The Anion Is the Cheap Part — the Cation and the Water Carry the Value

Every metal sulfate shares the same sulfate unit, and that unit is a bulk commodity the chemical industry moves by the tens of millions of tonnes. When you put a quotation for ferrous sulfate monohydrate next to one for magnesium sulfate, the price difference you are looking at is almost entirely the metal ion, the water it is carrying, and how clean the producing plant can make it. Two offers with the word “sulfate” in common are therefore not comparable at all — the sulfate part is the cheapest component in both. Even the same product name is not comparable until the hydrate state is fixed, because the hydrate is the product identity. A specification sheet that does not state the hydration state unambiguously is an invitation to buy water at metal prices.

Water of Crystallisation Is Identity, Not Moisture

The CAS number changes with the hydrate. Anhydrous magnesium sulfate is CAS 7487-88-9 at MW 120.37; the monohydrate known as kieserite is a different registry entry (the magnesium sulfate hydrate page lists CAS 22189-08-8) at MW 138.38. The same pattern runs through the whole family. Ferrous sulfate monohydrate (MW 169.92) carries about 32.9% Fe, while the familiar pale-green heptahydrate (CAS 7782-63-0, MW 278.01) carries about 20.1% Fe — 39% less metal per kilogram, purely from bound water. Nickel sulfate exists as an anhydrous salt (CAS 7786-81-4, MW 154.75, about 37.9% Ni) and as a hexahydrate (MW 262.85, about 22.3% Ni); our listings carry the hydrate grade under CAS 15244-37-8 and the high-purity anhydrous salt under CAS 7786-81-4.

This matters for two lines on the certificate. The assay basis must match the hydrate: “98% assay” quoted as anhydrous equivalent is a different product from 98% as the hydrate. And the bound water is a fixed fraction of each hydrate — about 13.0% of a magnesium sulfate monohydrate, 41.1% of a nickel sulfate hexahydrate, 45.4% of a ferrous sulfate heptahydrate — so a “moisture ≤0.5%” line can only ever refer to free moisture sitting on the crystal surface, not to the water of crystallisation.

Compare the Nutrient, Not the Salt

Once the hydrate is fixed, the honest way to compare two offers is price per kilogram of active metal. Anhydrous magnesium sulfate at 20.2% Mg looks cheap next to kieserite at about 17.6% Mg until you remember what each one does: the monohydrate is the form agronomists specify for soil because it dissolves slowly and stores without hardening, while the anhydrous salt serves laboratories and fully dissolved feed solutions. Manganese sulfate hydrate (MnSO₄·H₂O) delivers roughly 32.5% Mn and is the standard micronutrient and battery-precursor manganese source. Ammonium sulfate (MW 132.14) is the interesting exception in the family: it is priced against nitrogen sources, carrying about 21.0% N, but it also brings roughly 24.2% S, and buyers who credit the sulfur content in their cost model frequently find it beats urea on delivered cost per unit of nutrition.

Purity Tiers Follow the Destination

The same hydrate is sold in grades that differ by an order of magnitude in price, and the grade makes sense only against the end use. Fertilizer and industrial grades tolerate co-ions; an electroplating nickel bath does not, because parts-per-million levels of cobalt, copper, zinc or organics interact with brightener systems and change deposit stress. Battery precursor grades go further: the nickel sulfate feeding NCM production is specified down to tens of ppb for sodium, potassium, calcium, magnesium and magnetic foreign matter, because those elements end up inside the cathode lattice. Downstream of the nickel salts sit cobalt hydroxide — the pink-to-red powder whose tap density and magnetic impurity limits decide whether it can enter a precursor line — and in lithium circuits lithium sulfate monohydrate is the workhorse feed for carbonate precipitation routes, where calcium and magnesium co-precipitation is precisely the failure mode to control.

When Solubility Is the Safety Story

Barium sulfate (MW 233.39) is the extreme case of an application defined by what it does not do. With a solubility product around 1×10^-10, it passes through the human digestive tract essentially unchanged, which is why pharmaceutical X-ray contrast media are made from it — and why the same compound weights drilling fluids at densities of 4.2–4.5 g/cm³. Soluble barium salts are toxic, so the release item that matters on a barium sulfate certificate is acid-soluble barium, and a batch with elevated acid-soluble content can be perfectly white and perfectly unusable for food-contact or medical-adjacent applications. Beryllium sulfate tetrahydrate (MW 177.14, melting around 270°C) is freely soluble in water, and that is exactly the problem: soluble beryllium compounds are associated with chronic beryllium disease. Handling and containment, not chemistry, dominate the delivered price — treat any quotation that ignores them as incomplete.

Specialty Sulfates: Tanning, Plating and Rare-Earth Catalysis

Chromic sulfate (Cr₂(SO₄)₃, MW 392.18) is the chrome tanning workhorse, and the parameter that decides fit is basicity — the degree of hydroxyl complexing — quoted at levels such as 33% or 42% rather than assay alone. The salt itself is a purple solid whose aqueous solution turns green on heating — normal hydrolysis, not contamination. Rhodium(III) sulfate is the standard rhodium source for decorative and electrical plating baths, supplied and stored under inert gas at 2–8°C; a certificate that omits rhodium concentration basis (metal vs salt) cannot be priced. Samarium sulfate (the octahydrate, total formula weight about 733) and its rare-earth siblings are bought on total rare-earth purity and individual rare-earth impurity distribution, a specification structure closer to battery materials than to conventional inorganic salts.

Two Sulfates Quoted on a Different Basis — and the RFQ Checklist

Titanium oxysulfate, the feedstock of the sulfate route to titanium dioxide, is customarily quoted on its TiO₂ content basis rather than the salt assay, so always ask which basis a number refers to before comparing offers. Aluminum sulfate (MW 342.15, melting point about 770°C) has been quoted for a century on an Al₂O₃ equivalent basis — “17% alum” means 17% Al₂O₃, not 17% Al₂(SO₄)₃ — and iron-free grades for paper and water-treatment use carry their own iron specification. A short checklist closes most sulfate disputes before they start:

1. Hydrate CAS number and molecular weight, stated on the certificate, not just the name.
2. Assay basis: as hydrate or as anhydrous equivalent.
3. Bound-water arithmetic done once, in writing, so free-moisture limits are not confused with water of crystallisation.
4. Application-specific impurities: Co/Cu/Zn for plating, Na/K/Ca/Mg and magnetic foreign matter for battery grades, iron for aluminium sulfate.
5. Acid-soluble barium for barium sulfate; basicity for tanning chromium salts.
6. Concentration basis for plating salts (metal vs salt) and TiO₂ basis for titanium oxysulfate.
7. Packing and storage: inert-gas or cold-chain requirements where the page specifies them, and hygroscopic grades in sealed liners.

None of these items costs anything to request. All of them are cheaper to fix before the purchase order than after the first rejected batch.