes open-system water loss above 100 °C, formation of metaboric acid HBO₂, and eventual B₂O₃. Three metaboric forms have different melting points. The same page distinguishes crystalline oxide melting from amorphous oxide softening. [Product sheet, pages 3–4](https://a.storyblok.com/f/143307/142d613abd/optibor-boric-acids.pdf)

The November 2022 SDS instead states that decomposition prevents a defined melting point over 25–1000 °C. The documents do not supply matching heating rates, atmosphere, or thermal analysis methods. Their entries cannot be reduced to one universal equilibrium melting point. The 171 °C entry is retained as a source-specific listing, not a process threshold. [SDS, sections 9–10](https://s3-us-west-1.amazonaws.com/agrian-cg-fs1-production/pdfs/Optibor_TG_MSDS.pdf)

Derived chemistry: H₃BO₃ → HBO₂ + H₂O and 2 HBO₂ → B₂O₃ + H₂O express overall stoichiometric water loss. These equations do not identify every intermediate phase, transformation rate, or volatilized species. Oxide-equivalent reporting does not mean that the original crystalline powder is a physical mixture of free B₂O₃ and water.

## Hazard documentation and evidence limits

The U.S. Borax November 2022 SDS, document US-US-OPT-EN, classifies Optibor TG and TP as reproductive toxicity Category 2. A separate manufacturer statement dated May 2020 describes EU CLP Category 1B classifications for listed French-produced Optibor grades and Category 2 self-classification for specified U.S.-produced industrial grades in other regions, with exceptions. These are dated, region- and product-specific statements. They do not establish current legal compliance in every jurisdiction. [SDS, sections 1–2 and 16](https://s3-us-west-1.amazonaws.com/agrian-cg-fs1-production/pdfs/Optibor_TG_MSDS.pdf), [U.S. Borax hazard classification statement](https://www.borax.com/customer-support/product-stewardship/hazard-classifications)

This reference covers substance identity, two supplier grades, selected physical properties, and limited aqueous and thermal chemistry. The SDS is a manufacturer-authored copy hosted by Agrian; the manufacturer's current SDS search page did not expose a downloadable revision in the retrieved text. The two specification sheets carry version codes, not calendar dates. The NBS paper is historical primary research, and the Canadian assessment remains explicitly an updated draft. No paid standard, current lot analysis, formulation performance study, or customer process qualification underlies this page.

Related context: [Surface-finishing processes](../../processes/finishing/index.md).