Cosgard (Geogard 221)

Overview

Cosgard is the French name for what is sold elsewhere as Geogard 221. It is the same product — a clear, slightly yellow liquid combining benzyl alcohol, dehydroacetic acid, and water into one preservative.

Both ingredients are nature-identical, meaning they exist in nature (benzyl alcohol in jasmine and ylang-ylang, dehydroacetic acid in some flowers) but are made synthetically for cosmetic use. This is why Cosgard carries an Ecocert and Cosmos approval — a big selling point for hobbyists building “natural” product lines. It is paraben-free, formaldehyde-free, and approved for use on babies and children.

The thing to understand about Cosgard is how pH affects its performance. The system is approved across pH 2 to 7, and that is the working window you can rely on. The reason for the upper limit is that dehydroacetic acid works in its undissociated (uncharged) form, and the fraction of molecules in that form shrinks as the pH climbs. At pH 4-5 the bulk of the dehydroacetic acid is active; by pH 6 a meaningful share has shifted to the dissociated form; by pH 7 most of it has, and the benzyl alcohol is doing the heavy lifting on its own. So while the approved range is pH 2-7, the sweet spot for full broad-spectrum performance is pH 4-6.

For most leave-on skincare (creams, serums, lotions, body butters) this is easy — they naturally sit at pH 4.5-5.5. For formulas that drift higher (some shampoos, conditioners, gel-cream hybrids), you can still use Cosgard up to pH 7 but pair it with a co-preservative or reduce the pH if you can. Above pH 7, switch to a different system entirely (Optiphen, Liquid Germall Plus, Geogard ECT).

What it does in a formula

Primary role: broad-spectrum preservation in water-based products across pH 2-7. Benzyl alcohol handles bacteria; dehydroacetic acid handles yeasts and moulds. The system is strongest at pH 4-6 and weakens as you approach pH 7.

Secondary role: benzyl alcohol contributes a faint floral-vanilla aroma. Some people like it, some find it a little nail-polish-remover. At 0.6-1% in a finished product, it is usually subtle.

How to use

Use at 0.6-1% of the total formula. Effective concentrations can be as low as 0.2% in optimal conditions, and 0.6% is a reliable standard starting rate.

Add to the cool-down phase, below 40°C.

Check your pH. This is non-negotiable with Cosgard. After your formula is finished and cooled, dip a pH strip or use a meter. Cosgard is approved for the pH 2-7 range, but is at its strongest between pH 4 and 6. If you land above 6, you can either accept the softer performance (with a co-preservative for insurance) or adjust the pH down with a few drops of lactic acid or citric acid solution (a 10% citric acid solution lets you adjust drop by drop without overshooting). If you are above pH 7, switch to a different preservative entirely.

The benzyl alcohol portion has poor water solubility on its own, but the manufacturer’s blend (with a bit of water and the dehydroacetic acid) is designed to dissolve in cosmetic emulsions. In purely watery formulas — a clear toner with no glycols or solubilisers — it can cause cloudiness. Add it to oil phase or solubilise with a small amount of polysorbate 20.

Best for / Worst for

Best for: facial serums, light lotions, body creams, light gel-cream emulsions, anything that sits naturally at pH 4-6. Especially good for products marketed as “natural” or “Ecocert” because of its certification status.

Worst for: anything above pH 7. That rules out most solid shampoo bars (often pH 7-9), un-pH-adjusted liquid shampoos, and high-pH hair masks. Between pH 6 and 7 it still works but with reduced strength — pair with a co-preservative or drop the pH if you can.

Also avoid in fully anionic-surfactant systems with carbomers — anionics can cause discoloration in Cosgard-preserved products (mitigated by adding a small amount of tocopherol or other antioxidant).

Common pitfalls

Not checking pH: by far the most expensive mistake. A hobbyist makes a “natural” lotion, preserves with Cosgard at pH 8, ships it to friends, and three weeks later there is pink slime in the jar. Cosgard works across pH 2-7, but above 7 it is the wrong tool — and even between 6 and 7 you want a co-preservative for insurance. Buy pH strips or a $20 pH meter before you use Cosgard.

The scent: benzyl alcohol carries a faint floral-solvent note that some people find off-putting. If you are making unscented baby products, do a sniff test first.

Anionic discoloration: Cosgard can yellow over time in formulas with anionic thickeners or surfactants. A pinch of vitamin E (tocopherol, not tocopheryl acetate) at around 0.2% prevents this.

Mixing it into the heated water phase: do not. Benzyl alcohol is somewhat volatile and the cool-down phase exists for exactly this kind of ingredient.

Assuming “Ecocert” means “won’t cause irritation”: benzyl alcohol is a known sensitiser for some people, and EU regulations require it to be listed if above a certain threshold. Patch test.

Substitutes

• Geogard ECT — same family (also Ecocert), works up to pH 8, but brings a stronger marzipan/almond scent.
• Optiphen — not natural-certified, but works up to pH 8 and is more forgiving of pH drift.
• Liquid Germall Plus — works across pH 3-8, much lower use rate (0.5%), but releases trace formaldehyde and is not eco-certified.
• Euxyl K 903 — similar benzyl-alcohol-based family, slightly stronger and broader pH window.