| HS Code | 434652 |
| Product Name | Potassium Ferrocyanide |
| Iupac Name | Potassium hexacyanoferrate(II) |
| Common Synonyms | Yellow prussiate of potash; potassium hexacyanoferrate(II); tetrapotassium hexacyanoferrate |
| Chemical Formula | K4[Fe(CN)6] (anhydrous); K4[Fe(CN)6]·3H2O (trihydrate) |
| Molar Mass | 368.35 g/mol (anhydrous); 422.39 g/mol (trihydrate) |
| Appearance | Yellow crystalline solid or crystalline powder |
| Odor | Odorless |
| Density | 1.85 g/cm3 (trihydrate) |
| Melting Point | 70 °C (trihydrate loses water of crystallization) |
| Decomposition Temperature | Above 400 °C |
| Solubility In Water | Soluble; approximately 28 g/100 mL at 20 °C |
| Solubility In Ethanol | Insoluble |
| Solubility In Ether | Insoluble |
| Crystal Structure | Monoclinic |
| Ph | Slightly alkaline in aqueous solution |
| Cas Number | 13943-58-3 (anhydrous); 14459-95-1 (trihydrate) |
| Ec Number | 237-722-2 |
As an accredited Potassium Ferrocyanide factory, we enforce strict quality protocols—every batch undergoes rigorous testing to ensure consistent efficacy and safety standards.
| Packing | Potassium Ferrocyanide packaged in a 500 g amber glass bottle with tight-fitting screw cap, desiccant, and hazard label. |
| Container Loading (20′ FCL) | Potassium Ferrocyanide chemical (25 kg bags/drums) is properly palletized, loaded, secured, and sealed in a 20′ FCL container for export. |
| Shipping | Potassium ferrocyanide is generally not classified as dangerous goods for transport. Ship in well-sealed, properly labeled containers to prevent moisture and contamination. Keep away from strong acids, oxidizers, and heat. Follow applicable regulations and provide SDS. For air, sea, or road, verify carrier requirements. |
| Storage | Store potassium ferrocyanide in a cool, dry, well-ventilated area in a tightly sealed, labeled container. Keep away from strong acids, oxidizers, and heat or ignition sources, because acid contact can release toxic hydrogen cyanide. Protect from moisture and direct sunlight. Maintain good housekeeping, avoid dust, and store separately from incompatible materials. Follow all local regulations and relevant SDS guidance. |
| Shelf Life | Potassium ferrocyanide has an indefinite shelf life if stored dry, sealed, and protected from light and oxidizing agents. |
In sodium chloride crystallization lines, potassium ferrocyanide trihydrate is metered as a dilute aqueous solution into the centrifuge feed or onto the belt-dried crystal stream to modify crystal habit and reduce interparticle bridging. The additive is not a hydrophobic coating; it operates by selective adsorption on sodium chloride crystal surfaces during growth, producing hopper-shaped or star-shaped crystals with fewer flat contact planes and lower bulk adhesion. In vacuum salt plants using pusher centrifuges and fluidized-bed dryers, the solution is injected at 0.5–2.0% w/w into the thickened slurry, with final residual concentration in finished salt controlled below 20 mg/kg as anhydrous K4[Fe(CN)6] under EU 1333/2008 Annex II food category 12.1.1. The Codex Alimentarius GSFA counterpart lists sodium, potassium and calcium ferrocyanides at 14 mg/kg as anhydrous sodium ferrocyanide equivalent; formulators exporting to multiple jurisdictions must therefore align the dosing setpoint with the strictest destination limit, typically 14 mg/kg, even though EU permits 20 mg/kg. Dosing pumps with flow verification accuracy of ±2% and in-line conductivity sensors are required at the centrifuge feed because post-drying retroactive correction is not feasible once the crystal surface has been modified. In production experience, batch-to-batch variance in upstream brine organic residues shifts the effective adsorption capacity and can depress anti-caking performance at the same dosing rate, requiring feed brine total organic carbon to be maintained below 10 mg/L. Terminal products include food-grade fine salt for table use, free-flowing low-sodium salt blends, and food processing salt used in dry seasoning mixes and cheese brines.
| Jurisdiction / instrument | Food category | Maximum residual | Control method |
|---|---|---|---|
| EU 1333/2008 Annex II E 536 | Salt and salt substitutes | 20 mg/kg as anhydrous K4[Fe(CN)6] | ICP-MS after ashing |
| Codex GSFA 12.1.1 | Salt | 14 mg/kg as anhydrous ferrocyanide | UV/VIS after Fe(III) complexation |
| Food Chemicals Codex monograph | Food-grade potassium ferrocyanide trihydrate | Assay 99.0–102.0% | Potentiometric titration |
Blue fining with potassium ferrocyanide targets soluble Fe3+ and Cu2+ in clarified white and rosé wines by converting them into insoluble mixed metal hexacyanoferrate precipitates, which are then removed by bentonite fining and filtration. The reaction with Fe3+ produces insoluble Fe4[Fe(CN)6]3, while Cu2+ forms Cu2[Fe(CN)6]; the mixed precipitate is removed as a blue-brown lees fraction. The treatment is governed by EU Regulation 2019/934 Annex I Part A Table 2 and the OIV International Code of Oenological Practice, which set a maximum addition of 20 g/hL and require that no residual ferrocyanide remains in the finished wine. Because wine pH typically falls between pH 2.9 and pH 3.6, the dosing solution must be prepared within 1 h of use and kept at 10–15 °C to minimize ligand exchange that could release free cyanide. The treatment sequence begins with a laboratory fining trial at 0.5–20 g/hL to determine the minimum effective dose; the production batch is then dosed through a venturi injection loop at 10–15% of the tank volume per hour, followed by addition of 40–80 g/hL bentonite slurry to adsorb excess ferrocyanide and flocculate the blue precipitate. Separation is performed with a plate-and-frame filter precoated with kieselguhr at 1.5–2.5 bar differential pressure; rotary vacuum filtration may be substituted for lees-heavy tanks. A post-treatment spot check with acidified ferric chloride must remain negative for Prussian blue formation before final sterile filtration. Terminal products are clarified white wines, rosé wines, and sparkling wine base lots destined for second fermentation. The treatment is not applied to red wines because the presence of anthocyanin-tannin aggregates reduces precipitate filterability and risks color stripping.
The pigment synthesis route from ferrocyanide is not a simple stoichiometric precipitation; it is a nucleation-controlled process in which the Fe3+ feed rate, temperature, and acid concentration determine primary particle size, tint strength, and filter cake compressibility. Potassium ferrocyanide trihydrate and ferric chloride are reacted in a molar ratio of 3.0 mol K4[Fe(CN)6]·3H2O to 4.0 mol FeCl3, with a practical ferric excess of 5–10 mol% to force complete ferrocyanide consumption and prevent residual cyanide-bearing liquor in the discharge. In a high-shear reactor at 40–60 °C and pH 2.0–3.0, the ferric chloride solution is added below the liquid surface at a rate not exceeding 0.5 mol Fe3+/min per kg batch mass to avoid local supersaturation spikes that generate gelatinous, poorly filterable precipitates. After oxidation and acid aging for 2–4 h, the suspension is filtered on a membrane filter press to 20–30% solids, washed with demineralized water until conductivity falls below 500 µS/cm, dried in a tray dryer at 80–120 °C, and micronized in a jet mill. Commercial grades used in printing inks and masterbatches exhibit BET specific surface areas between 35 m²/g and 120 m²/g, oil absorption values of 30–45 g/100 g measured by ISO 787-5, and volatile matter below 3.0% at 105 °C per ISO 787-2. Terminal product categories include solvent-based and water-based printing inks, artist-grade watercolor pigments, thermoplastic masterbatch for packaging, and cementitious surface colorants.
| Process variable | Setpoint range | Effect of exceeding upper limit |
|---|---|---|
| FeCl3 feed rate | ≤0.5 mol Fe3+/min per kg batch | Gelatinous precipitate; filter blinding |
| Reaction temperature | 40–60 °C | Agglomeration beyond 60 °C; loss of tint strength |
| Aging pH | pH 2.0–3.0 | Incomplete ferrocyanide consumption below pH 2.0; HCN risk |
| Wash conductivity end point | <500 µS/cm | Soluble chloride carryover; reduced coating compatibility |
Oxidative conversion of potassium ferrocyanide to potassium ferricyanide is carried out in continuous bubble column reactors or plate-type electrolytic cells, where the ferrocyanide/ferricyanide redox couple is driven by chlorine gas or anodic oxidation. The chlorine route follows the stoichiometry 2 K4[Fe(CN)6] + Cl2 → 2 K3[Fe(CN)6] + 2 KCl, requiring 1.0 mol Cl2 per 2.0 mol potassium ferrocyanide; because excess chlorine oxidizes the product further or generates chlorate, the gas flow is controlled by oxidation-reduction potential at +350 mV to +450 mV versus Ag/AgCl. The reaction medium is adjusted to pH 8.0–9.5 with potassium hydroxide, maintained at 30–50 °C, and agitated with compressed air at 0.5–1.0 vvm. Under these conditions the conversion reaches 98–99% within 60–120 min; residual ferrocyanide is monitored by spectrophotometric absorbance at 420 nm after complexation with ferric iron. The oxidized liquor is then passed through a crystallizer with internal draft tube at 5–10 °C and the resulting K3[Fe(CN)6] crystals are centrifuged, washed with ice-cold demineralized water, and dried under vacuum at 40–50 °C. Photographic and reagent grades are tested against ISO 3622:1985 for potassium ferricyanide and ISO 6353-2:1983 R 126 for assay, chloride, and hexacyanoferrate(II) content. Terminal uses include photographic bleach baths, blueprint paper sensitizers, electroplating bath additives, and analytical redox reagents.
Reagent-grade potassium ferrocyanide trihydrate is dissolved to 5% w/v in freshly boiled and cooled demineralized water for use as a spot-test reagent for Fe3+; the solution is buffered to pH 6.0–7.0 with acetate buffer because acidic samples below pH 2.5 can release HCN from the complex. The reagent is applied dropwise to acid-digested water, food, or soil extracts; a Prussian blue precipitate indicates ferric iron with a visual detection limit near 1 mg/L. For quantitative work, the reaction is used in conjunction with UV/VIS absorbance at 690 nm in a standard addition calibration over 0.5–10 mg/L Fe. Reagent specifications are defined by ISO 6353-2:1983 R 127, which sets minimum assay at 98.0%, chloride below 0.01%, sulfate below 0.01%, and water-insoluble matter below 0.005%. Laboratory production batches of test kits are filled into amber glass bottles under nitrogen to prevent slow photodecomposition; shelf-life testing at 25 °C and 60% RH supports 24-month stability when stopper integrity is maintained. The reagent is incompatible with strong acids, zinc dust, and oxidizing acids, and must be segregated from free cyanide reagents in storage. Terminal products include laboratory test kits for iron in water, histology Prussian blue stains, and colorimetric paper test strips used in field water testing.
Non-food sodium chloride for de-icing and industrial brine conditioning is treated with potassium ferrocyanide under a separate compliance envelope that permits higher dosing than food-grade salt but imposes wastewater cyanide limits. In rock salt processing, the additive is sprayed as a 1–3% w/w aqueous solution onto crushed salt before the fluidized-bed dryer at a rate set to leave 25–100 mg/kg as K4[Fe(CN)6] in the final bulk product; dosing is adjusted by belt-scale feed mass rather than by fixed volume because rock salt moisture varies from 0.8% to 4.5% and shifts adsorption efficiency. The treated crystals are dried to below 0.1% moisture at 130–180 °C and screened to the intended gradation. For de-icing salt, specification compliance is generally verified by ASTM sodium chloride specifications, while environmental discharge limits under the receiving facility’s industrial wastewater permit require total cyanide below 0.2 mg/L in plant runoff, measured by ISO 6703-1:1984 or equivalent. Workers handling concentrated ferrocyanide solutions use chemical goggles and nitrile gloves; the solution must not be acidified with sulfuric acid during line cleaning because HCN evolution can occur. Terminal products include rock salt for winter road maintenance, water softener pellet salt, hide-curing salt, and bulk industrial salt for chlor-alkali brine makeup. The additive does not suppress ice formation; it maintains free-flowing storage and metering under humid site conditions.
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Potassium ferrocyanide is supplied as a yellow crystalline solid with the stoichiometric formula K4[Fe(CN)6]·3H2O and a molecular weight of 422.39 g/mol. The trihydrate is identified by CAS 14459-95-1, while the anhydrous form used in low-moisture industrial feedstocks is CAS 13943-58-3. Commercial product lines are classified as technical grade, food-grade E536, or analytical reagent grade, and the distinction is controlled by chloride, sulfate, water-insoluble matter, lead, arsenic, and mercury limits rather than by a single product name. The water of crystallization in the trihydrate accounts for 12.79 wt% of the formula weight and serves as the reference basis for assay calculation in food-additive specifications.
There is no universal model designation for potassium ferrocyanide; the material is described by grade and by particle-size-controlled variants. Standard packaging for dry blending and salt milling lines includes 25 kg polyethylene-lined multi-wall sacks and 500–1,000 kg flexible intermediate bulk containers with antistatic liners. Technical grade is used in brine crystallization, pigment synthesis, and ferricyanide production; food-grade E536 is specified for salt and salt substitutes under Commission Regulation (EU) No 231/2012; reagent grade is supplied against compendial or national analytical specifications such as GB/T 1274-2011. A technical-grade lot is not automatically interchangeable with E536, because residual heavy-metal and chloride levels affect both regulatory status and downstream brine chemistry.
The food-grade product is regulated as an anti-caking agent, not as a functional crystal-growth aid in non-food salt; the distinction matters when a food-grade lot is used in an industrial evaporator. Food-grade lots under EU additive specification are tested for lead, arsenic, and mercury, with limits expressed in milligram per kilogram units. Reagent-grade material under GB/T 1274-2011 is used in analytical laboratories where trace chloride and sulfate interference is unacceptable; suppliers typically provide particle-size distribution data, but this is not a compliance parameter.
Table 1. Grade and specification matrix for potassium ferrocyanide trihydrate.
| Parameter | Unit | Technical grade | Food-grade E536 | Reagent grade |
|---|---|---|---|---|
| Assay as K4[Fe(CN)6]·3H2O | wt% | ≥ 98.0 | ≥ 99.0 | ≥ 99.0 |
| Water-insoluble matter | wt% | ≤ 0.02 | ≤ 0.005 | ≤ 0.005 |
| Chloride as Cl | wt% | ≤ 0.05 | ≤ 0.01 | ≤ 0.005 |
| Sulfate as SO4 | wt% | ≤ 0.05 | ≤ 0.005 | ≤ 0.005 |
| Lead as Pb | mg/kg | ≤ 5 | ≤ 1 | ≤ 1 |
| Arsenic as As | mg/kg | ≤ 3 | ≤ 1 | ≤ 1 |
| Mercury as Hg | mg/kg | ≤ 1 | ≤ 1 | ≤ 1 |
Technical-grade limits are representative supplier release criteria, not harmonized food-additive standards. Food-grade limits are derived from the EU food-additive specification; reagent-grade values reflect typical compendial limits for analytical use. The lower chloride and sulfate ceilings in reagent grade matter when the material is used in analytical precipitation or electrochemical work, where residual anionic loading can shift detection limits and contaminate reference electrolytes.
In vacuum salt production, potassium ferrocyanide is applied at 5–20 mg/kg on a dry salt basis. The additive is prepared as an aqueous solution and injected into clarified brine or sprayed onto centrifuged salt before drying. The ferrocyanide ion adsorbs onto the {100} faces of NaCl and retards cube-edge growth, producing hopper-shaped and dendritic crystals with reduced interparticle contact. This crystal habit shift is the primary anti-caking mechanism, not a surface coating; the additive becomes incorporated at the crystal surface and cannot be removed by dry sieving.
The dosing window is narrow in EU food salt because the maximum permitted level in salt and salt substitutes is 20 mg/kg as anhydrous potassium ferrocyanide under Commission Regulation (EC) No 1129/2011. Below 5 mg/kg, caking control is unreliable in fine vacuum salt with specific surface area above 0.5 m2/g; above the regulatory limit, the lot is non-compliant. Production lines therefore use positive-displacement dosing pumps with mass-flow verification and hold batch-to-batch addition at ±10% of setpoint. In a forced-circulation evaporator, injection into clarified brine at 60–80°C gives more uniform distribution than spraying onto damp salt, because solubility is higher and the solution disperses before the crystallization front.
In brine crystallization, the effect of potassium ferrocyanide is observed in the crystal shape rather than in the final salt chemistry. A treated vacuum salt lot typically shows a shift from well-formed cubic crystals to hopper-shaped crystals with stepped or dendritic faces. The resulting bulk density is commonly 0.05–0.10 g/cm3 lower than that of untreated cubic salt, so packaging lines set weight-based fill targets rather than volume-based targets. The lower bulk density can increase bulk truck volume by a similar percentage; logistics planning uses the treated-salt bulk density after dryer classification.
Caking tendency is evaluated in a controlled humidity chamber at 25°C and 50% relative humidity for 72 h, followed by measurement of cake break strength in a uniaxial compression cell. The accepted target is plant-specific and normally specified as a maximum break force for a 50 mm diameter compact. The anti-caking effect is not instantaneous; it develops as hopper crystals are produced in the evaporator. Salt that is dry-blended with ferrocyanide powder after crystallization shows weaker caking control than salt crystallized in the presence of the additive, because the powder does not achieve the same crystal-surface incorporation.
Finished salt is tested for ferrocyanide by reaction with ferric ion in acid solution to form Prussian blue and absorbance measurement at approximately 700 nm; the method is calibrated against standard ferrocyanide solutions. In production, the test is used as a batch-release check. The additive level is reported as anhydrous potassium ferrocyanide, not as trihydrate mass, to avoid overstatement of the active complex concentration.
Food-grade potassium ferrocyanide also replaces sodium ferrocyanide in low-sodium salt substitutes where additional sodium intake is undesirable. In potassium chloride-based table salt substitutes, the potassium cation contributes to the product’s potassium content rather than adding sodium. The addition level remains bounded by the same 20 mg/kg limit. Potassium ferrocyanide trihydrate contains 37.0 wt% potassium, whereas sodium ferrocyanide decahydrate contains 19.0 wt% sodium; for a dosage of 20 mg/kg of potassium ferrocyanide trihydrate, the elemental potassium addition is approximately 7.4 mg/kg. This is low relative to the potassium chloride carrier and does not drive formulation changes.
The selection among ferrocyanide salts is made on molecular weight, cation content, and regulatory status. Potassium ferrocyanide trihydrate, CAS 14459-95-1, formula weight 422.39 g/mol, is the potassium salt; sodium ferrocyanide decahydrate, CAS 13601-19-9, formula weight 484.06 g/mol, is the sodium equivalent. The sodium salt contains 19.0 wt% sodium and is used where a lower-cost sodium cation is acceptable. Potassium ferrocyanide contains 37.0 wt% potassium, which makes it preferable in low-sodium salt substitutes but may be undesirable in chloride-limited industrial brines. Calcium ferrocyanide, E538, introduces divalent calcium and is less commonly used because calcium can precipitate sulfate and carbonate scales in brine circuits.
Table 2. Comparative data for potassium ferrocyanide, sodium ferrocyanide, and potassium ferricyanide.
| Property | K4[Fe(CN)6]·3H2O | Na4[Fe(CN)6]·10H2O | K3[Fe(CN)6] |
|---|---|---|---|
| CAS | 14459-95-1 | 13601-19-9 | 13746-66-2 |
| Formula weight | 422.39 g/mol | 484.06 g/mol | 329.24 g/mol |
| Cation content | 37.0 wt% K | 19.0 wt% Na | 35.6 wt% K |
| Iron redox state | Fe(II) | Fe(II) | Fe(III) |
| Food-additive status in EU | E536, salt and salt substitutes at 20 mg/kg | E535, salt and salt substitutes at 20 mg/kg | None |
| Principal industrial role | Crystal habit modifier, anti-caking agent, pigment precursor | Crystal habit modifier, anti-caking agent | Analytical reagent, cyanotype, oxidizing agent |
Potassium ferricyanide, CAS 13746-66-2, is the oxidized Fe(III) complex and participates in different chemistry. It is used in cyanotype, blueprint, and as an outer-sphere electron acceptor; it does not provide the same crystal habit modification in NaCl and is not approved as a food additive. When potassium ferrocyanide is oxidized by chlorine or hypochlorite in brine, the resulting ferricyanide has reduced anti-caking performance and can impart yellow-to-brown discoloration. This is why dechlorination is critical before the ferrocyanide injection point.
Potassium ferrocyanide remains stable in neutral and alkaline aqueous solution, but the complex decomposes under strongly acidic conditions with release of hydrogen cyanide. The operational limit in food-grade brine systems is generally pH above 6; below pH 2, measurable HCN evolution occurs and requires scrubbed ventilation. Acid cleaning of dosing lines must be followed by flushing to neutral pH before reintroducing the ferrocyanide solution. Storage vessels are vented, and the product is kept away from strong oxidizers such as chlorine, hypochlorite, permanganate, and concentrated nitric acid because oxidation to ferricyanide alters performance and creates side products.
In salt plants that chlorinate recirculated brine, residual chlorine is reduced to below the detection limit of a DPD free-chlorine test before the ferrocyanide addition point. Sodium bisulfite is used for dechlorination, and the oxidation-reduction potential of the brine is monitored. When ferrocyanide is added to a high-chlorine residual, the anti-caking effect is not only reduced, but Prussian blue pigment formation can occur if free iron is present. This creates blue specks in the final salt and is a known failure mode on production lines with corroded piping upstream of the injection point.
Analytical reagent grade potassium ferrocyanide is used as a qualitative reagent for ferric ion. In acidified aqueous samples, Fe(III) reacts with ferrocyanide to produce Prussian blue, with a visual detection limit near 1 mg/L for ferric iron; spectrophotometric validation is required for quantitative reporting. The reaction is also used in pigment production, where an acidic ferric chloride solution is metered into potassium ferrocyanide solution at pH 3–4 and temperature below 45°C. The resulting precipitate is filtered, washed, and controlled for particle size because the color strength of ferric ferrocyanide pigments is dependent on primary particle aggregation and residual soluble salts.
