What Is Ferrous Ammonium Sulfate?
Ferrous ammonium sulfate, also known as diammonium iron bis(sulfate) or Mohr’s salt, is a crystalline compound that combines iron, ammonium and sulfate ions. It is created by reacting iron sulfate with ammonium sulfate in water, then allowing the solution to cool so pale green crystals form. Because it is relatively stable in air compared with other iron(II) salts, it became popular in the late 1800s for laboratory work and soon found its way into industrial and cosmetic applications.
In cosmetics, ferrous ammonium sulfate is valued for its ability to curb unwanted microbial growth. Manufacturers often add it in small amounts to water-based products where bacterial contamination can be a concern. You may spot it on ingredient lists for facial toners, sheet mask essences, soothing gels and certain rinse-off treatments that need a little help staying fresh without relying on heavier preservative systems.
Ferrous Ammonium Sulfate’s Cosmetic Benefits/Uses
The main role of ferrous ammonium sulfate in skin care is as an antimicrobial agent. By keeping bacteria in check it helps extend the shelf life of a formula and supports product safety, which in turn protects the skin from potential irritation caused by spoiled products.
Who Can Use Ferrous Ammonium Sulfate
Because ferrous ammonium sulfate is used at very low levels as a preservative rather than an active treatment, it is generally considered suitable for all skin types, including dry, oily, combination and sensitive skin. People with extremely reactive or allergy-prone skin may wish to watch for any early signs of irritation, as they would with any new ingredient.
The compound is mineral based and does not contain animal-derived components, so it is acceptable for vegans and vegetarians.
Current safety data do not flag specific concerns for pregnant or breastfeeding women when the ingredient is used topically in cosmetic amounts. That said this is not medical advice, and anyone who is pregnant or nursing should discuss their entire skincare routine with a doctor to be sure every product is suitable.
Ferrous ammonium sulfate does not increase photosensitivity, so it will not make skin more prone to sunburn. Standard daytime sun protection habits still apply for overall skin health.
Potential Side Effects/Adverse Reactions
Reactions to topical ferrous ammonium sulfate vary from person to person. The points below outline potential side effects only. When the ingredient is used correctly in a well-formulated product most people will not notice any problems.
- Mild skin irritation such as temporary redness or stinging, more likely in those with very sensitive skin
- Allergic contact dermatitis in rare cases for individuals who have a specific sensitivity to iron salts
- Temporary surface staining of skin or fabrics if the product is not fully absorbed before contact with clothing or towels
- Color shift in the formula if combined with ingredients that strongly acidify or oxidize, which could signal reduced preservative efficiency
If any unwanted reaction occurs discontinue use and seek advice from a qualified healthcare professional.
Comedogenic Rating
Rating: 0 / 5
Ferrous ammonium sulfate is a highly water-soluble inorganic salt that is used in very small amounts and leaves no oily or occlusive residue on skin, which means it has virtually no ability to block pores. Because it stays dissolved in the watery portion of a formula and is rinsed or absorbed away without forming a film, it earns the lowest possible comedogenic rating.
Suitable for those who are prone to acne or breakouts.
As with any preservative the ingredient does its work behind the scenes rather than sitting on the skin, so its interaction with sebum and dead cells that can trigger comedones is minimal.
Summary
Ferrous ammonium sulfate functions mainly as an antimicrobial preservative that keeps bacteria from multiplying in water-based cosmetics. By releasing iron ions in a controlled way it disrupts microbial activity and helps products stay fresh and safe.
It is a bit of a niche choice compared with big-name preservatives like phenoxyethanol or parabens, yet formulators who want a simple mineral option still reach for it, especially in toners or masks aimed at sensitive users.
Used at the low concentrations allowed in cosmetics it is considered safe for all skin types with only isolated reports of irritation or allergy. As with any new skincare product, patch testing is a smart step to make sure your skin agrees with the formula.