*2.1.4.3 pH adjusters*

In addition to the chemical stability of the ingredients and the formulation itself, pH is a crucial consideration for topical moisturisers. Not only is the absolute pH value important, but the buffer capacity is also crucial to the skin's natural acid mantle. The buffer capacity describes the ability of a formulation to keep the pH value almost constant or as close to the skin's natural pH as possible [65, 66]. This can be achieved by adding pH adjusters to the formulation [66]. The natural pH of the skin surface of most parts of the body is slightly acidic and in the range of pH 4.1–5.8 [66], a feature that can have significant impacts on how the skin reacts to the product. It is a generally accepted fact that the use of alkaline or non pH-balanced products such as soaps, cleansers and creams will lead to skin barrier impairment with a concomitant pH increase in both healthy and diseased skin. The duration of this increase in skin pH depends on skin condition, frequency of application and the composition of the product. Therefore, every skincare product is a potential skin surface pH modifier

and the pH of such products must be adjusted to a physiological pH during its development [67]. Some of the most commonly used pH adjusters for topical moisturisers include aminomethyl propanol and citric acid. Aminomethyl propanol is a synthetically produced pH adjuster that is classed as an aliphatic alcohol. It is commonly used in topical formulations due to its safety profile when used in low concentrations [68, 69]. Citric acid is a weak alpha hydroxy acid (AHA) that is naturally occurring in plants and animals. The majority of citric acid comes from citrus fruits, like oranges, lemons, grapefruit and limes. When used and applied in small amounts, it serves as an effective pH adjuster [70].
