Sodium Selenite CAS 10102-18-8: Chemical Properties, Specifications, and Industrial Uses

Anhydrous sodium selenite, designated by CAS registry number 10102-18-8 and EC number 233-267-9, is the disodium salt of selenious acid with the formula Na₂SeO₃ and a molar mass of 172.94 g/mol. The stoichiometric selenium content is 45.66 wt%, derived from the 78.96 g/mol atomic weight of selenium, and this value is used arithmetically as a release criterion for commercial lots. The product is a white to off-white crystalline powder with a density of approximately 3.1 g/cm³ for the anhydrous form; the pentahydrate, CAS 26970-82-1, has a formula weight of 263.02 g/mol and a theoretical selenium content of 30.02 wt%. Aqueous solutions are alkaline because the selenite anion undergoes hydrolysis; the pH of a 10% solution is typically between 10.5 and 11.5 when measured with a calibrated glass electrode. Solubility in water is high, with published values of 85 g/100 mL at 25 °C for the anhydrous salt and 95 g/100 mL at 20 °C for the pentahydrate, sufficient for the preparation of concentrated stock solutions in closed mixing vessels. The compound is essentially insoluble in ethanol and nonpolar solvents, which restricts solvent-based formulation options. On heating, sodium selenite releases selenium dioxide and sodium oxide; the decomposition pathway is highly dependent on furnace atmosphere and the presence of reducing or oxidizing agents, as described in the glass manufacturing section below. Anhydrous grades are hygroscopic and require sealed packaging to maintain assay; exposure to relative humidity above 60% leads to moisture uptake and caking in bulk bags.

When aqueous stock solutions are prepared, the resulting solution should be stored in sealed high-density polyethylene or stainless steel vessels; contact with mild steel or galvanized surfaces is not recommended because selenite can be reduced to red elemental selenium by base metals, producing visible deposits and lowering active Se(IV) concentration. The addition of reducing agents such as ascorbic acid, glucose, or sulfite triggers precipitation of amorphous red selenium, a reaction exploited deliberately in nanoparticle synthesis but unwanted in feed or electroplating make-up tanks. Strong acidification below pH 2 converts selenite to selenious acid; if the solution is subsequently heated or brought into contact with reducing metallic surfaces, toxic hydrogen selenide formation is possible. Industrial stock solutions in water should therefore be kept at pH 9–11 and should not be mixed with acidic concentrates or amine-based additives, which can alter redox state and create uncontrolled precipitation. Due to the acute toxicity of the solid and its solutions, handling stations in feed premix plants and glass batch houses are equipped with high-efficiency particulate air filtration and closed transfer systems. Liquid dosing systems should use metering pumps with perfluoroelastomer seals, and the solution tanks should be equipped with level sensors that prevent overflow into floor drains because the compound is toxic to aquatic organisms with long-lasting effects.

What Controls Selenium Retention During Sodium Selenite Additions to Flint Container Glass?

In continuous container glass production, iron oxide impurities from silica sand and cullet create the green-blue absorption band that is neutralized by complementary pink transmission from elemental selenium. Sodium selenite is added as a minor batch component at the batch house weigh station, either directly into the raw material mixer or through an automated micro-dosing auger. The compound decomposes in the batch before or during initial melting; Se(IV) is reduced to Se(0) only when the batch redox number and localized oxygen partial pressure are sufficiently reducing. If the batch oxidation state remains high, selenium is volatilized as selenium dioxide or remains as colorless selenite species, and color compensation is lost. Furnace atmosphere, batch carbon additions, cullet ratio, and fining agent selection all alter selenium retention. Measured laboratory melts for similar selenium compounds have shown that selenium retention can range from below 10% to above 50% depending on redox, and the practical consequence is that furnace color trim requires frequent fiber-optic or spectrophotometric feedback from drawn container samples. Typical selenium metal additions in flint container glass are between 0.03 wt% and 0.15 wt%; sodium selenite is charged so that the selenium supplied falls within the same range after accounting for loss. A regenerative side-port furnace operating at 1450–1550 °C with forehearth temperatures of 1080–1150 °C represents the downstream thermal environment in which the glass must retain the desired selenium redox state. In excessively reducing melts, iron selenide formation can shift the color from pink to brown; this is a known process conflict when high cullet ratios introduce variable organic contamination.

Grade selection between sodium selenite and elemental selenium involves several production-scale considerations. Sodium selenite provides a defined selenium assay per unit mass, does not require handling of fine selenium metal powder with its associated dust explosion and toxicity limitations, and dissolves in the aqueous phase of the batch during early heating, which can improve distribution. However, the additional sodium oxide introduced by sodium selenite alters the glass basicity and acts as a flux; batch reformulation may be required to maintain the viscosity-temperature curve measured by beam-bending viscometry according to ISO 7884-2. When reducing agents such as blast-furnace slag or carbon-based furnace dust are used to increase selenium retention, the redox number of the batch must be controlled within a narrow range. Excessive reducing conditions also cause amber chromophore formation, especially when sulfate fining passes are present. In a typical container glass plant, the batch-to-batch variation in selenium retention is monitored by comparing drawn samples against a master color standard using a spectrophotometer; adjustments to sodium selenite dosing are made at the batch weigh scale in increments no greater than 10% of the previous setting to avoid cycling. A cullet-to-batch ratio above 60% commonly destabilizes color because organic contaminants in post-consumer cullet create localized reducing zones and because the selenium already present in recycled glass is partly oxidized or volatilized during remelting. Published data for this specific configuration is limited because furnace operating conditions and cullet chemistry vary between plants, but the control principle is consistent across continuous glass operations.

In feed additive manufacturing, sodium selenite is converted into a microingredient premix because the target selenium concentration in complete feed is in the range of 0.1–0.5 mg/kg under European Union and United States regulatory frameworks. A typical sequence begins with a 1% selenium premix, corresponding to 10,000 mg Se/kg, prepared by blending sodium selenite with calcium carbonate or wheat flour in a stainless-steel ribbon mixer. For a final complete-feed target of 0.3 mg/kg, the 1% premix must be dosed at 30 g per metric tonne; therefore the weigh-scale readability and dust extraction system are critical. A micro-ingredient dispenser with a readability of ±1 g gives a theoretical dispensing error of approximately 3.3% at this target, which is acceptable only if the premix itself is homogeneous. Blending validation is performed by collecting 10 evenly spaced core samples from the ribbon mixer after a mixing time determined by tracer studies, and the coefficient of variation for selenium concentration should be below 10% as measured by hydride-generation atomic absorption spectrometry or inductively coupled plasma mass spectrometry. AOAC 986.15 and EN 17053 provide analytical frameworks for selenium in feed matrices; microwave-assisted acid digestion in closed polytetrafluoroethylene vessels is the preferred sample preparation route because it minimizes volatile selenium loss. The United States Food and Drug Administration regulation at 21 CFR 573.920 permits sodium selenite as a source of selenium for broiler chickens, laying hens, turkeys, swine, sheep, and beef cattle, with a maximum total selenium concentration of 0.3 mg/kg in complete feed. European Union regulation 1831/2003 sets a maximum total selenium content of 0.5 mg/kg for complete feed at 12% moisture for most species, with lower limits applied in specific pet food and companion animal categories. The process conflict in premix plants is carryover: because sodium selenite is highly toxic and active at part-per-million levels, any residual selenium in a mixer, elevator leg, or dust filter can contaminate subsequent non-selenium batches. Dedicated equipment is preferable; if shared equipment is used, sequential flushing with a selenium-free carrier and verification by inductively coupled plasma mass spectrometry are required before changeover.

In aqueous dilution systems used for liquid feed supplementation, sodium selenite solutions are alkaline and should not be combined with acidified molasses or organic acid blends because selenium dioxide species can form and the mixture may generate toxic vapors under heat. The solution is also incompatible with reducing sugars in long residence-time tanks; slow reduction to red selenium creates sedimentation and dosing irregularity. In dry feed manufacturing, sodium selenite should be stored in sealed bags or fiber drums in a ventilated room at relative humidity below 60%; moisture uptake causes caking and non-uniform flow through micro-ingredient dosing augers. Batch-to-batch variance is controlled by salt action, with release limits confirming that selenium content is between 45.0% and 46.0% on an anhydrous basis, which corresponds to an assay of not less than 98.5% Na₂SeO₃ when residual moisture is negligible. The stoichiometric relationship between assay and selenium content is used by quality-control laboratories to reject material that has been diluted with carriers or damaged by exposure to moisture. In high-throughput feed mills, sodium selenite premix is added through an automated micro-dosing system with agitation in the hopper; the dosing auger is validated at the intended feed rate to prevent bridging, and the hopper is vented through a dust collector with a sealed cartridge filter to prevent selenium-containing dust from reaching the mill atmosphere.

When Sodium Selenite Is Used in Electrochemical Deposition Baths

Electrochemical synthesis of selenide semiconductor films uses sodium selenite as the Se(IV) precursor because it is readily water-soluble and can be co-dissolved with cadmium sulfate, indium chloride, or zinc sulfate to form a stable acidic electrolyte. The reduction of SeO₃²⁻ to Se²⁻ requires a multi-electron transfer at the cathode, and the deposition potential is shifted by pH; published bath formulations for CdSe and CuInSe₂ typically maintain pH between 2.0 and 3.0 and use a current density of 1–20 mA/cm² at temperatures from 60 °C to 90 °C. At more negative potentials, selenide formation competes with hydrogen evolution, and the local pH near the cathode can rise enough to precipitate metal hydroxides. The addition of a supporting electrolyte such as sodium sulfate at 0.5 M reduces ohmic drop and improves thickness uniformity across the substrate. Because Se(IV) is a moderately strong oxidizing agent in acidic media, the bath must be prepared with deionized water and filtered through inert polypropylene or polytetrafluoroethylene components; contact with copper or brass immersion heaters can cause electroless reduction of selenium onto the metal surface, which depletes the bath and contaminates the heater. Ventilation must be designed for selenium species, and an electrolytic cell hood with a scrubbed exhaust is used to prevent worker exposure above the 0.2 mg/m³ selenium occupational exposure limit. The process is used in pilot-scale production of photovoltaic absorber layers and thin-film thermoelectric materials; published data for specific industrial cell configurations is limited because bath composition and electrode geometry are often proprietary. However, the electrochemical behavior of sodium selenite in acidic media is well documented, and the main operational boundary is that bath pH and deposition potential must be controlled simultaneously to avoid amorphous red selenium deposits instead of the desired selenide film.

Sodium selenite is a laboratory and pilot-scale precursor for colloidal selenium and selenium-containing organic intermediates. In aqueous reduction processes, ascorbic acid or glutathione reduces Se(IV) to amorphous red selenium; the reaction is carried out between 60 °C and 80 °C at pH 4–6, with capping agents such as polyvinylpyrrolidone or sodium citrate added to control particle growth. The resulting hydrodynamic particle size, measured by dynamic light scattering, typically falls between 20 nm and 200 nm depending on reducing-agent concentration and temperature, and the particles can be converted to gray trigonal selenium by extended heating. This route is used because sodium selenite has a defined selenium oxidation state and high water solubility, allowing accurate stoichiometric control in batch reactors with glass-lined or polytetrafluoroethylene contact surfaces. In organic synthesis, sodium selenite is used to prepare selenocysteine analogs, selenium-containing heterocycles, and catalysts for oxidation reactions; these reactions are conducted in fume hoods with scrubbed exhaust because of the acute inhalation toxicity of the solid and potential release of volatile selenium species. The compound should not be combined with strong reducing agents in acidic media outside a closed reactor, as the formation of hydrogen selenide is possible when Se(IV) is reduced under low pH conditions. For agricultural biofortification trials, sodium selenite is sometimes applied as a dilute foliar spray, but the application rate must be validated by selenium recovery in plant tissue and soil residual monitoring; published agronomic data are variable because selenium bioavailability depends on soil redox state, organic matter, and competing sulfate concentration.

Regulatory Compliance Matrix for Feed and Technical Grade Shipments

Shipments of sodium selenite require documentation that spans transport, occupational safety, and feed additive controls. The United Nations transport classification for sodium selenite is UN 2630, Selenites, n.o.s., Class 6.1, Packing Group I, which triggers stringent packaging and segregation requirements. The European CLP classification includes Acute Tox. 2, H300; Acute Tox. 1, H330; STOT RE 1, H372; Aquatic Acute 1, H400; and Aquatic Chronic 1, H410. In the United States, the occupational exposure limit for selenium compounds is 0.2 mg/m³ as selenium for an 8-hour time-weighted average under OSHA PEL 29 CFR 1910.1000 Table Z-1; the ACGIH TLV is also 0.2 mg/m³ as selenium. Analytical results in the certificate of analysis should include the test method used for selenium, such as ICP-MS after closed-vessel digestion or hydride-generation atomic absorption spectrometry, and should report the result against the theoretical selenium content of 45.66 wt% for the anhydrous salt. The matrix below summarizes the principal compliance anchors for international movement and use.