Zinc sulfate

Identity and hydrate scope

Ideal ZnSO₄ contains 40.50% Zn by mass. The monohydrate contains 36.43% Zn and the heptahydrate 22.74% Zn. These formula values differ from salt assay and delivered lot results. The public USP preview distinguishes the anhydrous identity and defines the mono- and heptahydrates within the monograph scope. USP preview

Values and identifiers in this table come from the USP preview. “White vitriol” is an alias on the ILO/WHO heptahydrate card. ICSC 0349

Other documented forms include ZnSO₄·6H₂O and ZnSO₄·4H₂O. Ding and colleagues' original 2025 study used powder X-ray diffraction (PXRD), thermogravimetry and vibrational spectroscopy to distinguish hydrates. Its detailed results describe the tetrahydrate-rich sample as a mixture containing smaller amounts of hexahydrate and monohydrate, qualifying the abstract's broader purity wording. Samples with an average composition of four waters per ZnSO₄ could instead contain hexahydrate and monohydrate. Composition alone therefore did not identify a single crystalline phase in those experiments. Ding et al., §§3.2–3.3

Elemental zinc, salt content and formula water

Calculated reference values, not measurements: using the molar masses above and zinc atomic weight 65.38 from CIAAW (reported as 65.38(2)), the ideal zinc mass fraction is 65.38 divided by the formula molar mass. The ZnSO₄-equivalent fraction is 161.44 divided by that mass. Formula-water fraction is the remaining fraction for the named ideal hydrate. Formula masses; CIAAW zinc

These rounded deductions assume the stated pure formula. Formula water is not a measured moisture value or loss on drying. Neither total mass loss nor a nominal hydrate label alone establishes the phases present; the cited experiments combined composition and PXRD evidence. The table does not predict dissolved species, activity or process performance. Experimental distinction

A percentage “as ZnSO₄” differs from the same percentage “as Zn.” For a mass fraction explicitly expressed as ZnSO₄ equivalent, its elemental-zinc equivalent is that fraction multiplied by 65.38/161.44, approximately 0.4050. This is a reporting-basis conversion, not a preparation or dosing calculation. A percentage without a named analyte and mass/volume basis remains ambiguous.

Property values and their boundaries

Synthesis: solid density, bulk density including interparticle voids, and solution density describe different volumes. None of these sources supplies a concentration-dependent solution-density model. The heptahydrate solubility figures remain discrepant: 54 g/100 mL converts arithmetically to 540 g/L, not 965 g/L. Their available fields do not establish matching solute-equivalent, solvent-volume or final-solution-volume conventions or experimental methods; no reconciliation or universal saturation curve follows. ICSC; manufacturer page

Thermal terminology

ICSC 0349 and Merck's heptahydrate page list 100 °C as a melting point. ICSC 1698 instead reports anhydrous decomposition at 680 °C. The freshly retrieved monohydrate SDS lists decomposition at 240 °C and no melting-point data. These entries concern different forms and different named events. ICSC heptahydrate; Merck; ICSC anhydrous; monohydrate SDS

Ding et al. distinguish thermogravimetric mass loss from calorimetric events and crystalline-phase identification. Their results include alternate hydrate pathways and mixed phases; a single catalog melting point cannot represent that whole behavior. Dehydration removes formula water, whereas decomposition of the sulfate is a different chemical change. This reference supplies no heating profile or conversion procedure. Ding et al., §§3.1–3.2

Specifications, SDS entries and named lots

The USP preview defines monohydrate content as 89.0–90.4% ZnSO₄, corresponding to 99.0–100.5% monohydrate; heptahydrate content is 55.6–61.0% ZnSO₄, corresponding to 99.0–108.7% heptahydrate. These are published definition ranges on stated equivalent bases, not elemental-zinc percentages or proof of any lot's composition. Only the public preview was read, not the subscriber-only analytical monograph. S1

Merck's 1.08883 page separately lists a 99.5–103.0% complexometric assay range. A public Alfa Aesar certificate for product 33399, named lot N01F011, lists a 99.0–103.0% assay limit and a 100.44% result. Its pH row reports 5.01 against 4.4–6.0 for a 5% solution at 25 °C. The certificate is undated; it is a named-lot example, not evidence for currently supplied material. The assay row does not identify its method. Merck specification fields; one-page certificate

Synthesis: an equivalent assay above 100% does not mean more than 100% elemental zinc or certify absence of impurities. Specification ranges, individual test results, formula composition and SDS composition declarations answer different questions. The Canadian SDS explicitly disclaims being a quality specification or warranty. S7

Hazard context and coverage limits

The Canadian monohydrate SDS, revised 19 December 2025, classifies acute oral toxicity as category 4 and serious eye damage/eye irritation as category 1 under its stated Canadian WHMIS framework. Its other-hazards section identifies long-lasting aquatic toxicity. These are dated, regional, named-product statements, not a worldwide compliance determination or a process-compatibility assessment. S7, §§1–2

This article covers selected identities and source-specific data. It does not establish an exhaustive hydrate/polymorph catalog, equilibrium speciation, analytical moisture/assay methods, material compatibility, application suitability or customer qualification. Surface-finishing context is separate from substance identity: see Surface-finishing processes.

Source versions and access limits

Research reviewed 30 October 2026 PDT. The USP preview and both ICSC HTML cards were freshly retrieved. ICSC card dates are October 2001 (0349) and November 2007 (1698), not new property determinations. CIAAW's zinc page identifies its atomic-weight recommendation as dating from 2007.

The Merck page was readable through the web reader; a separate direct HTTP request returned 403. It is undated and was not freshly reproduced by that request. The Thermo Fisher certificate and original research PDF were freshly downloaded and relevant PDF pages visually inspected. The research publisher's article page was unavailable to the web reader, but its PDF asset was accessible directly. The Canadian SDS was freshly downloaded and its relevant pages visually inspected: revision 7, nine pages, 19 December 2025. The web reader returned an older eight-page revision dated 22 May 2024; the freshly retrieved PDF governs the SDS claims here. No current-lot certificate, paywalled standard or complete solution-property dataset was verified.