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Cosmetic preservative selection: pH window, water phase and packaging

A preservative is chosen against three things you already know: whether the formula contains water, the pH of its water phase, and how the consumer will contaminate the pack. This guide sets out how those three constrain the shortlist — and why nothing on this page can tell you that a formula is preserved.

The short answer

Water availability decides whether the question exists at all. pH decides which weak acids still have an undissociated fraction to work with. Packaging decides how hard the system is asked to work in use. Everything after that is verified by challenge testing the finished product in its final pack — not by the ingredient list.

Step 1 · Where is the water?

Microbial growth needs water that is available to the organism, not simply water on the formula sheet.

Emulsion — cream or lotion
Water phase plus oil phase. Both phases and the interface have to be considered.
Aqueous leave-on — toner, serum, mist
Mostly water, left on the skin, often packed with a dip or spray exposure.
Rinse-off surfactant — shampoo, body wash
High water, surfactant-rich, usually a closed pack and short skin contact.
Wet wipes or a high-exposure aqueous product
Substrate and lotion together; the substrate can adsorb part of the system.
Anhydrous — balm, oil, stick, powder
No added water. The first question is contamination control, not preservative choice.

An anhydrous product is not automatically safe: it is a product where the control is keeping water out. If wet hands, condensation or a damp raw material can introduce water, the formula is treated as a water-containing product and the pH question returns.

Step 2 · What pH does to a weak acid

Weak acids act largely in the undissociated form. The published pKa is the pH where the acid is half undissociated; each pH unit above it cuts that fraction roughly tenfold.

System	Published pKa	pH 4.0–4.5	pH 4.5–5.5	pH 5.5–6.5	pH > 6.5
Sorbate system (potassium sorbate)	≈ 4.76	76.4 %	36.5 %	5.4 %	0.6 %
Benzoate system (sodium benzoate / benzoic acid)	≈ 4.20	47.1 %	13.7 %	1.6 %	0.2 %
Dehydroacetic acid system	≈ 5.27	91.3 %	65.1 %	15.7 %	1.8 %

Percentages are the undissociated acid fraction at each band's representative pH, from 1 ÷ (1 + 10^(pH − pKa)). The fraction explains why a system weakens with pH. It is not a measure of antimicrobial performance in a real formula.

Step 3 · Routes that are not governed by a pH window

These are the reason a formula that cannot be acidified still has a structure to work with.

Phenoxyethanol system
INCI: Phenoxyethanol
A glycol ether, not a weak acid, so its behaviour is not governed by a pKa window. This is why it is customarily used where the formulation pH cannot be pushed down.
Typical published range: 0.5 – 1.0 % in published cosmetic formulation literature — context only, never a dose.
Benzyl alcohol system
INCI: Benzyl Alcohol
An aromatic alcohol used both as a preservative component and as a solvent for poorly soluble partners such as dehydroacetic acid.
Typical published range: 0.5 – 1.0 % in published cosmetic formulation literature — context only, never a dose.
Chelant support (EDTA salts)
INCI: Disodium EDTA · Tetrasodium EDTA
A chelant is not a preservative. It sequesters metal ions carried in by water and raw materials, which is why it is customarily discussed alongside a preservation system rather than instead of one.
Typical published range: 0.05 – 0.2 % in published cosmetic formulation literature — context only, never a dose.

Step 4 · What the pack changes

The same formula faces different in-use contamination depending on how the consumer reaches it.

Airless or pump
The consumer does not put fingers into the bulk product.
Tube or closed bottle
Limited exposure, but the orifice and the residual head-space still matter.
Open jar
Re-inoculated on every use, with water carried in from wet hands.

No use level is calculated for your formula. Ranges shown are typical published formulation ranges reported in cosmetic-science literature, given only so a shortlist can be discussed, and never as a recommended dose.

Permitted use, concentration limits and labelling conditions for cosmetics sold in Canada are set by the Health Canada Cosmetic Ingredient Hotlist for the specific substance and product type. Check the current Hotlist entry; an absence of restriction is not a permission to dose.

Verify against your own stability and challenge testing of the finished product in its final packaging. Nothing here substitutes for that test.

Take the same logic through your own numbers

The selector applies the arithmetic above to your product type, pH band, pack and claim constraint, and shows what each remaining route needs from the rest of the formula.

Open the Preservative System Selector

Source page: Cosmetic Preservative Selection — pH Window, Water Phase and Packaging

Use this when

You are choosing between weak-acid systems and non-pH-dependent systems for a new formula.
A formula moved pH during development and you need to know whether the existing system still applies.
You are replacing a preservative because of a claim constraint and need to understand the trade you are making.
You need to brief a supplier or a laboratory in the same language they use.

How this decision is structured

Water first: microbial growth needs available water. An anhydrous formula raises a different question from an emulsion.
pH second: weak acids act mainly in the undissociated form, so the published pKa sets the pH band where the system still has something to give.
Packaging third: an open jar re-inoculates the product on every use; an airless pump does not present the same exposure.
Chelants and multifunctionals are treated as partners, not preservatives — they change what the system has to cope with.
Everything ends at challenge testing of the finished product in its final pack.

What this page does not determine

This guide does not state that any system will preserve your formula. Only challenge testing of the finished product does that.
It does not publish a permitted concentration. Permitted use and labelling in Canada are set by the Cosmetic Ingredient Hotlist for the specific substance and product type.
It does not make a safety, irritation, sensitisation or 'hypoallergenic' claim, and it makes no therapeutic claim.
Typical published use-level ranges are context, not a dose. Your dose comes from your own testing.
It does not evaluate compatibility with your actives, fragrance, packaging material or process.

Relevant Allzone products

These are catalogue identities relevant to this decision. A listing here is not a statement that the material is suitable for your formula, and it is not a permitted-use, safety or performance claim.

Ask for a quotation or a specification

Where this comes from, and what it cannot tell you

Published physical-chemistry constants (acid dissociation)
Supports: Sorbic acid pKa ≈ 4.76, benzoic acid pKa ≈ 4.20, dehydroacetic acid pKa ≈ 5.27. The undissociated fraction of a weak acid falls as pH rises above its pKa.
Limitation: The undissociated fraction explains why a system weakens with pH. It does not quantify antimicrobial performance in a real emulsion.
Universal · Physical constants; stable.
ISO 11930 — Cosmetics, evaluation of the antimicrobial protection of a cosmetic product
Supports: That preservation is established by challenge testing the finished product in its final packaging, not by the identity of a preservative on an ingredient list.
Limitation: A test result applies to the exact formula, batch, pH and pack that was tested. Nothing on this page predicts a pass.
International · Standard method; confirm the revision your laboratory runs.
Health Canada — Cosmetic Ingredient Hotlist
Supports: That restricted and prohibited substances in cosmetics sold in Canada are defined by the Hotlist, and that the Hotlist entry — not this page — is the authority for any concentration limit or labelling condition.
Limitation: The Hotlist states restrictions. It never states that an unrestricted ingredient is effective, safe in your formula, or approved for your product.
Canada · Consult the current published Hotlist before every formulation decision.