Svante Arrhenius is credited with first elucidating the nature of acids and bases (he also gave us the Arrhenius Equation which factors into the use of increased temperature in formulation stability studies). His definition was a bit narrow and was later expanded on by two chemists for whom the current definition of acids and bases is named, Johannes Brønsted and James Lowry. By the Brønsted-Lowry definition, acids are proton donors and bases are proton acceptors.
The pH scale is a compact way to represent the acidity of a solution. The pH of a solution is the negative log of the hydronium ion (H3O+scale based on 10, the pH changes by 1 for every power of 10 change in hydronium ion concentration. A solution of pH 3 has 10 times the concentration a solution of pH 4 and 100 times that of a solution of pH 5. And because it is a negative log scale the pH decreases as hydronium ion concentration increases. The lower the pH of a solution, the more acidic it is. A pH of 7 is considered neutral and higher pHs are more basic.
One important thing to keep in mind is that measurement of pH is limited to aqueous solutions. Measurement of pH in alternate systems would require the development of new pH scales. Because of this, certain cosmetic formulations like water in oil creams do not have a pH but the pH of the water phase can be measured prior to emulsification or upon breaking the emulsion.
Why is pH Important for formulating Cosmetics?
The normal skin surface pH is slightly acidic and ranges between values from around pH 4 to pH 6. A study by Lambers et al. reported that the skin surface is close to an initial pH of 4.7, an average calculated from 330 subjects, showed virtually no change when the application of water or cosmetics was restricted for 24 hours. The skin surface pH can also vary by body site and can be changed significantly with the application of soaps, cosmetics or even rinsing with alkaline water.
Some of the most important anti-aging ingredients used in cosmetics treatments are alpha hydroxy acids. While the percentage of alpha hydroxy acid is important, the pH of the final product has a big impact on efficacy. Bringing a formulation to a more neutral pH decreases efficacy. However it can also be useful when formulating for sensitive skin types.
pH is also important to variety of other cosmetic ingredients including polymeric thickeners, dyes, and certain preservatives. So be sure to understand the pH limitations of individual ingredients and the overall formulation when developing cosmetic products.