Zinc Sulphate Monohydrate vs Heptahydrate: Compare the Zinc You Buy
Two zinc sulphate offers can have different prices per tonne and still reverse order when compared per kilogram of zinc. The first check is therefore not which hydrate is “better”, but which chemical form, zinc assay and reporting basis each quotation actually describes. A technically acceptable offer must also meet the application's impurity and process requirements; a favourable zinc calculation cannot replace those checks.
Separate hydrate identity, zinc content and salt purity
Zinc sulphate monohydrate is ZnSO4·H2O, CAS 7446-19-7; the heptahydrate is ZnSO4·7H2O, CAS 7446-20-0. The additional crystallization water increases the mass associated with the same amount of zinc. Chemical identity is corroborated by Sigma-Aldrich's monohydrate reference and Merck's heptahydrate reference. Their reagent specifications are not specifications or certifications for MOS material.
“Zinc content” and “zinc sulphate purity” use different numerators. A high percentage of the named salt does not mean that the same percentage of the product is elemental zinc. A dry-basis result also cannot be used as an as-supplied concentration without the relevant water correction. Ask for the analyte, unit, method and reporting basis before comparing certificates.
| Procurement check | Monohydrate | Heptahydrate | What the buyer should do |
|---|---|---|---|
| Chemical form | ZnSO4·H2O; CAS 7446-19-7 | ZnSO4·7H2O; CAS 7446-20-0 | Match the named hydrate across quotation, label and lot documents. |
| Current MOS catalogue zinc limit | Zn ≥35% | Zn ≥21.5% | Treat these as catalogue limits, not measured results for an offered batch. Confirm the contractual reporting basis. |
| Same-mass substitution | Delivers zinc according to its documented assay | Delivers zinc according to its documented assay | Recalculate the charge; do not replace one tonne with one tonne by name alone. |
| Other acceptance requirements | Require separate qualification | Require separate qualification | Agree impurity limits, grade, handling and process acceptance independently of zinc content. |
The current monohydrate product record and heptahydrate product record provide the starting catalogue specifications. Neither a CAS number nor a zinc limit establishes approval for food, feed, pharmaceutical or fertilizer use in a particular destination.
Put both quotations on one zinc basis
For a comparable as-supplied zinc mass fraction, the arithmetic is straightforward: contained zinc = product mass × zinc fraction; delivered cost per kilogram of zinc = comparable delivered product cost ÷ contained zinc. Use the same currency, delivered destination, included charges and assay basis. Compare acceptable offers only.
If a quotation gives only a dry-basis assay, an unspecified “purity” or a typical value, leave the normalized comparison unresolved until the basis is clarified. A result labelled “Zn” needs its own reporting basis; the hydrate name does not supply that missing information. Do not mix a minimum for one offer with an optimistic typical result for another and call that an equivalent commercial comparison.
Qualify the replacement at equal zinc input
A useful controlled comparison starts with the same target zinc input, a documented grade and representative samples. Keep the process water, addition sequence, mixing, relevant temperature and observation period consistent. Record the actual product mass and any water added separately. For a solution process, check the prepared solution against agreed concentration, clarity or residue criteria appropriate to that application; visual clarity alone does not establish chemical purity.
Different product charges can change the overall formulation mass and water balance. Include the hydrate and any independently reported free water in the buyer's formulation calculation rather than assuming all water has the same specification meaning. The extra mass is a formulation consideration, not evidence that one material dissolves faster or performs better.
Stop qualification when identity or assay basis is unresolved, an agreed contaminant limit is exceeded, or the representative trial fails the buyer's acceptance criteria. Do not compensate for an unacceptable impurity result by lowering the price or adjusting the zinc dose. Route the discrepancy through the quality decision before releasing material for use.
Request an offer that can actually be compared
Send the intended application and destination, exact hydrate, required zinc basis and acceptance limits, sample or batch quantity, packaging and delivery basis. Request the current grade TDS/SDS and a specimen COA, then agree when the shipping-lot results will be available. Identify whether a reported value is batch-tested, calculated or typical; the chemical COA checklist explains how to review the record.
Use the monohydrate specification and inquiry route for that exact form, or reference the heptahydrate record when requesting the alternative. Ask for comparable evidence before deciding; this guide does not promise current stock, a destination-specific certification or a universal replacement ratio.
Sources
- Zinc sulfate monohydrate product identity — Sigma-Aldrich
- Zinc sulfate heptahydrate product identity — Merck
Sources checked on 2026-10-24.
