A practical guide to boric acid, why its assay can read above 100%, how it differs from borax, and how it is supplied, from a US-based distributor.
Boric acid (H3BO3, CAS 10043-35-3) is a white crystalline powder supplying roughly 17.5% boron, or 56.3% as B2O3. Its largest markets are borosilicate glass and glass fiber, and ceramic frits and glazes, where it fluxes the melt and improves thermal shock resistance. It is also a boron micronutrient source. Stratus Chemical supplies technical grade in bags and supersacks across the United States.
| Property | Value |
|---|---|
| Chemical name | Boric acid (orthoboric acid) |
| Synonyms | Orthoboric acid, boracic acid, hydrogen borate, sassolite |
| CAS number | 10043-35-3 |
| Molecular formula | H3BO3 |
| Molecular weight | 61.83 g/mol |
| EC / EINECS | 233-139-2 |
| Appearance | White crystalline powder |
| Boron content | Approximately 17.5% B, 56.3% B2O3 |
| Grades supplied | Technical grade, powder and granular |
| Packaging | Bags, supersacks |
| Documentation | COA & SDS provided on request |
The figures below are representative of technical grade boric acid powder. They are indicative rather than guaranteed: specifications vary by source and by lot, and the certificate of analysis for your material governs.
| Component | Content |
|---|---|
| B2O3 | 56.25 – 56.80% |
| Equivalent H3BO3 | 99.92 – 100.89% |
| SO4 | 300 ppm max |
| Cl | 5 ppm max |
| Fe | 4 ppm max |
Particle size for the powder grade:
| Size | Content |
|---|---|
| +1.000 mm | 0.00% max |
| −0.125 mm | 45.00% min |
A boric acid certificate showing equivalent H3BO3 at, say, 100.89% looks like an error and is not. It is worth understanding before someone rejects a good lot.
The measured value is the boron oxide content, B2O3. The H3BO3 figure is then calculated back from it. Boric acid gives up water readily, converting toward metaboric acid, so a sample that has lost even a trace of moisture carries slightly more boron per unit weight than pure boric acid would. Expressed back as equivalent boric acid, the arithmetic runs past 100%.
This is why B2O3 is the more meaningful line on the certificate. If you are comparing two suppliers, compare on B2O3, not on the calculated acid figure.
Both supply boron, and the choice usually turns on sodium rather than on boron.
Boric acid is the free acid. It is mildly acidic in solution and introduces nothing but boron, hydrogen, and oxygen into the system. Borax is sodium tetraborate, an alkaline salt that brings sodium with it.
In glass and ceramics that difference is decisive. Sodium acts as a flux in its own right and changes the thermal expansion and durability of the finished product, so a borosilicate formulation that has been balanced without it cannot simply substitute borax. Where the sodium is acceptable or actively wanted, borax is generally cheaper per unit of boron and is the sensible choice.
In agriculture the acidity of boric acid can matter on already-alkaline soils, and its higher solubility suits foliar and fertigation programs.
The largest use by volume. Boron oxide lowers the thermal expansion of glass, which is what makes borosilicate glass resistant to thermal shock, and it improves chemical durability. Continuous glass fiber for insulation and reinforcement, textile fiberglass, and heat-resistant labware all depend on it.
Boric acid acts as a flux, lowering the melting point of a glaze so it matures at a workable temperature, and improves gloss, hardness, and resistance to crazing. It is used where a sodium-free flux is needed.
Used in fluxes for brazing, soldering, and welding, where the borate melt dissolves surface oxides and shields the joint from atmospheric oxidation, and in metal treatment baths.
Boric acid and borates are used in cellulose insulation and in wood and textile treatments, where on heating they form a glassy char layer that limits the supply of oxygen and fuel to the flame.
Boron is an essential plant micronutrient with an unusually narrow window between deficiency and toxicity, narrower than any other micronutrient. Deficiency shows in brassicas, sugar beet, alfalfa, and tree fruit as hollow stem, cracked stem, and poor seed set. Because the margin is small, application rates should follow a soil or tissue test rather than a standing prescription. See our crop micronutrients guide for how boron sits alongside the other micronutrients.
Boric acid is used in electroplating baths as a buffer, particularly in nickel plating, in the manufacture of other boron compounds, and as a neutron absorber in nuclear applications.
Store boric acid dry in closed original packaging, off damp floors. The material is stable, non-flammable, and does not degrade in storage; the routine problem is caking after moisture pickup.
Handle the dust with the ventilation and personal protection specified in the Safety Data Sheet, and avoid dry sweeping of spills. Classification and workplace requirements differ by jurisdiction, so work from the Safety Data Sheet for your grade and confirm the current position for your site.
Boric acid is used chiefly in borosilicate glass and glass fiber manufacture, which is its largest market, and in ceramic frits and glazes where it acts as a flux and improves thermal shock resistance. It is also used in metallurgical fluxes and welding, in flame retardant formulations, in electroplating baths, and as a boron micronutrient source in agriculture. Stratus Chemical supplies technical grade boric acid across the United States.
The CAS number for boric acid is 10043-35-3. Its molecular formula is H3BO3 and its molecular weight is 61.83 g/mol. Its EC number is 233-139-2. It is also known as orthoboric acid, boracic acid, hydrogen borate, and in its natural mineral form as sassolite.
Because the H3BO3 figure is usually calculated from the measured boron oxide content rather than determined directly. Boric acid readily loses a little water on drying, converting toward metaboric acid, which raises the boron concentration of the sample slightly. When that boron is then expressed back as equivalent H3BO3, the arithmetic can exceed 100%. A range such as 99.92 to 100.89% equivalent H3BO3 is therefore normal and is not an error on the certificate.
Boric acid is the free acid, H3BO3, and is mildly acidic in solution. Borax is sodium tetraborate, an alkaline sodium salt that also supplies sodium along with the boron. The choice usually comes down to whether sodium and alkalinity are wanted. Glass and ceramic formulations that cannot tolerate added sodium specify boric acid; formulations where the sodium is acceptable or useful often use borax, which is generally cheaper per unit of boron.
Boric acid contains approximately 17.5% boron by weight, corresponding to roughly 56.3% as boron oxide, B2O3. Boron specifications are quoted on either basis depending on the industry, with glass and ceramics generally working in B2O3 and agriculture generally in elemental boron, so check which basis a specification uses before comparing products.
Store boric acid dry in closed original packaging, off damp floors, as the powder cakes if it takes on moisture. It is stable, non-flammable, and does not degrade in storage. Handle the dust with the ventilation and personal protection specified in the Safety Data Sheet, and avoid dry sweeping of spills. Classification and workplace requirements differ by jurisdiction, so confirm the current position for your site.
Stratus Chemical supplies boric acid in bags and supersacks, in technical grade powder and granular forms, with a certificate of analysis provided for each shipment. Tell us the grade, particle size, and volume and we will confirm pricing and availability within one business day.
Building a micronutrient program? See zinc sulfate, copper sulfate, and the crop micronutrients guide. Looking for something else? Browse all products or request a quote.
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