Many chemicals are sold in both anhydrous and hydrated forms, and a kilogram of each does not contain the same amount of substance. Roughly half of a tonne of magnesium sulfate heptahydrate, for example, is water. If the form is not specified, buyers can receive the wrong material or compare prices that are not comparable. This matters when evaluating quotations, preparing formulations and placing orders.
What is a hydrate?
A hydrate is a salt that holds a fixed amount of water (water of crystallisation) in its crystal structure. The formula shows it with a dot: MgSO₄·7H₂O means seven water molecules per formula unit. This water is part of the product. It is not moisture or an impurity. The anhydrous form contains no water of crystallisation. Each form has its own CAS number (see the CAS number guide).
Common forms and their active content
"Anhydrous equivalent" is the share of anhydrous substance in one kilogram of the hydrate. The last column shows how much hydrate corresponds to 1 kg of the anhydrous substance. Figures are calculated from molecular weights.
| Substance | Form | CAS | Anhydrous equivalent | Per 1 kg anhydrous |
|---|---|---|---|---|
| Calcium chloride | Anhydrous, CaCl₂ | 10043-52-4 | 100% | 1.00 kg |
| Dihydrate, CaCl₂·2H₂O | 10035-04-8 | 75.5% | 1.32 kg | |
| Magnesium sulfate | Anhydrous, MgSO₄ | 7487-88-9 | 100% | 1.00 kg |
| Heptahydrate, MgSO₄·7H₂O | 10034-99-8 | 48.8% | 2.05 kg | |
| Magnesium chloride | Anhydrous, MgCl₂ | 7786-30-3 | 100% | 1.00 kg |
| Hexahydrate, MgCl₂·6H₂O | 7791-18-6 | 46.8% | 2.14 kg | |
| Sodium sulfate | Anhydrous, Na₂SO₄ | 7757-82-6 | 100% | 1.00 kg |
| Decahydrate, Na₂SO₄·10H₂O | 7727-73-3 | 44.1% | 2.27 kg | |
| Sodium carbonate | Anhydrous, Na₂CO₃ (soda ash) | 497-19-8 | 100% | 1.00 kg |
| Decahydrate, Na₂CO₃·10H₂O | 6132-02-1 | 37.0% | 2.70 kg | |
| Sodium acetate | Anhydrous, CH₃COONa | 127-09-3 | 100% | 1.00 kg |
| Trihydrate, CH₃COONa·3H₂O | 6131-90-4 | 60.3% | 1.66 kg | |
| Zinc sulfate | Anhydrous, ZnSO₄ | 7733-02-0 | 100% | 1.00 kg |
| Monohydrate, ZnSO₄·H₂O | 7446-19-7 | 90.0% | 1.11 kg | |
| Heptahydrate, ZnSO₄·7H₂O | 7446-20-0 | 56.1% | 1.78 kg | |
| Copper sulfate | Anhydrous, CuSO₄ | 7758-98-7 | 100% | 1.00 kg |
| Pentahydrate, CuSO₄·5H₂O | 7758-99-8 | 63.9% | 1.56 kg | |
| Iron(II) sulfate | Anhydrous, FeSO₄ | 7720-78-7 | 100% | 1.00 kg |
| Heptahydrate, FeSO₄·7H₂O | 7782-63-0 | 54.6% | 1.83 kg |
How to compare prices
To compare quotations for different forms, divide each price by its anhydrous equivalent:
Price on anhydrous basis = Price per tonne ÷ Anhydrous equivalent
Example: if magnesium sulfate heptahydrate costs 100 units per tonne, its price on an anhydrous basis is 100 ÷ 0.488 ≈ 205 units. An offer for anhydrous magnesium sulfate below 205 units is therefore cheaper per unit of active substance.
The actual content of commercial grades also depends on the assay. Use the figure on the certificate for your calculation. Calcium chloride, for instance, is usually sold in two grades: flakes contain about 74–77% CaCl₂ (mainly dihydrate), and granules or powder contain about 94–97% CaCl₂ (mainly anhydrous).
Which form to choose
- Freight and storage: with a hydrate you also pay to ship the water. For long-distance imports where freight is a large share of the cost, the anhydrous form can be more economical. Compare on an anhydrous basis with freight included.
- Moisture uptake: most anhydrous forms absorb moisture from the air quickly and can cake. Keep packaging closed and store the product in a dry place. Anhydrous forms are preferred as drying agents (desiccants).
- Heat of solution: anhydrous calcium chloride and magnesium sulfate release heat when they dissolve in water. Add large quantities to water gradually.
- Recipe compatibility: use the form your formulation or process instruction was written for. If you switch to another form, recalculate the quantity with the ratio in the table.
- Ease of use: some hydrates, such as copper sulfate pentahydrate and iron(II) sulfate heptahydrate, dissolve easily and are the common commercial forms in agriculture and water treatment.
When ordering or requesting a quotation
- Give the form, formula and CAS number together with the product name, for example "Magnesium sulfate heptahydrate, MgSO₄·7H₂O, CAS 10034-99-8".
- State the basis for your minimum content: as supplied, or on an anhydrous basis.
- On delivery, check the Certificate of Analysis (CoA) and confirm which form the assay refers to (see the SDS, CoA and TDS guide).
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