Potassium hydroxide is a highly alkaline ingredient that is used in small amounts in cosmetics and personal care products to establish and hold the pH of a product.
Potassium hydroxide, also known as lye and caustic potash, is an inorganic compound with the formula KOH. It can be found in pure form by reacting sodium hydroxide with impure potassium. Potassium hydroxide is often used interchangeably with sodium hydroxide for numerous applications, for instance, in the manufacture of pulp and paper, textiles, soaps and detergents, and as a drain cleaner. However, sodium hydroxide is preferred because of its lower cost. Potassium hydroxide is soluble in water and is hygroscopic.
Potassium hydroxide is noteworthy as the precursor to most soft and liquid soaps, but can also be used in the formulation of bath products, cleansing products, fragrances, foot powders, hair dyes and colors, makeup, nail products, personal cleanliness products, shampoos, shaving products, depilatories, skin care products, and suntan products.
The main function of potassium hydroxide in cosmetics and personal care products is to establish and hold the pH of a product. In chemistry, pH stands for ‘potential hydrogen’ and it refers to the level of acidity or alkalinity in a given solution. The pH scale ranges from 0 to 14. A pH of 7 is neutral. A pH less than 7 is acidic. A pH greater than 7 is basic.
The skin’s normal pH is slightly acidic (typically between 4 and 6). This acidity of the skin is termed the “acid mantle” and is maintained by sebaceous glands, sweat glands, and normal skin flora. The acid mantle provides a film of amino/lactic acids and oils that effectively protect skin from environmental factors (bacteria, pollutants) that contribute to premature aging and irritation.
It is important to regulate the pH of cosmetics and personal care products so as not to shift the skin’s normal pH too far from normal. On one hand, if a product is too acidic it may irritate the skin or cause a stinging sensation. On the other hand, a product that is too alkaline is detrimental because it depletes the skin of vital, natural lipids. Additionally, a disrupted acid mantle will not allow for products to absorb into the skin as well. This is why pH adjusters like potassium hydroxide are used in cosmetic formulations.
In higher concentrations, potassium hydroxide can aggravate skin, even if used in rinse-off products. In fact, a study on skin products found that using an alkaline cleanser, even once, can do significant damage to the skin. Alkaline cleansers alter the skin’s natural bacterial flora composition and the activity of the enzymes in the upper layers of skin since these both have an optimal pH level. Furthermore, the damage is cumulative: the longer you use the product, the more damage it does to your skin.
The United States Food and Drug Administration (FDA) has approved potassium hydroxide as GRAS (Generally Recognized as Safe) as a direct food additive.
Concentrated potassium hydroxide is a strong irritant and corrosive to the skin, eyes, respiratory tract, and gastrointestinal system if ingested. The severity of effects caused by potassium hydroxide is a function of the concentration, the pH, the length of tissue contact time, and local conditions and skin type.
Potassium hydroxide is approved for use in cosmetics and personal care products in varying concentrations: 5% by weight in nail cuticle solvents, 2% by weight in hair straighteners for general use, 4.5% by weight in hair straighteners for professional use, up to a pH 12.7 in depilatories, and up to pH 11 in other uses as a pH adjuster.
References: Wikipedia “Potassium Hydroxide”, Chemists Corner “The Importance of pH in Cosmetic Formulation”, SebaMed “Too Acidic, Too Alkaline: The Different Dangers” 2016, Cosmetics Info “Potassium Hydroxide”