Sodium persulfate

Identity and related substances

United Initiators’ NPS TDS lists typical 99.9% w/w assay and 6.7% w/w active oxygen. Ideal Na₂S₂O₈ gives about 6.72% active oxygen on the formula basis. These are different reporting quantities. Sodium persulfate (disodium peroxydisulfate) is CAS 7775-27-1, EC 231-892-1, 238.11 g/mol in Merck’s listing. ILO/WHO ICSC 1136, May 2002 Merck 106609

The S₂O₈²⁻ ion has an oxygen–oxygen linkage. Ammonium and potassium persulfates have different counterions and formula masses. Sigma-Aldrich 71890, structure field United Initiators APS ILO/WHO ICSC 1133, November 2002

Peroxymonosulfate is another distinction, not an alternative spelling. Sigma-Aldrich 12600 is a potassium monopersulfate triple salt, 2KHSO₅·KHSO₄·K₂SO₄, CAS 70693-62-8; its listed concentrations use separate KHSO₅ and active-oxygen bases. Neither that mixed-salt formula nor its assay basis identifies Na₂S₂O₈. Sigma-Aldrich 12600, US English product page.

Typical values, specifications and lot results

These are different kinds of evidence. A typical value describes a producer's product; a specification states a product limit; a lot result reports a particular sample. An SDS composition entry is neither an analytical certificate nor a complete impurity balance.

United Initiators' NPS TDS revision 1, marked “Valid from: 02/12/2025” (date string retained without interpreting its order), lists these typical values: assay approximately 99.9% w/w, active oxygen approximately 6.7% w/w, moisture <0.03% w/w, acid calculated as H₂SO₄ approximately 0.01% w/w, and iron approximately 0.5 mg/kg. Silicic acid is an optional addition. These figures do not establish the composition of every sodium persulfate grade. The TDS disclaimer does not warrant specific properties. NPS TDS, pp. 1–2.

Merck's undated 106609 EMSURE for analysis product page separately labels the following as specifications, not typical or measured lot values:

The same page lists identity and loss-on-drying criteria. Its certificate section requires a lot number; no lot certificate underlies the values above. “Heavy metals as Pb” is a reporting convention distinct from the separate lead limit. Merck 106609, Specifications and Certificates of Analysis.

The IQ English page for Sigma-Aldrich 71890, retrieved for this draft, displays ≥98% (RT) in its heading and assay field, despite a search title suggesting ≥99.0%. It lists chloride and each of Ca, Cd, Co, Cu, Fe, Ni, Pb and Zn at ≤50 mg/kg. The page does not expand “RT” in the inspected assay field. Its pH range 2.5–4 lacks a concentration there. This is a dated observation of a regional catalog display, not a claim that all historical 71890 specifications changed. Its certificate widget reported a retrieval problem; no current lot result was obtained. Sigma-Aldrich 71890, IQ English.

Reporting bases and formula deductions

Nonprocedural deductions: Using 238.11 g/mol for pure Na₂S₂O₈, one formula unit contains one peroxide linkage. An active-oxygen equivalent corresponding to one oxygen atom gives 100 × 16.00 / 238.11 ≈ 6.72 mass%. This is an equivalent reporting convention, not the total elemental oxygen fraction (approximately 53.8%). The formula contains no water of crystallization. These calculations use the formula and mass above; they are not measurements of product composition, activity, reaction yield or available oxidizing capacity. Merck identity data.

If an assay is explicitly a Na₂S₂O₈ mass fraction on an as-received basis, multiplying that fraction by 6.72% gives a formula-based equivalent, under the additional assumption that other components contribute no active oxygen. An iodometric assay does not by itself establish a full species analysis. A dry-basis fraction and an as-received fraction are related by the non-water mass fraction only when the water determination and assay share compatible definitions and the non-water composition is unchanged. Loss on drying is not automatically a selective water measurement. These are reporting relationships, not preparation or dosing calculations.

Mass-unit equivalence is less ambiguous: 1 mg/kg = 1 mass ppm, and 0.0005 mass% = 5 mg/kg. Thus a typical iron value of 0.5 mg/kg and a specification maximum of 5 mg/kg can coexist without either predicting a lot result. Missing impurity entries do not establish zero content; separate upper limits cannot be summed into an actual composition.

Physical properties and unresolved differences

The NPS TDS reports approximate bulk density 1250 kg/m³; solubility approximately 515, 545, 605 and 680 g/L at 10, 20, 40 and 60°C, respectively; and typical pH approximately 5.0 at 1% and 4.0 at 10% in water. Its pH footnote links initial pH to certified sulfuric acid content. The percentage convention and solubility denominator are not further defined in that table. NPS TDS, p. 1.

The NPS Germany English SDS, version 3.6, revision 9 January 2026, instead reports 730 g/L at 25°C, pH 4.5 at 10 g/L, bulk density 1200 kg/m³, and relative density 1.68 at 20°C, while marking density undetermined. NPS DE/EN SDS, §9, pp. 11–12.

The older ILO/WHO card lists 55.6 g/100 mL at 20°C, density 1.1 g/cm³ without a bulk/crystal qualifier, and decomposition at 180°C. Arithmetic converts its solubility number to 556 g/L on the same original volume basis; that does not establish whether the denominator is water or final solution. ICSC 1136, May 2002.

Synthesis: The solubility entries cannot establish one reconciled curve without compatible methods and denominator definitions. A temperature difference alone does not resolve the TDS/SDS discrepancy. Bulk density includes interparticle void space; relative density is dimensionless. The inspected documents do not establish a verified crystal-density value. Their numbers cannot be averaged or silently relabeled as that property.

The SDS describes decomposition before melting and a 180°C SADT, measured by UN H.4, defined for the tested package size. The TDS warns that moisture or impurities can reduce SADT. A card's generic decomposition entry and a package-dependent SADT are different property descriptions even when their numbers coincide. Neither establishes a universal thermal onset or operating limit. NPS SDS, §9; NPS TDS, footnote.

Hazard and evidence scope

The Germany English NPS SDS v3.6, under EU CLP, classifies oxidizing solid category 3, acute oral toxicity category 4, skin and eye irritation category 2, respiratory and skin sensitization category 1, and respiratory single-exposure target-organ toxicity category 3. Its composition table states ≤100% w/w disodium peroxodisulphate; that is not a 100% purity guarantee. These are dated regional product disclosures, not global compliance conclusions. NPS SDS, §§2–3.

Reviewed on 30 October 2026 PDT. The original NPS PDFs were downloaded and their relevant tables, footnotes and disclaimers visually inspected. Catalog and ILO pages were inspected through the web reader; those views may be cached and do not prove a live catalog revision or lot composition. Legacy FMC/PeroxyChem pages returned fetch errors and are excluded from the numerical evidence. No original experimental study, current lot certificate or independently verified crystal-density measurement was obtained. This reference covers identity and property-reporting limits; application performance, customer capability and operating instructions remain outside its scope.