Ferric chloride

Identity and material forms

Ideal FeCl₃ contains 34.43% elemental Fe. FeCl₃·6H₂O contains 20.66% Fe and 60.01% FeCl₃ equivalent. A liquid specified at 40.0 mass% FeCl₃ equivalent therefore corresponds to 13.77 mass% Fe if all iron belongs to that ferric component. These are formula calculations. Anhydrous ferric chloride is CAS 7705-08-0, about 162.20 g/mol. Thermo Fisher anhydrous specification

The deliquescent hexahydrate is CAS 10025-77-1, 270.29 g/mol. Its six waters belong to the crystal formula. Thermo Fisher hexahydrate specification

Terminology synthesis: ferric denotes iron in oxidation state +3; ferrous denotes +2. FeCl₂ and FeCl₃ therefore identify different salts. A ferrous-iron impurity result is not a ferric-chloride assay. Other hydration states are outside this page's verified property coverage. Thermo Fisher identity and ferrous-iron entry

A commercial aqueous product is a mixture. Hill Brothers SDS CF01000 lists ferric chloride at 39–44%, hydrochloric acid below 5%, and water below 60%, all by weight. Its ferric-chloride ingredient retains CAS number 7705-08-0. That ingredient number does not describe the complete mixture or imply an anhydrous product. The three ranges do not define one exact composition. Hill Brothers SDS, section 3

Grade, assay, and impurities

The following are product specifications, not universal grade definitions or measured lot results.

Sources: Thermo Fisher anhydrous specification, Thermo Fisher hexahydrate specification, Wako specification.

The Thermo hexahydrate sheet gives no separate water determination. Its upper assay of 102% is an analytical specification on the hydrate basis, not a literal mass fraction above unity. The sheet does not explain the method or cause of results above 100%. Thermo Fisher hexahydrate specification

Wako separately lists tests for oxychloride and free chlorine, with pass/fail wording rather than numerical limits. Its appearance test is marked as an additional Wako test outside the cited JIS requirements. The sheet references JIS K 8142:2024; this reference does not independently verify the standard's full content. Wako's product page calls its assay titrimetric. Wako specification and footnote, Wako product page

Synthesis: an assay, an acidity result expressed as HCl, an iron(II) limit, and a water determination describe different quantities. Neither a grade name nor an SDS ingredient range establishes all of them. No current lot certificate of analysis was examined, and the specifications do not establish a particular customer's material composition or suitability.

Formula-equivalent mass bases

The following calculations use 162.20 g/mol for FeCl₃, 270.29 g/mol for FeCl₃·6H₂O, and 55.845 g/mol for Fe. CIAAW gives iron's standard atomic weight as 55.845(2). Anhydrous formula mass, hydrate formula mass, CIAAW iron

Derived stoichiometry, for ideal pure formula compositions:

Derived example: a hypothetical solution containing exactly 40.0% by mass FeCl₃ equivalent contains 0.400 × 0.3443 ≈ 13.77% by mass ferric iron, provided all iron belongs to that ferric component. Conversely, ferric-iron mass multiplied by about 2.904 gives FeCl₃-equivalent mass. Total iron cannot replace ferric iron when appreciable iron(II) is present.

These are accounting equivalents, not a description of discrete FeCl₃ molecules in water. They do not determine free acid, water content, or impurities in a commercial mixture. A percentage by mass is grams per 100 g product. A concentration in g/L additionally needs a volume denominator and temperature-matched solution density. Neither crystal density nor powder bulk density supplies that conversion.

Density, solubility, and thermal entries

Merck's undated product page for anhydrous 8.03945 reports density 2.89 g/cm³ at 25 °C and bulk density 1000 kg/m³. Bulk density includes spaces between particles. The page gives no packing method for its bulk value. Its solubility entry is 920 g/L with a hydrolysis qualifier, without a temperature or explicit solvent-versus-solution volume basis. The same page reports 306 °C with decomposition under melting point. These qualifiers prevent treating the values as one unqualified set of equilibrium properties. Merck 8.03945 properties

ILO/WHO ICSC 1499, dated April 2004, is headed anhydrous ferric chloride but lists 37 °C as melting point. Its note explicitly identifies an apparent melting point caused by loss of crystal water. The card also reports decomposition above 200 °C, density 2.9 g/cm³, and solubility of 92 g/100 mL at 20 °C with a reaction qualifier. It does not explicitly identify whether that volume is water or final solution. ICSC 1499, physical properties and notes

Wako's hexahydrate page separately lists 37 °C as melting point and 280 °C as boiling point, without thermal method, pressure, or decomposition qualification. Synthesis: the repeated 37 °C value does not repair the mismatch between the ICSC heading and its hydrate-related note. These listings do not establish a single reversible melting or boiling transition for every ferric-chloride form. Wako hexahydrate properties

MG Chemicals' 415 TDS, version 2.3 dated 20 February 2026, lists solution density 1.4 g/mL, boiling point 110 °C, and crystallization entries of −26 °C for 37% FeCl₃ and −12 °C for 40% FeCl₃. It does not specify density temperature, boiling pressure, or the percentage basis beside those entries. These are commercial-solution data, not solid-salt constants. MG 415 TDS, property table

Synthesis: reactive solubility entries are not measurements of unchanged salt molecules in water. Their unclear volume bases prevent an exact conversion to saturated mass percentage here. Concentration-dependent crystallization, decomposition, and loss of hydration water are different phenomena. No heating rate, phase diagram, or validated thermal transformation model is established by these listings.

Aqueous chemistry and acidity

Merck reports pH 1 at 20 °C for 200 g/L in water for product 8.03945. That is a stated concentration and temperature, not a universal pH of ferric chloride. Merck product properties

Khoe and colleagues' 1986 original study examined iron(III) hydrolysis at 25 °C in 1.0 mol/dm³ sodium perchlorate, potassium nitrate, and potassium chloride media. Its public abstract reports different hydrolysis constants for those media and both mononuclear and polynuclear hydrolyzed species. Only the abstract was available in the retrieved publisher text. No numerical constant or detailed experimental method from the inaccessible full paper is adopted here. Khoe et al., DOI 10.1039/DT9860001901, abstract

Synthesis: hydrolysis makes dissolved iron chemistry dependent on medium as well as total composition. A conditional equilibrium constant at finite ionic strength is not automatically a thermodynamic constant at infinite dilution. The cited abstract does not establish species fractions in a concentrated commercial solution. Formula-equivalent arithmetic alone therefore cannot predict its pH, dissolved species distribution, or corrosion performance.

Source versions and coverage limits

The two locally retrieved Thermo specifications print 29 October 2026. Their print dates differ from search-indexed copies. Wako's retrieved specification has revision footer 18 April 2024 and a separate top date of 31 October 2026. The latter is a source artifact date, not evidence of a future review or revision. Thermo anhydrous sheet, Thermo hydrate sheet, Wako sheet.

The Wako and Merck product pages have no verified publication date. Sources were reviewed 30 October 2026 PDT.

Hill Brothers' SDS is revised 21 February 2024 and states a U.S. OSHA/GHS basis. Its product-specific hazard classifications include oral toxicity Category 4, skin irritation Category 2, serious eye damage Category 1, and corrosivity to metals Category 1. Those dated classifications do not establish current legal status in every region. An SDS is not a performance specification. Hill Brothers SDS, sections 2 and 16

Coverage is limited to selected supplier products, formula-based deductions, source-specific physical entries, and bounded hydrolysis evidence. No current lot analysis, complete impurity balance, hydration-state measurement, concentrated-solution speciation model, or application performance study underlies this page.

Related context: Surface-finishing processes.