Concentration, Acidity and Neutralization.(acidity in cosmetics)

In the past, aestheticians, dermatologists and other skin care professionals paid little attention to acidity and neutralization. Only cosmetic chemists dealt with acidity and neutralization in cosmetic production or development. Recently alpha hydroxyl acids (AHAs) have become a very popular technology in the cosmetic industries. The concept of concentration, acidity and neutralization are closely involved in AHA technology. Therefore, these terms are often mentioned by dermatologists, aestheticians, other skin care professionals and even some consumers.

Concentration, acidity and neutralization are technical terms in chemistry pertaining to some basic chemistry knowledge. Aestheticians, cosmetologists and even dermatologists usually do not have a clear understanding of these terms. There is substantial confusion and misinformation about these terms in the skin care market. This article will provide a simple and understandable depiction of these chemistry terms for aestheticians and skin care professionals. Acid is a substance which is able to release hydrogen ions. An acid molecule can be expressed by HA. Through ionization HA becomes hydrogen ion (H+) bearing a positive charge and a corresponding acid anion (A-) bearing negative charge. The ionization is usually achieved in a polar solvent such as water. The process is shown in Figure 1.

Acids are commonly classified as strong or weak, to indicate the approximate degree of ionization. The strong acids include hydrochloric acid, nitric acid, and sulfuric acid among others. These strong acids have 100% of the molecules ionized in a dilute aqueous solution. In contrast, weak acids have small portions of the molecules ionized in solution. For instance, acetic acid is the active ingredient in vinegar which gives it a distinct odor and sour taste. There is only 0.42% of acetic acid molecules ionized (4.2 molecules per 1000 molecules). The ionization of a strong acid and a weak acid is shown in Figure 2.

In the cosmetic industry, the concentration traditionally used is weight-percent, i.e., % (w/w).

The concentration in weight-percent is defined as: % (w/w) = Weight of an ingredient Total weight.

For instance, there are 5 grams of glycolic acid (100% pure) in 50 grams of solution. The concentration (w/w) is 550 = 10% (w/w). This is illustrated in Figure 3. Often, the …