Sodium Selenite Analytical Grade

Characterisation of sodium selenite analytical grade begins with the anhydrous-salt identity Na₂SeO₃, CAS 10102-18-8, EC 233-267-9, molecular weight 172.94 g·mol⁻¹. The commercially supplied analytical reagent is commonly the pentahydrate Na₂SeO₃·5H₂O, molecular weight 263.01 g·mol⁻¹, containing 30.0% selenium on the hydrated form and 45.7% selenium on the anhydrous basis. Lot release documentation for analytical grade material specifies assay on an anhydrous basis after drying at 105 °C, with acceptance windows generally set at 98.0% minimum and iodometric titration results commonly between 99.0% and 100.5%. Impurity ceilings for chloride, sulfate, selenate, iron, and heavy metals are lower by a factor of 5–20 than technical grade product because the material must function as a trace-level selenium source, selective growth-medium component, and redox reference reagent. Typical certificate-of-analysis parameters include insoluble matter ≤0.005%, chloride ≤0.005%, sulfate ≤0.010%, selenate expressed as SeO₄ ≤0.05%, iron ≤0.001%, and heavy metals as Pb ≤0.002%; these are determined by turbidimetric, colorimetric, or inductively coupled plasma optical emission spectrometry procedures aligned to the general methods of ISO 6353-1 or equivalent reagent compendia. The powder is packaged in double-sealed HDPE drums with internal polyethylene liners; if a container remains open for longer than 24 h at relative humidity above 60%, assay should be revalidated by loss-on-drying because the hydration state influences accurate weighing.

Physical inspection establishes a white crystalline powder with no visible caking. The material is freely soluble in water and insoluble in ethanol. A 5% aqueous solution is slightly alkaline, typically pH 8–9; the exact pH is lot-dependent and reported on the certificate of analysis. For trace-metal analysis, arsenic, cadmium, mercury, and nickel are screened by inductively coupled plasma mass spectrometry using the sample preparation framework of Ph. Eur. 2.4.20 where pharmacopoeial alignment is required. The analytical grade product is not a direct dietary ingredient unless the relevant compendial monograph is applied.

Anhydrous residue is determined by drying at 105 °C for 2 h; the theoretical water content of the pentahydrate is 34.25%, and a deviation greater than ±0.5% indicates contamination or altered hydration state. Selenate content is determined by ion chromatography with suppressed conductivity detection; chloride and sulfate are determined by turbidimetry after dissolution in 1:1 nitric acid; iron is measured by colorimetry with 1,10-phenanthroline. Heavy metals as Pb are determined by sulfide precipitation or inductively coupled plasma optical emission spectrometry. The product code suffix AR or ACS is supplier-specific and does not replace lot-specific certificate-of-analysis documentation.

What Distinguishes Analytical Grade Sodium Selenite from Feed and Technical Grades?

Grade separation is enforced through impurity ceilings and selenium assay windows rather than through a single nominal purity value. Technical grade sodium selenite is typically released at 95.0% minimum assay because the principal use is glass batch melting, where chloride and sulfate are diluted into the silicate matrix or partially volatilised. Feed grade product is specified on a selenium-content basis of 45.0% minimum for the anhydrous form, and contaminant ceilings for arsenic, cadmium, and lead are set by regional premix regulations; chloride is less tightly controlled because it does not define the biological endpoint. Analytical grade material additionally restricts the anion and heavy-metal burdens because chloride can shift the electrochemical potential of selenite enrichment broth, iron can alter glass redox states, and sulfate can interfere in ion chromatography or turbidimetric method development.

In addition, the analytical grade lot is typically qualified by trace-level ICP-MS screening for transition metals that influence glass colour or microbial inhibition. The technical grade counterpart may carry recoverable dust and broader particle-size distribution; this is acceptable for large-scale furnace charging but not for preparation of 4.0 g·L⁻¹ selective broth where dissolution time and local pH gradients affect reproducibility. Analytical grade material is therefore not an arbitrary quality label: it is a set of measurable constraints that reduce batch-to-batch variance in downstream aqueous redox and microbial-growth applications.

In selective enrichment media conforming to ISO 6579-1:2017, analytical grade sodium selenite is incorporated at 4.0 g·L⁻¹ in selenite cysteine broth together with pancreatic digest of casein 5.0 g·L⁻¹, lactose 4.0 g·L⁻¹, disodium phosphate 10.0 g·L⁻¹, and L-cystine 0.01 g·L⁻¹; pH is adjusted to 7.0 ± 0.2 at 25 °C. In this formulation, selenite acts as a redox-selective inhibitor: Salmonella spp. reduce selenite to elemental selenium and tolerate the resulting intracellular stress, while many competing enteric organisms are suppressed. Concentration control is critical. Below 3.5 g·L⁻¹, selectivity may be lost through overgrowth of non-target flora; above 5.0 g·L⁻¹, the broth may inhibit Salmonella after extended incubation. Analytical grade material is preferred because chloride and heavy-metal contaminants above the specified ceilings can alter the electrochemical potential of the medium and change the time window for visible selenium reduction.

Subculture timing follows the selective enrichment step at 12–18 h on xylose lysine deoxycholate agar or bismuth sulfite agar; incubation beyond 24 h is not recommended because the broth loses differential selectivity and accumulates red-orange elemental selenium precipitate. ISO 11133:2014 requires growth-promotion testing with a Salmonella-positive control and selectivity testing with a non-target strain such as Escherichia coli for each medium batch. Prepared selenite cysteine broth should not be autoclaved after selenite addition; excessive thermal exposure can precipitate selenium and reduce the effective concentration. In a production-scale quality control laboratory, the dry powder is weighed on a calibrated balance with readability 0.01 g and dissolved in warm purified water at 40–50 °C with gentle agitation, then the solution is dispensed aseptically into sterile containers without terminal steam sterilisation.

The following compliance anchors apply when analytical grade sodium selenite is used across the indicated applications.

Container-Glass Decolorisation and Selenium Retention Limits

In soda-lime-silica glass melting, iron present in silica sand at 0.02–0.10% Fe₂O₃ on sand mass produces a green chromophore associated with Fe²⁺ coordination. Sodium selenite analytical grade is weighed into the batch to contribute a complementary red-pink selenium chromophore, and the addition is normally expressed as selenium metal equivalent. Published glass-literature ranges for selenium addition in container glass are typically 0.01–0.20 kg Se·t⁻¹ of glass, but the exact setpoint is furnace-dependent and cannot be transferred directly from one line to another. Selenium retention in the final matrix is incomplete and commonly falls between 20% and 40%; the remainder is volatilised or removed with flue-gas particulates. This retention window is narrow enough that the chloride and heavy-metal profile of analytical grade material becomes relevant: chloride can form volatile salts during melting and reduce selenium retention, while iron and nickel impurities alter the final colour balance.

A controlled laboratory trial is performed in a 5–10 kg box furnace with ramp at 5 °C·min⁻¹ to 1450 °C, soak 2 h, and rapid quench on a stainless steel plate; dry raw materials are blended in a 10 L V-shell blender for 15 min before charging. The resulting glass is sectioned and measured by X-ray fluorescence for selenium retention, while optical transmittance is measured spectrophotometrically across 380–780 nm. If retained selenium exceeds the target, a pink or grey cast develops; if retention is too low, the green iron chromophore remains insufficiently compensated. Published data for specific production furnace configurations is limited, so trial melts are required whenever batch composition, cullet ratio, or furnace pressure changes.

For glass batch dosing on a continuous line, the analytical grade powder is typically pre-dispersed in a 1:10 sand concentrate to prevent feeder segregation; dust extraction with local exhaust ventilation is mandatory because the compound is toxic by inhalation and ingestion. The material is not an exact substitute for selenium metal or selenide frit: it enters the batch in the +4 oxidation state and its melting behaviour differs from elemental selenium. When the process requires a more oxidised selenium source, sodium selenate is used, but that product may shift the batch redox number differently and is outside the analytical grade specification covered here.

For calibration and trace-metal work, sodium selenite analytical grade can serve as a selenium source only after correction for hydration state. Dissolution of 1.000 g of the anhydrous salt in 1000 mL of 2% v/v nitric acid yields a stock containing 456.6 mg·L⁻¹ selenium; dissolution of 1.000 g of the pentahydrate yields 300.2 mg·L⁻¹. Ignoring the certificate-of-analysis hydration state invalidates ICP-OES or ICP-MS calibration at trace levels. For anion chromatography method development, the analytical grade product is used to generate selenite ion standards in deionised water; the selenate peak should be monitored because even a 0.05% selenate impurity can produce a measurable secondary peak at 1 mg·L⁻¹ total selenium. Unlike sodium selenate, which carries selenium in the +6 oxidation state, sodium selenite is the preferred reagent when reduction to elemental selenium is required, such as in selective enrichment broth and in mild reductive synthesis of selenium nanoparticles. Selenium dioxide is also a +4 selenium source but is more volatile and more acidic, and its deliquescent handling characteristics make it less convenient for aqueous reagent preparation.

Operational boundaries include avoidance of strong reducing agents such as ascorbic acid or sodium borohydride in acidic solution, because rapid precipitation of elemental selenium lowers available selenite activity and changes the reaction stoichiometry. Conversely, the compound should not be combined with oxidising agents that convert it to selenate unless the analytical method specifies selenate equivalence. Analytical grade sodium selenite is a laboratory and industrial process reagent; it does not carry a direct dietary monograph and should not be used for animal or human supplementation unless the appropriate feed or pharmaceutical grade is applied under local registration. Where a feed or premix application is required, the feed-grade product with selenium assay of 45.0% minimum and the relevant contaminant documentation should be used.

In iodometric titration, sodium selenite is assayed through its reaction with potassium iodide in acidic solution to liberate iodine, which is then titrated with standard sodium thiosulfate. The stoichiometry releases 2 mol I₂ per 1 mol selenite, consuming 4 mol sodium thiosulfate per 1 mol selenite in the subsequent titration. A standard 0.1 mol·L⁻¹ Na₂S₂O₃ solution is used, and the starch endpoint is sharp under controlled pH. This redox chemistry is also exploited in laboratory demonstrations of selenium colloid formation but should not be run without fume hood extraction and secondary containment, because the reaction releases iodine vapour and generates selenium-containing waste that must be handled as hazardous waste.