Ferrous sulfate

Ferrous sulfate is the iron(II) salt of sulfuric acid and one of the iron sources permitted in EU food supplements. It carries 36.8 % elemental iron on an anhydrous basis, so 272 mg of anhydrous ferrous sulfate supplies 100 mg of iron. This page covers the grades manufacturers buy, its EU status and its behaviour in tablets, capsules and powders.

Grades and forms

Ferrous sulfate powder is traded in more than one hydration state. The heptahydrate forms the familiar greenish crystals. Dried (exsiccated) ferrous sulfate has lost most of its water of crystallisation and carries more iron per gram.

Physical forms include crystals, granules and fine powder. Coated or encapsulated grades are also used where the iron must be kept apart from other ingredients.

What decides the choice:

• Iron per gram: a dried grade gives a smaller fill mass for the same label claim.
• Format: fine powder blends evenly in capsules and premixes; granules flow better on tablet presses.
• Storage behaviour: the heptahydrate loses water in dry air and oxidises in moist air, so it is more sensitive to storage conditions.
• Taste and interactions: coated grades reduce contact with sensitive actives and mask the salty, metallic taste.

The specification sheet and certificate of analysis state the hydration state, iron content and particle size of each lot. For bulk supply, name the grade you need in the quote form.

EU regulatory status

Ferrous sulphate is listed as a source of iron in Annex II of Directive 2002/46/EC, so it may be used in food supplements throughout the EU. It is also listed in Annex II of Regulation (EC) No 1925/2006 for adding iron to other foods. The guide to permitted vitamin and mineral forms explains how the annexes work.

Harmonised EU maximum amounts of iron in food supplements have not been adopted. Member states apply their own limits and notification procedures, summarised in the maximum levels guide.

The label declares the amount of iron, not of the salt, per daily portion. Any claim falls under Regulation (EC) No 1924/2006. Authorised wording is in the EU Register of nutrition and health claims and explained in the health-claims guide.

Formulation notes

Elemental arithmetic. Divide the iron claim by 0.368 to get the anhydrous salt mass. A 100 mg iron claim needs 272 mg of anhydrous ferrous sulfate. A 14 mg claim needs about 38 mg. Hydrated grades carry less iron per gram, so recalculate with the iron content on the certificate of analysis. The elemental content calculator does this per lot.

Oxidation and moisture. Iron(II) oxidises to iron(III) in the presence of air and moisture, and the material turns yellow-brown. The salt is hygroscopic. Keep containers closed, control room humidity and choose low-moisture excipients.

Interactions. Iron ions catalyse the oxidation of ascorbic acid, vitamins A and E and unsaturated oils. Polyphenol-rich botanical extracts darken in contact with iron. Separate granulation, coated iron or bilayer tablets limit contact in multi-nutrient products.

Taste. Ferrous sulfate has a saline, metallic taste in water. Powders and chewables need coated grades or taste masking; tablets are usually film-coated. In capsules, the low milligram mass for typical claims leaves room for diluents that protect against moisture.

How it compares to other forms

Of the forms with stated values, ferrous sulfate gives the most iron per gram. Ferrous gluconate needs nearly three times the powder mass for the same claim. The ferrous fumarate vs ferrous sulfate comparison and the iron forms guide cover the trade-offs in more detail.