pH in Cosmetics Explained
Why pH matters, the correct range for each product type, and how to adjust it
pH is one of the most important parameters in cosmetic formulation. It affects skin compatibility, ingredient stability, preservative efficacy, emulsifier performance, and active ingredient activity. Getting pH right is not optional — it is a fundamental step in producing a safe, effective, and stable cosmetic product.
The pH scale — a quick reminder
pH basics
pH runs from 0 to 14. pH 7 is neutral (pure water). Below 7 is acidic; above 7 is alkaline (basic).
The scale is logarithmic: pH 4 is 10× more acidic than pH 5, and 100× more acidic than pH 6.
Skin's natural pH: approximately 4.5–5.5 (acid mantle).
Target pH ranges by product type
| Product type | Target pH range | Why |
|---|---|---|
| Facial cleanser | 4.5–6.0 | Minimize acid mantle disruption; near skin pH |
| Body wash / shampoo | 5.0–6.5 | Hair cuticle stays closed below 6.5; scalp is similar to facial skin |
| Leave-on facial serum | 4.5–6.0 | Preserve acid mantle; maintain active ingredient stability |
| Body lotion / cream | 5.0–6.5 | Skin compatible; emulsifier stability |
| Eye-area product | 6.0–8.0 | Tear duct fluid is pH 7.4; near-neutral reduces irritation |
| Vitamin C (L-ascorbic acid) serum | 2.5–3.5 | Required for efficacy; intentionally acidic |
| AHA / BHA exfoliant | 3.0–4.0 | Acids must be in un-ionized form to penetrate; requires low pH |
| Conditioner (leave-on or rinse-off) | 4.0–5.5 | Cationic conditioners deposit better on slightly acidic hair |
| Antiperspirant / deodorant | 3.5–5.0 | Aluminum actives require acidic environment |
| Cold process soap | 8–10 | Inherently alkaline; not pH-adjustable in the conventional sense |
How to measure pH correctly
- Use a calibrated pH meter, not pH strips. Strips are inaccurate in surfactant-containing products and are not reliable enough for cosmetic formulation.
- Calibrate before each use with pH buffer solutions (4.0 and 7.0 buffers are standard). A meter that has not been calibrated gives unreliable readings.
- Measure pH at room temperature — pH readings change with temperature. Allow the formula to cool to ~25°C (77°F) before measuring.
- Rinse the electrode with distilled water between measurements; dry gently with a lint-free cloth.
- Store the pH meter electrode in storage solution (not distilled water) when not in use.
Surfactants confuse pH strips
pH strips in a surfactant solution read incorrectly due to the electrostatic charge from surfactant molecules affecting the dye. Even high-quality strips cannot be trusted in body wash, shampoo, or any foam-producing product. Use a meter only.
How to adjust pH
| To adjust | Use | How to use |
|---|---|---|
| Lower pH (make more acidic) | Citric acid solution (10–20%) | Add dropwise; mix; measure; repeat. Prepare a 10% solution in distilled water. |
| Lower pH (make more acidic) | Lactic acid (80% or 90%) | Dilute before adding; more gradual than citric; commonly used in skincare |
| Lower pH (make more acidic) | Glycolic acid solution | For AHA formulas; adds exfoliant activity in addition to pH adjustment |
| Raise pH (make more alkaline) | Sodium hydroxide solution (10%) | Extremely strong — add very slowly; safety precautions apply |
| Raise pH (make more alkaline) | Triethanolamine (TEA) | Milder; used in carbomer gels; some sensitization concern at high levels |
| Raise pH (make more alkaline) | Sodium bicarbonate solution | Mild; limited adjustment range; good for delicate formulas |
Always dilute before adding
Never add concentrated acids or bases directly to a formula. Prepare 10–20% solutions in distilled water first, then add drop by drop. This gives you control and prevents overshooting your target.
Pro Tip
Adjust pH during the cool-down phase, below 40°C (104°F), after all other ingredients have been added. Adding fragrance, preservative, or actives can shift pH — always do a final check after everything is incorporated.
How pH affects key ingredients
| Ingredient | pH sensitivity | Notes |
|---|---|---|
| L-ascorbic acid (vitamin C) | Degrades rapidly above pH 4 | Keep at 2.5–3.5 for efficacy |
| Niacinamide | Converts to nicotinic acid above pH 7 or below pH 4 | Keep at pH 5–7 |
| Retinol | Degrades in acidic and alkaline conditions | Ideal pH 5–7; avoid with AHAs |
| AHAs (glycolic, lactic) | Only active in un-ionized form below pH 4 | Above pH 4.5, most exfoliant activity is lost |
| Phenoxyethanol | Most effective below pH 6.5 | Still functional at pH 8 but efficacy reduces |
| Organic acid preservatives (benzoic, sorbic) | Ineffective above pH 6.5 | Pair with pH management strategy |
| Carbomers / Carbopol | Thickening activated between pH 5–10 by neutralization | Must neutralize to activate; triethanolamine or NaOH |
| Olivem 1000 emulsifier | Loses stability below pH 5.5 | Avoid highly acidic formulas with this emulsifier |