Hydrofluorosilicic acid: identity and specification, not policy

One substance, many names

The aggregated public record for fluorosilicic acid gives one current registry number, CAS 16961-83-4, and the formula H₂SiF₆. Its synonyms include hexafluorosilicic acid, fluorosilicic acid, hydrofluosilicic acid, hydrofluorosilicic acid and silicofluoric acid, and the record lists three deprecated CAS numbers — 1309-45-1, 12672-67-2 and 1202864-64-9 — alongside the current one. Among its removed synonyms is “HFS (CHRIS Code),” the abbreviation that also turns up in product names. The same record carries two separate CAMEO Chemicals datasheets, one for “Fluorosilicic acid” and one for “Hydrofluorosilicic acid (25% or less).”

For a solicitation that means writing the name, the CAS number and the formula together, and checking that a certificate or safety data sheet carries the current registry number rather than a retired one. A spelling is not an identity. None of those names on its own says anything about concentration, impurity content or certification.

A directory search has to try every name

NSF’s search for certified drinking water treatment chemicals offers “Fluorosilicic Acid,” “Fluosilicic Acid” and “Hydrofluosilicic Acid” as three separate chemical-name entries. On September 14, 2026 all three returned the same result page. Inside it, entries were filed under all three headings, and trade designations used further variants, including “Hydrofluorosilicic Acid” and the bare abbreviation “FSA,” with a product function of Fluoridation and a maximum use in mg/L for each product.

The practical lesson is about lookup, not chemistry. Search by company, facility and trade designation, confirm the entry names the function the system uses, and record the date; do not rely on a single chemical-name query to prove that a listing does or does not exist. The maximum use on an entry is a certification ceiling for that product. It is not a dose or a target, this brief reproduces no figure from it, and comparing it with a system’s design is the engineer’s task. How a certification lookup fits into a bid is covered in the first guide of this collection, and nothing here states that any Alliance product is certified or listed.

“Twenty-three percent” of what, by what measure

The Hazardous Substances Data Bank entries on the same record give a density table at 17.5 °C — “5% solution, 1.0407; 10% solution, 1.0834; … 25% solution, 1.2235; 30% solution, 1.2742; 34% solution, 1.3162” — and describe the article of commerce as “marketed as aqueous solution only /5, 10, 15, 20, 25, 30, and 34%/,” with “Grades: technical; CP.” None of those entries is dated, and the table does not restate its basis. Trade designations in the certification listings commonly carry a bare “23%” or “23-25%.”

A percentage for this acid can describe the acid on a mass basis, a fluoride content, or something else a particular standard defines, and those are different numbers with a relationship fixed by formula weight. Write the species, the mass or volume basis, the reference temperature and the method on the solicitation, and ask whether a quoted figure is typical, a guaranteed limit or a measured lot result. A density reading is a check against a table at that table’s temperature, not an assay; the same argument for hydrometer readings is made at length in the oil and gas collection under “Twenty degrees Baumé is a hydrometer reading,” and is not repeated here.

Where it comes from is why trace constituents are specified

The same record describes the manufacturing routes in undated entries: the crude acid is made by reacting silicon tetrafluoride, “obtained from mfr of phosphate fertilizers or phosphoric acid,” with water, and a purified acid “by distillation of crude fluosilicic acid or by reacting pure silica with hydrofluoric acid.” Another entry notes that material “recovered from phosphate fertilizer operations” is marketed for fluoridation of municipal water. A route recovered from a mineral process is the reason a specification for this acid controls trace constituents at all.

Which constituents, at what limits, by which methods and to which reporting limits is set by the document the buyer invokes. AWWA’s catalogue lists B703-24, Fluorosilicic Acid, as the current edition of its commodity standard; it is sold by AWWA and was not read, so nothing here paraphrases it. The aggregated record also contains a secondary summary of “the American Water Works Association standard for fluosilicic acid” with no edition given. An undated summary of an unidentified edition is not the standard, and this brief reproduces nothing from it. Obtain the edition the specification invokes, cite its clauses by number, and ask for a lot certificate that reports each controlled constituent with its method, units and reporting limit — a sample COA is not a lot certificate.

Handling is a boundary, and the safety data sheet sets it

In the Hazardous Materials Table at 49 CFR 172.101, the entry “Fluorosilicic acid” carries hazard class 8, identification number UN1778 and packing group II. The CAMEO Chemicals datasheet on the aggregated record describes “a colorless fuming liquid with a penetrating pungent odor. Corrosive to metals and tissue,” and an HSDB entry records that it “attacks glass and stoneware.” It is corrosive and fluoride-bearing, and the exposure controls, protective equipment and first aid for a particular product are on that product’s safety data sheet. This brief gives none of them.

The design standards treat storage as its own subject. The 2022 Ten States Standards require at §5.8.7 that all fluoride compounds “conform to the applicable AWWA Standards and be ANSI/NSF Standard 60 certified,” that storage and day tanks for fluosilicic acid be “sealed and vented to the atmosphere outside the building,” and that a separate room for its storage and feed be provided “unless allowed by the reviewing authority.” Those are the engineer’s provisions. They are quoted so that the container, venting connection and delivery point written into a solicitation match the installation that will receive it, and classifying any particular shipment remains the offeror’s determination.

Who sets fluoridation policy, as of September 14, 2026

Community water fluoridation is contested and the policy position is changing, so this brief states only dated facts about who decides, and takes no position. EPA’s August 3, 2026 release states: “Decisions on whether to add fluoride to drinking water are guided by public health recommendations from HHS and the Centers for Disease Control and Prevention (CDC) and are ultimately made at the state or local level,” and that EPA “does not make policy recommendations to state or local systems to add fluoride to drinking water.” EPA’s role under the Safe Drinking Water Act is the maximum contaminant level, which it records as set in 1986 and most recently reviewed in 2024.

That federal position is itself under review. EPA released a preliminary assessment plan and literature survey on January 22, 2026, with public comment open until February 27, 2026 under docket EPA-HQ-OW-2025-3823, and on August 3, 2026 released its Protocol for Developing the Fluoride Human Health Toxicity Assessment, EPA-822-D-26-001. The agency states that a draft assessment will be released for public comment and that, when final, the assessment “will inform potential revisions to federal drinking water regulations.” It was not complete on the date this brief was written. Neither the protocol nor the underlying studies were read, and CDC’s community water fluoridation pages returned HTTP 403 to an unauthenticated request on September 14, 2026 and were not read either.

Utah names the acid

Utah’s H.B. 81, Fluoride Amendments, from the 2025 General Session, repeals and reenacts Utah Code §19-4-111 as a prohibition. It defines “fluoride” as a chemical compound that contains the fluoride ion and is used to fluoridate drinking water, including “(a) fluorosilicic acid; (b) sodium fluorosilicate; or (c) sodium fluoride.” It then provides that “A person may not add fluoride to water in, or water that will be introduced into, a public water system,” and that “A political subdivision may not enact or enforce an ordinance that requires or permits the addition of fluoride” to such water. The enrolled bill states that it takes effect on May 7, 2025.

This is quoted for what it is: a dated statute naming the substance this brief describes. Whether it has been amended since was not checked, and its application to any system or purchase is a question for that system and its counsel.

Florida defines the additive instead

Florida took a different drafting route. Chapter 2025-22, enacted as CS/CS/CS/SB 700 and approved by the Governor on May 15, 2025, added a definition at Florida Statutes §403.852(19): a “water quality additive” is “any chemical, additive, or substance that is used in a public water system for the purpose of: (a) Meeting or surpassing primary or secondary drinking water standards; (b) Preventing, reducing, or removing contaminants; or (c) Improving water quality.” It added §403.859(8), which makes it a prohibited act to use “any additive in a public water system which does not meet the definition of a water quality additive.” The act takes effect July 1, 2025, except as otherwise provided. The enrolled text read for this brief does not use the word fluoride; how the definition applies to a particular additive is for Florida’s regulators and counsel to state, not for this library.

Other states’ legislation was not surveyed, and the status in any state can change between the day a specification is written and the day a contract is let. A solicitation for this acid should therefore record the governing state rule and the system’s authority to use the chemical on the date of issue, and name who confirmed it. This brief gives no fluoride level, target or dose, performed no original testing, and makes no statement that any Alliance product is suitable, approved, certified or recommended for fluoridation or any other use. Independent specialist review is pending.

• Do the name, CAS number and formula on the specification, the certificate and the safety data sheet all describe the same substance?
• Is the concentration stated as a percentage of which species, on which basis, at what temperature and by which method?
• Which edition of which standard sets the trace-constituent limits, and does the lot certificate report each with its reporting limit?
• On the date of issue, which state rule governs the system’s use of this chemical, and who confirmed it?

Requirement → evidence → decision boundary

An editorial checklist for your review—not a table of product specifications.

Revision record

1.0 · September 14, 2026 — Source-based editorial edition. No original testing or product qualification is claimed.

For utilities, public works departments, public agencies and their procurement officers writing solicitations for treatment, fluoridation, chloramination and winter-maintenance chemistry. This collection does not interpret procurement law for any agency, determine the certification, qualification or eligibility of any product, set a dose, feed rate, chlorine-to-ammonia ratio, fluoride level or application rate, take a position on community water fluoridation, or make a compliance, permit, transport or waste determination.