Sodium Metasilicate Pentahydrate

Commercial sodium metasilicate pentahydrate, CAS 10213-79-3, is a hydrated crystalline solid with nominal formula Na₂SiO₃·5H₂O and molar mass 212.14 g/mol. The material is supplied as white, low-dust granules or crystalline fines. The fixed stoichiometry gives a SiO₂/Na₂O molar ratio of approximately 1.0, an Na₂O content of 28.5–29.5 wt%, an SiO₂ content of 27.5–28.5 wt%, and water of crystallization at 42–43 wt%. Commercial differentiation is not based on a unified model number; instead, suppliers classify grades by granule-size distribution, apparent density, residual iron concentration, and water-insoluble matter. Low-iron fine grades are typically directed to detergent compounding and industrial cleaning, whereas granular grades are preferred in concrete densification and ceramic deflocculation where controlled dissolution is required. The hydrate releases water of crystallization when heated above approximately 72 °C, producing a sticky partial melt before final dehydration; this behavior influences dryer inlet temperature in spray-particle processing and the design of screw conveyors after hot-air exposure.

In aqueous systems, a 1.0 wt% solution of the pentahydrate at 25 °C exhibits pH 12.4–12.8. This places it above sodium carbonate and below sodium hydroxide on a delivered-alkalinity basis, while the silicate anion provides additional functions that caustic and soda ash cannot supply: precipitation of hardness ions, protection of sensitive metal surfaces under controlled conditions, peroxide stabilization, and formation of siliceous films on concrete. These functions support its use in heavy-duty alkaline cleaners, ceramic casting slips, cementitious surface treatments, and oxygen-bleach detergents.

What Distinguishes Sodium Metasilicate Pentahydrate from Anhydrous Metasilicate and Sodium Silicate Solutions?

Compared with anhydrous sodium metasilicate, CAS 6834-92-0, the pentahydrate carries roughly 42–43 wt% water and therefore delivers lower Na₂O per unit mass. This reduces the dissolution exotherm and lowers the local pH spike when the material is added to cold water or to a detergent slurry. Anhydrous metasilicate, by contrast, carries approximately 50–51 wt% Na₂O and is favored where shipping cost per active alkali or minimum water burden is critical, but its rapid dissolution can generate localized gel particles in mixers with weak agitation. Sodium silicate solution, commonly supplied at 37–40 wt% solids with a SiO₂/Na₂O weight ratio between 2.0 and 3.3, is lower in alkalinity and higher in polymerizable silica. The solution form avoids dry-handling but introduces large water freight and requires tank heating in cold storage to avoid viscosity increase. Pentahydrate sits between these forms: it has a fixed fully alkaline ratio and is a dry material, but retains sufficient hydration to dissolve more gently than the anhydrous salt.

Against sodium hydroxide, the metasilicate offers a less aggressive pH curve at equal additions and contributes silica-based corrosion inhibition, but it cannot achieve the same peak pH for saponification or carbon removal. In immersion degreasing, replacement of part of the sodium hydroxide with metasilicate is used to maintain the same free-alkalinity target while reducing attack on soft metal components. Reformulation often begins at 20–40% substitution on total Na₂O and is adjusted by plant coupon weight-loss testing.

In alkaline industrial cleaning, the pentahydrate is typically formulated at 0.5–2.0 wt% in immersion tanks and 0.25–1.0 wt% in spray washers. At these levels, the pH remains above 11.5 and supplies sufficient saponification of fatty soils while silicate species precipitate calcium and magnesium from incoming hard water, reducing film formation on steel parts. On a continuous washing line treating stamped steel components, hard-water scale on downstream blow-off nozzles is reduced when metasilicate replaces part of the soda ash; however, quantitative deposit mass data for this specific configuration is limited. The same silica can form a protective layer on aluminium and zinc, but only within controlled temperature and concentration windows. Above 70 °C and 5 wt% metasilicate, hydrogen evolution from aluminium increases and the silicate film is no longer protective. For mixed-metal immersion cleaners, bath temperature is therefore held at 55–65 °C and free NaOH is kept below 0.5 wt% unless a dedicated etch is required.

In powder detergents, the hydrate is incorporated at 1–10 wt% depending on whether the product is a heavy-duty laundry powder or automatic dishwasher detergent. It functions as an alkaline builder, modifies powder structure during agglomeration, and reduces peroxide decomposition in oxygen-bleach systems by deactivating trace transition-metal ions. Batch addition is normally made after zeolite or sodium carbonate but before heat-sensitive bleaching agents. A low-shear ribbon blender operating at tip speeds below 2 m/s is sufficient to distribute the granules without excessive attrition. High-shear granulators with impeller tip speeds above 10 m/s can fracture the crystalline material and raise fines below 200 μm, which alters bulk density and flowability.

In hydrogen-peroxide bleaching of cotton and cotton-blend textiles, metasilicate pentahydrate is used as a stabilizer and alkali source. A continuous pad-steam line running at 60–80 m/min may dose metasilicate at 1–3 g/L in the saturator together with an organic stabilizer; the silicate portion lowers the decomposition rate of peroxide by deactivating trace iron and copper. At equivalent Na₂O, it produces less fabric strength loss than sodium hydroxide-only bleaching when tested by ISO 13934-1 tensile strength retention because the siliceous species moderates local alkalinity on the fiber. The hydrated form is preferred over the anhydrous form here because the lower dissolution exotherm reduces crystallite precipitation on the padding roll after line stops.

When Ceramic Slips Require Deflocculation Without Excessive Sodium Accumulation

In ceramic casting-slip preparation, sodium metasilicate pentahydrate is used as a deflocculant because the silicate anion adsorbs on clay particle edges and increases negative surface charge, reducing yield stress and water demand. Addition levels are typically 0.05–0.5 wt% on dry body mass, with the optimum determined by Brookfield viscosity at 20 °C or by flow-time testing with a 4 mm orifice flow cup. Overdosing produces an increase in viscosity and thixotropy rather than further fluidization; the practical operating window is frequently narrower than ±0.05 wt% around the optimum in a whiteware casting slip at 1.75 g/cm³ density. A plant-scale plunger pump in a ceramic slip line can show pressure fluctuations when the slip shifts from dispersed to flocculated at the overdosage boundary; production additions are therefore made as a pre-diluted 10 wt% solution rather than as dry granules to avoid overshooting. The Na₂O introduced with the metasilicate also contributes to total soluble alkali, which can affect plaster mold life through sulfate accumulation; mold degradation is observed when sulfate and alkali in the casting slip exceed local process thresholds.

Detergent-Grade Assay, Particle-Size, and Insoluble-Content Limits

Specification acceptance windows vary by producer and by end use. The following table summarizes typical commercial technical and low-iron detergent-grade parameters. Test methodology generally follows ASTM D501-03 for total alkalinity and moisture/ignition loss, with elemental iron by ICP-OES according to ISO 11885:2007 or an equivalent producer method.

Concrete surface densification with metasilicate is based on the reaction between silicate species and free calcium hydroxide in hardened cement to form calcium silicate hydrate, which reduces surface porosity and dusting. The pentahydrate is typically diluted to 10–20 wt% solids and applied at 0.2–0.4 L/m² depending on surface absorption and existing carbonation. The reaction is sensitive to carbonation: on highly carbonated surfaces the silicate consumes carbonate rather than forming a well-developed calcium silicate hydrate network, and published data for this specific configuration is limited. Excess unreacted metasilicate can produce white efflorescence on dark concrete, so the surface is rinsed after 20–40 min of wet dwell and before full drying.

Operational boundaries for storage and handling derive from the hydrate’s water release and moderate hygroscopicity. Storage in unlined bags at relative humidity above 60% can cause particle caking and reduce free-flow behavior; closed bags or silos with dehumidified transfer air are therefore used. The compound is incompatible with concentrated acids and with ammonium salts because acidification releases silicate gel and heat, while ammonia may be evolved under alkaline conditions. Dry mixing with aluminium powder or strong reducing agents is avoided due to the potential for hydrogen evolution in alkaline slurries. The product is alkaline, and wet or dry dust contact with eyes requires immediate rinsing according to the supplier safety data sheet.