Copper sulfate

Identity and hydrate forms

Ideal anhydrous CuSO₄ contains 39.813% Cu by mass. CuSO₄·5H₂O contains 25.451% Cu and 63.926% CuSO₄ equivalent. These figures follow from formula masses. Supplier assay and lot data are separate. Copper(II) sulfate is also called cupric sulfate. Aldrich 451657 Sigma-Aldrich 209198

The sources do not explain the small difference between pentahydrate mass values. The Advent certificate identifies the hydrated formula explicitly. The dot denotes water of crystallization, not a separately specified free-moisture allowance.

A 2024 experimental study distinguishes monohydrate CuSO₄·H₂O, trihydrate CuSO₄·3H₂O, and pentahydrate. Its monohydrate-to-pentahydrate hydration passed through trihydrate under the studied conditions. Phase stability depends on temperature and water-vapor pressure, while kinetics can preserve metastable phases. This observation does not establish one universal transformation path or temperature. CAS identifiers and commercial specifications for the lower hydrates are outside this reference's verified coverage. Cotti et al., theory, methods, and conclusion.

Assay and equivalent reporting

Salt assay, copper content, and trace-metal purity describe different quantities. The following are derived stoichiometric values for ideal pure material, using 159.61 and 249.68 g/mol above and the conventional copper molar mass 63.546 g/mol. CIAAW reports copper's standard atomic weight as 63.546(3). CIAAW copper.

Derived chemistry uses copper fraction = 63.546 / formula mass. The pentahydrate's CuSO₄ fraction = 159.61 / 249.68. These rounded factors express composition, not material additions or a process recipe. The 36.074% difference is theoretical bound water, not a measured loss-on-drying specification.

Conditional synthesis: if an assay is explicitly a mass fraction of pentahydrate in the supplied sample, its copper contribution equals that fraction multiplied by 0.25451. This interpretation fails when the result uses a dry, anhydrous-equivalent, or trace-metals basis, or includes copper-bearing impurities. The unqualified word "purity" does not resolve that basis.

Commercial specifications and lot evidence

Sigma-Aldrich 209198 is an ACS reagent crystal product with assay entries ≥98.0% and 98.0–102.0% as its ACS specification. Its listed impurity maxima are chloride 0.001%, calcium 0.005%, iron 0.003%, potassium 0.01%, sodium 0.02%, and nickel 0.005%. These are that product's stated limits, not measured lot results. 209198 properties. Synthesis: an analytical assay interval extending above 100% cannot be interpreted as a literal substance mass fraction above 100%.

Aldrich 451657 is an anhydrous powder labeled ≥99.99% on a trace-metals basis, with trace-metal analysis ≤100.0 ppm. That wording alone establishes neither total salt assay nor water content. It is not directly comparable with the hydrated reagent assay. 451657 properties.

Advent's AR pentahydrate COA, product 90147-500 G, batch F25J099, version 02, identifies manufacture in September 2025 and expiry in August 2030. It reports assay 99.63% against ≥99.5%. Selected impurity results illustrate the difference between a result and a limit. Original one-page COA, analysis table and remarks.

The COA remarks restrict certification to analysis at quality release. A less-than result is not zero. The certificate does not state assay method, uncertainty, or a separate water result. It represents that historical batch, not another supplier or a currently supplied lot. COA.

Physical properties and thermal labels

Synthesis: a solid's density and a powder's bulk density describe different volumes. The cited values do not establish package volume, porosity, or solution density. An unresolved solubility denominator prevents a reliable conversion to grams per 100 g water or percent by mass.

Thermal labels also differ. ICSC 1416 gives 110 °C as decomposition. Roth's P024 Malta SDS instead labels 30–110 °C as crystal-water release and >30 °C as decomposition associated with that release. These are not equivalent descriptions of sulfate destruction. ICSC, Roth SDS, section 9.

The anhydrous 451657 page itself lists 200 °C with a decomposition qualifier under melting point, yet describes oxide formation above 600 °C. Its fields do not resolve that discrepancy. None of these labels defines a verified heating schedule or operating limit. Aldrich properties and description.

Aqueous species and color

The 2007 IUPAC report treats CuSO₄(aq) as an associated aqueous species in equilibrium with Cu²⁺ and sulfate. It distinguishes contact and solvent-separated ion pairs and cautions that spectroscopy may detect them differently. Its environmental-water calculations depend on ligand composition, ionic strength, and temperature. They cannot be transferred unchanged to a concentrated formulation. Powell et al., sections 6.4 and 9, published report available through Murdoch University.

Synthesis: total copper expressed as Cu is an elemental accounting basis, not the concentration of free Cu²⁺ or metallic copper particles. Blue appearance alone establishes neither assay nor a unique dissolved species distribution. These sources provide no universal color-to-concentration or color-to-performance relationship.

Hazard scope and evidence limits

Roth's P024 SDS, Malta English version 9.2, revised August 4, 2025, classifies oral acute toxicity category 4, serious eye damage category 1, and acute and chronic aquatic hazard category 1 under EU CLP. This dated regional classification is not a global compliance determination or process-performance evidence. Roth SDS, sections 1–3.

ICSC 1416 is dated November 2023. The manufacturer web listings have no displayed technical revision date in the cited property sections and were accessed October 30, 2026 PDT. The original hydration study was published December 9, 2024. This reference establishes no customer material approval, formulation compatibility, production result, or current-lot certification. Surface-finishing processes.