Techniques · Intermediate ·
Why pH matters (and how to test it at home)
The acidity of your formula decides whether actives work, preservatives protect and skin stays comfortable.
Key takeaways
- Healthy skin is slightly acidic (about pH 4.7-5.75), and most leave-on products aim for pH 4.5-6.
- Preservatives and actives each have a working pH range.
- Dilute thick products 1:9 in distilled water before testing, and always test the same way.
- Adjust slowly, one drop at a time, and re-check after 24 hours.
What pH is
pH is a scale from 0 (very acidic) to 14 (very alkaline), with 7 neutral. Healthy skin has a slightly acidic surface, around pH 4.7-5.75, which helps protect it. Products far from that range can sting, dry or irritate.
Why it matters for your formula
- Actives only work in a window. Niacinamide prefers pH 5-6 (below 5 it can turn into nicotinic acid and cause flushing); L-ascorbic acid needs pH 2.5-3.5; AHAs such as lactic or glycolic acid work around 3.5-4.5.
- Preservatives have a working range. Sodium benzoate/potassium sorbate fails above pH 5.5, and sodium dehydroacetate above 6.5.
- Thickeners and emulsifiers can lose strength outside their range.
Typical targets
| Product | Target pH |
|---|---|
| Facial toner or mist | 4.5-5.5 |
| Lotion or cream | 4.5-6 |
| Shampoo | 4.5-6 |
| Conditioner (cationic) | 4-5.5 |
| Gentle body wash | 4.5-6.5 |
| Vitamin C serum (L-ascorbic acid) | 2.5-3.5 |
How to test
pH strips
Cheap and fine to start with. Choose narrow-range strips (for example 4.5-7) rather than 1-14, because they resolve smaller differences. Strips are hard to read in coloured or opaque products.
pH meter
More accurate. Calibrate it with two buffer solutions (commonly pH 4.01 and 7.00) before each session, rinse the probe with distilled water between samples, and store it in storage solution, never dry.
The dilution method
For creams and lotions, mix about 1 part product with 9 parts distilled water and test that. The reading differs a little from the neat product, so use the same method every time and compare like with like.
Adjusting pH
- To lower: citric acid or lactic acid solution, added drop by drop.
- To raise: L-arginine, or a dilute (about 10%) sodium hydroxide solution.
- Stir well and re-measure after each addition. It is easy to overshoot. The pH adjuster calculator turns a small test into a dose for the whole batch.
- Re-check after 24 hours, because some ingredients (zinc oxide, gluconolactone) make pH drift.