pH in Cosmetics
Why it matters, what range each product needs, and how to test and adjust it
pH is one of the most important variables in cosmetic formulation, and one of the most commonly skipped by beginners. It affects whether your preservative works, whether your active ingredients do anything, how your emulsion behaves, and how the product feels on skin. Understanding it doesn't require a chemistry background.
What pH Means
pH measures how acidic or alkaline a water-containing solution is. The scale runs from 0 (strongly acidic) to 14 (strongly alkaline), with 7 being neutral. Each number represents a 10-fold change — pH 4 is ten times more acidic than pH 5, and 100 times more acidic than pH 6.
| Reference | pH |
|---|---|
| Lemon juice | ~2 |
| Apple cider vinegar | ~3 |
| Healthy skin surface | 4.5–5.5 |
| Distilled water | 7.0 |
| Baking soda solution | ~8.3 |
| Cold process bar soap | 8–10 |
| Bleach | ~12 |
Skin's Natural pH and Why It Matters
Healthy skin maintains a slightly acidic surface — the acid mantle — at approximately pH 4.5–5.5. This acidity:
- Supports beneficial skin microbiome bacteria
- Keeps the skin barrier intact (ceramides and lipids structure)
- Inhibits growth of pathogens like Staphylococcus aureus
Products close to skin's natural pH (4.5–6.5) cause minimal disruption. Highly alkaline products (bar soap at pH 9–10) temporarily raise skin pH and can compromise the acid mantle, leading to tightness or dryness.
pH Targets by Product Type
| Product | Target pH | Notes |
|---|---|---|
| Body lotion / cream | 5.0–6.5 | Closest to skin pH |
| Face moisturizer | 5.0–6.5 | Range for most actives |
| AHA / BHA exfoliant | 3.0–4.5 | Must be acidic to work |
| Vitamin C serum | 2.5–3.5 | Acidic for stability |
| Shampoo | 4.5–6.0 | Supports cuticle closure |
| Body wash / hand wash | 5.0–7.0 | Surfactant-based |
| Bar soap (CP) | 8–10 | Alkaline; not adjustable |
| Deodorant | 4.5–6.0 | Matches underarm pH |
pH and Preservative Efficacy
Most synthetic preservatives require a certain pH range to function. Outside this range, they become partially or completely inactive.
| Preservative | Effective pH range |
|---|---|
| Phenoxyethanol | 3–8 |
| Sodium benzoate | 2.5–6.5 |
| Potassium sorbate | 3–6.5 |
| Germall Plus | 3–9 |
| Optiphen | 3–8 |
| Geogard ECT | 4–6 |
Always test pH of your finished formula after adding preservative and adjusting. Re-test at 2 and 4 weeks to check for pH drift.
How to Test pH at Home
Oils do not have a pH — despite what many charts online claim
pH measures hydrogen ion concentration in a water-based solution. A pure oil contains no water and cannot establish a pH. Placing a pH electrode directly in jojoba oil, argan oil, or any other carrier oil produces an electrode artifact — a number the meter generates from the slight moisture on the probe, not a meaningful reading. Any chart listing "jojoba pH 5" or "rosehip pH 6.5" is not based on valid measurements. Never test or attempt to adjust the pH of an anhydrous product or a pure oil.
pH strips (0.5 accuracy)
Sufficient for basic screening. Test the product directly — do not dilute with water first. Distilled water has a pH of 7.0 and will pull any acidic formula toward neutral, producing a falsely elevated reading. Only dilute if the product is too viscous to make proper contact with the strip, and treat the result as approximate.
Digital pH meter (0.01 accuracy)
Much more accurate. Requires calibration with buffer solutions (pH 4 and 7) before use. Rinse probe with distilled water between tests.
Frequently Asked Questions
Tip: Track pH in your formula
LotionMath shows the recommended pH range for your preservative and warns if your formula falls outside a safe range.