Phosphoric acid

Identity and phosphate terminology

Innophos’s SDS for 75% and 85% phosphoric acid products have freezing-point entries of −17.5 °C and 21.1 °C, respectively, with pressure and method unstated. The figures belong to the named concentrations. Orthophosphoric acid is H₃PO₄, CAS 7664-38-2, 97.9952 g/mol, InChIKey NBIIXXVUZAFLBC-UHFFFAOYSA-N. NIST Chemistry WebBook, SRD 69

Dihydrogen phosphate, hydrogen phosphate and phosphate describe successive proton loss. An acid assay leaves the equilibrium fractions unresolved. Sigma-Aldrich 345245, description

Condensed phosphates contain orthophosphate units joined by phosphoanhydride bonds. Linear polyphosphates, cyclic metaphosphates, and branched ultraphosphates have different connectivity. This distinction concerns chemical structure, not a change in the percentage notation for orthophosphoric acid. Dürr-Mayer and colleagues, 2021, introduction and Figure 1

Innophos identifies its separate polyphosphoric acid 105–108% product as CAS 8017-16-1. Section 3 reports greater than 99 weight % polyphosphoric acid, with a footnote about other ingredients below stated disclosure thresholds. Synthesis: the product-strength name and the SDS ingredient percentage answer different questions. Neither supplies a distribution of chain lengths. Innophos polyphosphoric acid SDS, September 20, 2024, sections 1 and 3

Aqueous concentration, purity, and water

Sigma-Aldrich 345245 specifies both 85 wt. % in H2O and 99.99% trace metals basis. Its technical response defines wt. % as mass relative to total mass. Synthesis: trace-metal purity is a separate analytical basis from acid mass fraction. It does not make this aqueous product 99.99 mass % H3PO4. For an ideal binary mixture at exactly 85 mass % acid, subtraction gives 15 mass % water. That deduction is not a water measurement for a commercial lot with impurities. Sigma-Aldrich 345245, concentration, assay, and weight-percent response

Innophos's June 29, 2025 SDS covers 75% and 85% FCC and technical products together. Section 3 gives 75–85 weight % phosphoric acid. Its footnote says the balance comprises nonhazardous ingredients or ingredients below the applicable cut-off. Synthesis: this disclosure range is not an exact lot assay or a complete impurity analysis. Innophos phosphoric acid SDS, sections 1 and 3

Product specification and historical lot evidence

Thermo Scientific catalog 201145000 names an 85 wt% solution in water. Its specification separates a NaOH titration range from individual impurity limits. These selected entries describe that product, not universal grade requirements. Thermo product specification

Source: Thermo specification, complete one-page table and amendment notice. The sheet does not give a detailed titration endpoint, uncertainty, or an explicit mass denominator for its ppm fields. Its generated print date reads October 31, 2026, although this copy was retrieved on October 30, 2026 PDT. This unexplained date is not treated as an effective revision date.

Sigma-Aldrich's certificate for product 04107, batch STBK8116, records quality release on July 8, 2022 and recommended retest in December 2026. The following are historical results beside that certificate's specifications. Sigma certificate, pages 1–2

Source: Sigma certificate, assay and impurity rows. The certificate does not explicitly declare the mass denominator or analytical method for these percentage fields. Its second page limits the conformity statement to release or subsequent retest. Synthesis: a less-than result is a reported bound, not zero. This certificate establishes neither another batch's composition nor a present measurement of STBK8116. The Thermo range and Sigma range apply to different products and are not competing measurements of one sample.

H3PO4 and P2O5 reporting bases

A phosphorus-equivalent conversion compares equal amounts of phosphorus. H3PO4 contains one phosphorus atom per formula unit, while the P2O5 reporting unit contains two. Sigma lists formula weights 98.00 for H3PO4 and 141.94 for P2O5. Sigma acid certificate, Sigma phosphorus pentoxide 298220, identity

Nonprocedural deduction: on the same total-product mass basis, P2O5 equivalent % = H3PO4 equivalent % × 141.94 / (2 × 98.00), approximately H3PO4 equivalent % × 0.7242. Thus, exactly 85.0% H3PO4 corresponds to about 61.6% P2O5 equivalent. Conversely, 100% P2O5 equivalent corresponds to about 138.1% H3PO4 equivalent. This is stoichiometric bookkeeping, not a preparation or transformation. Equivalent content alone does not establish free P2O5, actual orthophosphoric acid fraction, water content, or condensed-phosphate distribution.

Physical properties and their conditions

The NIST-JANAF liquid H3PO4 table, dated December 1971, gives a crystal–liquid transition at 315.500 K, or 42.350 °C, under its standard-state pressure of 0.1 MPa. This pure-compound reference is separate from an aqueous grade. NIST-JANAF H-086, transition row and table header

Innophos's aqueous SDS gives the following concentration-specific entries. Innophos SDS, section 9

Specific gravity is dimensionless. The absent reference-water temperature prevents an exact density conversion. The SDS supplies no viscosity value. Sigma 345245 separately lists density 1.685 g/mL at 25 °C, but also a literature melting point near 40 °C for its 85 wt% product. The melting-point basis and the density method and pressure are unstated. The melting point conflicts with Innophos's 85% value. No cause is established here. Innophos SDS, section 9, Sigma 345245, properties

Acid dissociation and model limits

Roger Bates's 1951 original NBS study reports pK1 = 2.148 ±0.005 at 25 °C for aqueous H3PO4 ⇌ H+ + H2PO4−. The analysis uses molal activities and extrapolation to infinite dilution, rather than a concentration quotient for concentrated commercial acid. The paper discusses uncertainty in the activity coefficient used for the hydrogen-ion correction. Its Table 5 reports K1 as 7.11 × 10−3 at 25 °C. Bates, sections 3–6 and Tables 3–5

Synthesis: this historical thermodynamic constant does not supply the activities or species fractions of an 85% product. This reference does not provide a verified second or third dissociation constant, concentrated-solution activity model, or numerical speciation diagram. Acid assay, titration response, and equilibrium dissociation remain distinct quantities.

Dated hazard evidence and coverage

The June 29, 2025 Innophos SDS cites US OSHA and EU frameworks and classifies its 75–85% products as metal-corrosive category 1, oral acute-toxicity category 4, skin-corrosive category 1B, and eye-damage category 1. This dated supplier classification does not establish global or current legal compliance. Innophos SDS, sections 1–2

Sources were reviewed on October 30, 2026 PDT. The catalog pages have no numbered technical revision. The historical lot certificate, dated SDSs, original dissociation study, and JANAF table retain their separate dates. Coverage excludes current-lot measurements, complete impurity mass balances, material-specific compatibility, and process performance. No cited grade establishes customer qualification or approved settings. The related Surface-finishing processes supplies broader context.