How to Compare Magnesium Sulphate Quotations — Hydration Form and Purity Basis

The trap: the same price for different products

Magnesium sulphate is traded in three hydration forms — heptahydrate (Epsom salt, MgSO₄·7H₂O), monohydrate (MgSO₄·H₂O) and anhydrous (MgSO₄). They have the same chemical name, the same CAS family and often a similar-looking price, but they contain very different amounts of magnesium per tonne.

Water is the difference. The more water of crystallisation a form carries, the less MgO you receive for the same weight of material. Comparing two quotations without knowing which form each refers to compares nothing useful.

What to pin down in every quotation

Ask for these five points in writing, on every offer you compare:

• Hydration form — heptahydrate, monohydrate or anhydrous, stated explicitly
• The basis of the assay figure — as-is or on a dried/anhydrous basis; the two are not the same number
• MgO content, since that is what agricultural and feed buyers are actually buying
• Water insoluble matter, which drives solubility in fertigation systems
• Whether the grade is agricultural, technical or feed, and which one your specification requires

Match the form to the application

Heptahydrate dissolves readily and suits fertigation, foliar application and bath-salt use. Monohydrate is commonly used in feed and as a feedstock for granulation, where less free water is an advantage. Anhydrous is chosen where water content must stay low, or as a chemical feedstock.

Choosing the wrong form is not a quality failure — it is a specification failure, and it shows up as a yield or handling problem long after the container has arrived.

Also check the practical side

Two suppliers can quote the same grade and still differ on whether the material arrives free-flowing or caked, on particle size consistency, and on whether the paperwork matches the specification. Ask for the technical data sheet before ordering and compare it against the certificate of analysis that comes with the shipment.

We quote magnesium sulphate in all three hydration forms and will state the form and the assay basis on the face of the quotation, so the comparison with your other offers is a real one.