Potassium sorbate is the potassium salt of sorbic acid, with chemical formula CH3CH=CH−CH=CH−CO2K. It is a white salt that is very soluble in water (58.2% at 20 °C) where it converts into sorbic acid, its active form.
Potassium sorbate is not a broad spectrum preservative for cosmetic use and should be combined with other preservatives. If potassium sorbate is used as a preservative, the pH of the finished product may need to be reduced for potassium sorbate to be effective. This is because potassium sorbate is the inactive salt form of sorbic acid. To be useful, the pH of the formulation must be low enough to release the free acid for useful purpose .
While sorbic acid is naturally occurring in some fruits (like the berries of the mountain ash), virtually all of the world’s production of sorbic acid, from which potassium sorbate is derived, is manufactured synthetically, and is a nature-identical compound chemically equivalent to the molecule found in nature. It is a widely used preservative in food production, but cannot be considered “natural”.
It is primarily used as a food preservative (E number 202). Potassium sorbate is effective in a variety of applications including food, wine, and personal-care products.
Sorbic acid is very pH dependent. While it shows some activity up to pH 6 (about 6%), it is most active at pH 4.4 (70%). At pH 5.0 it is 37% active. As sorbic acid, it is considered to be effective against mold, fairly effective against yeast and poorly effective against most bacteria. Sorbic acid is an unsaturated fatty acid and as such is subject to oxidation. It is also sensitive to UV light and may turn yellow in solution.
A chart of the relative activity of Sorbic Acid at various pH:
pH % Activity of Sorbic Acid
In its pure form, potassium sorbate is a skin, eye, and respiratory irritant.Concentrations up to 0.5% are not significant skin irritants.