Is hyaluronic acid acidic for the skin?

When applied topically, hyaluronic acid is safe and particularly well tolerated by all skin types. However, it is still advised against using a product containing it on areas where the skin has inflammations and infections.

Hyaluronic acid is present in the skin in the form of a salt, hyaluronate. Why? Because the pH of the skin is situated between 4.7 and 5.75, but the pKa (acid constant) of hyaluronic acid is between 3 and 4, depending on the molecular weight of the polymer. Now, if the environment in which the molecule is found is higher than its pKa, it becomes ionised and the oxygens of the hyaluronic acid lose their hydrogens to become O-. Thus, hyaluronic acid transforms into the base hyaluronate upon penetrating the skin, and therefore presents no risk of irritation, even though it is an acid.

Furthermore, some treatments already contain the basic form of hyaluronic acid, the ingredient sodium hyaluronate. Several benefits are associated with this use: its molecular weight is lower, allowing it to penetrate more deeply, and its basic pH enables it to blend better with other ingredients, making the task easier for formulators.