Both are zinc sulphate. They differ in how much water of crystallisation they carry, and therefore in how much zinc is in each bag. Everything below uses the assay figures published on our own product pages.
The short answer. Zinc sulphate monohydrate is 33% Zn. Zinc sulphate heptahydrate is 21% Zn. A 50 kg bag of monohydrate therefore carries about 16.5 kg of zinc; a 50 kg bag of heptahydrate carries about 10.5 kg. Compare the two on price per kg of zinc, not price per kg of bag, or you will misjudge them by roughly half.
Zinc sulphate crystallises with water attached to it. The monohydrate holds one molecule of water (ZnSO₄·H₂O), the heptahydrate holds seven (ZnSO₄·7H₂O). That water is inert weight. It is part of the crystal, it is paid for by the kilogram, and it is not zinc.
This is why the trade in India refers to them by their zinc content rather than their chemical names. “Zinc sulphate 33” is the monohydrate. “Zinc sulphate 21” is the heptahydrate. If a supplier quotes you “zinc sulphate” without a number, the quote is incomplete and cannot be compared with anything.
| Monohydrate | Heptahydrate | |
|---|---|---|
| Trade name | Zinc sulphate 33 | Zinc sulphate 21 |
| Molecular formula | ZnSO₄·H₂O | ZnSO₄·7H₂O |
| Zinc content published on our page | 33% Zn | 21% Zn |
| Standard packing | 50 kg net, HDPE bag | 50 kg net, HDPE bag |
| Zinc in one standard bag | approx. 16.5 kg | approx. 10.5 kg |
| Bags to deliver 1,000 kg of zinc | approx. 61 | approx. 96 |
Zinc per bag and bags per tonne of zinc are arithmetic on the published assay and the standard 50 kg packing. They are indicative. The guaranteed analysis on the product page and the batch certificate of analysis govern.
Divide the delivered price per kg by the zinc fraction. At the published assays that is price ÷ 0.33 for the monohydrate and price ÷ 0.21 for the heptahydrate. The result is your cost per kg of contained zinc, and it is the only number on which the two grades can honestly be set against each other. Our nutrient cost comparison calculator does this arithmetic for you and shows its working.
Freight follows the same logic. Moving a tonne of heptahydrate moves roughly 790 kg of material that is not zinc. Over distance, that is a real cost that a per-kilogram price hides.
That depends on your process, and it is a question we would rather answer against your actual situation than in the abstract. The things that decide it are how the product is applied, what else it is blended with, how it must dissolve, what your own specification or a customer specification requires, and how far the material has to travel. Tell us those and we will tell you which grade we would send, and why.
Taskon Industries manufactures both. Zinc Sulphate Monohydrate and Zinc Sulphate Heptahydrate each have a full specification, physical properties and safety summary on their own page.
Four things let us quote without a round of questions: the grade you want (33 or 21, or tell us the zinc percentage you need), the quantity, the delivery location, and any specification you have to meet. Send those and we will come back with a price, the packing and the documents that travel with the consignment.
Not sure which grade suits your process? Describe the application and we will tell you what we would supply, and why.
Send the grade, the quantity and the delivery location and we will quote against them.