# Phosphoric acid

Orthophosphoric acid identity, aqueous grades, assay and phosphorus reporting bases, conditional physical data, and source limitations.

Reviewed: 2026-10-30

Tags: chemicals, acids, substances

## 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](https://webbook.nist.gov/cgi/formula?ID=C7664382&Mask=20&Units=SI)

Dihydrogen phosphate, hydrogen phosphate and phosphate describe successive proton loss. An acid assay leaves the equilibrium fractions unresolved. [Sigma-Aldrich 345245, description](https://www.sigmaaldrich.com/GB/en/product/aldrich/345245)

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](https://www.nature.com/articles/s41467-021-25668-3.pdf)

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](https://www.innophos.com/-/media/innophos-com/customer-center-files/phosphates/public/unique/001/ppa-105-to-108-tech-sds.pdf?rev=d892c286a21d48ee9f3c45f07a33247f)

## 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](https://www.sigmaaldrich.com/GB/en/product/aldrich/345245)

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](https://www.innophos.com/-/media/innophos-com/customer-center-files/phosphates/public/unique/001/phos-acid-75-to-85-sds.pdf?rev=a98bfc901d5f4ea2b6f88d3bfe5e77e6)

## 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](https://assets.thermofisher.com/chem-specs-pdf/retrievePdf?countryCode=cn&erp_type=CN_E1_LCD&rootSku=20114&sku=201145000)

| Reported attribute | Product specification |
| --- | --- |
| Titration with NaOH | 84.5–86.5% |
| Chloride | ≤5 ppm |
| Sulfate | ≤50 ppm |
| Arsenic | ≤1 ppm |
| Sodium | ≤500 ppm |

Source: [Thermo specification, complete one-page table and amendment notice](https://assets.thermofisher.com/chem-specs-pdf/retrievePdf?countryCode=cn&erp_type=CN_E1_LCD&rootSku=20114&sku=201145000). 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](https://www.sigmaaldrich.cn/certificates/Graphics/COfAInfo/fluka/pdf/PDF738398.pdf)

| Test | Certificate specification | Batch result |
| --- | --- | --- |
| Assay | 85.0–88.0% | 85.5% |
| Fluoride | ≤0.0005% | <0.0005% |
| Sulphate | ≤0.005% | <0.005% |
| Iron | ≤0.001% | <0.001% |

Source: [Sigma certificate, assay and impurity rows](https://www.sigmaaldrich.cn/certificates/Graphics/COfAInfo/fluka/pdf/PDF738398.pdf). 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](https://www.sigmaaldrich.cn/certificates/Graphics/COfAInfo/fluka/pdf/PDF738398.pdf), [Sigma phosphorus pentoxide 298220, identity](https://www.sigmaaldrich.com/US/en/product/sial/298220)

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](https://janaf.nist.gov/tables/H-086.html)

Innophos's aqueous SDS gives the following concentration-specific entries. [Innophos SDS, section 9](https://www.innophos.com/-/media/innophos-com/customer-center-files/phosphates/public/unique/001/phos-acid-75-to-85-sds.pdf?rev=a98bfc901d5f4ea2b6f88d3bfe5e77e6)

| Property | 75% product | 85% product | Basis retained from source |
| --- | --- | --- | --- |
| Melting/freezing point | −17.5 °C | 21.1 °C | Pressure and method unstated |
| Vapor pressure | 5.7 mmHg | 2.2 mmHg | 20 °C; method unstated |
| Specific gravity | 1.57 | 1.69 | 25 °C; reference-water temperature unstated |

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](https://www.innophos.com/-/media/innophos-com/customer-center-files/phosphates/public/unique/001/phos-acid-75-to-85-sds.pdf?rev=a98bfc901d5f4ea2b6f88d3bfe5e77e6), [Sigma 345245, properties](https://www.sigmaaldrich.com/GB/en/product/aldrich/345245)

## 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](https://nvlpubs.nist.gov/nistpubs/jres/47/jresv47n3p127_a1b.pdf)

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](https://www.innophos.com/-/media/innophos-com/customer-center-files/phosphates/public/unique/001/phos-acid-75-to-85-sds.pdf?rev=a98bfc901d5f4ea2b6f88d3bfe5e77e6)

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](../../processes/finishing/index.md) supplies broader context.