What pH should a shampoo, mask or conditioner be?
Short answer
Shampoo should sit at pH 4.5–5.5. Conditioning masks and balms go lower, pH 4.0–4.5, because the acidity both seals the cuticle and lets cationic conditioners work.
Explanation
Hair is naturally slightly acidic, and a product matching it helps the cuticle lie flat. The book sets shampoo at pH 4.5–5.5 and adjusts with citric or lactic acid as one of the last steps in the formula.
Conditioning products go further down. The intensive repair mask targets pH 4.0–4.5, adjusted with a 10% citric acid solution below 35 °C. There are two reasons for the lower figure: the acidic pH helps seal the hair cuticle, and cationic conditioners such as BTMS-50 need it in order to carry the positive charge that makes them deposit on the negatively charged hair shaft. Formulate the same mask at neutral pH and the conditioner simply rinses away.
pH is adjusted after the actives and before, or alongside, the preservative — and it must be measured on the finished, cooled product, since viscosity and temperature both shift the reading.
What to do
- Adjust pH near the end of the cool-down, below 35–40 °C.
- Use a 10% citric acid solution rather than dry crystals so you can add it a drop at a time.
- Measure the pH of the finished, cooled batch, not the hot one.
Source: The Big Book of Homemade Hair Care — The Shampoo Matrix: Beyond Basic Cleansing · The Intensive Repair Hair Mask