How Much Iron Is in 98% Ferrous Sulphate Heptahydrate? The 19.7% Fe Calculation

The Core Stoichiometric Equation

One of the most persistent misunderstandings in chemical procurement is confusing the purity of a chemical compound with the concentration of its constituent elements. When a supplier's Certificate of Analysis declares 'Assay: 98.2% Min', what does this mean for the process engineer who needs to dose 10 mg/L of active iron?

Let us examine the molecular breakdown of Ferrous Sulphate Heptahydrate 98% Min using atomic weights from the periodic table:

• Iron (Fe): 1 × 55.845 g/mol = 55.845 g/mol
• Sulfur (S): 1 × 32.065 g/mol = 32.065 g/mol
• Oxygen in sulfate (O₄): 4 × 15.999 g/mol = 63.996 g/mol
• Water of crystallization (7H₂O): 7 × 18.015 g/mol = 126.105 g/mol

Summing these components yields the total molar mass of pure FeSO₄·7H₂O:

Molecular Weight = 55.845 + 32.065 + 63.996 + 126.105 = 278.011 g/mol

Calculating the Theoretical Pure Iron Fraction

To determine how much of that total mass is elemental iron:

Iron Fraction = 55.845 / 278.011 = 0.20087 (or 20.087% by weight)

In 100% chemically pure ferrous sulphate heptahydrate, only 20.09% of the weight is iron metal. The remaining 79.91% consists of sulfate ions (SO₄²⁻) and crystal water (7H₂O) integrated into the solid crystalline matrix.

Scaling for 98% Commercial Assay

Commercial technical grades are certified to at least 98.0% FeSO₄·7H₂O purity, with minor allowances for moisture and trace minerals:

Elemental Fe = 20.087% × 0.980 = 19.685% ≈ 19.7% Fe

Therefore, every 25 kg bag of Ferrous Sulphate Heptahydrate 98% contains approximately 4.92 kg of elemental iron (25 kg × 0.19685).

The Risk of Dosing Confusion in Water & Agronomy

If an agronomist prescribes 2 kg of elemental iron per hectare and mistakenly orders 2 kg of ferrous sulphate heptahydrate, the crop will only receive 0.39 kg of iron, failing to correct the chlorosis. Conversely, assuming 98% iron in wastewater leads to under-dosing coagulant by a factor of five. Always base chemical dosing on elemental iron requirements divided by 0.197.

Understand the differences between hydration grades: Ferrous Sulphate Heptahydrate vs Monohydrate.

Avoid costly assay and dosing miscalculations: Common Ferrous Sulphate Buyer Mistakes and How to Avoid Them.