September 6, 2022
To understand the importance of the pH of a cosmetic product we have to know what it represents at a chemical level and what role it plays in our skin. Once we know the importance of this parameter we have to know how we can measure and adjust it.
pH stands for hydrogen ion potential. In 1909 it was defined by Søren Sørense as the result of the negative logarithm in base 10 of the hydrogen ion activity. Through approximations, we can speak of the proton activity as its molar concentration, [H^+]. Later this formula was redefined with the molar concentration of hydronium ions, [H_(3 ) O^+]. The pH formula is represented as:
pH = -log[H+ ] = -log[H3O+ ] = -log[H3O+ ]
In other words, pH is understood as the degree of acidity or basicity of an aqueous solution. It is measured on a scale from 0 to 14, where 0 is the most acidic pH value, 7 is the neutral pH value and 14 is the most basic pH value. For very acidic substances we can be at pH below 0 and in the case of very strong bases we can exceed pH 14, but this is unusual.
Substances with pH below 7 are called acidic, and those with pH above 7 are called bases (or alkaline), as we can see in the illustration. For example, sodium bicarbonate has a high pH in aqueous solution, it behaves like a base. Vinegar, on the other hand, has a low pH in aqueous solution, it behaves like an acid. We also have neutral substances, with an intermediate pH, as is the case of pure water.
pH scale from 0 to 14, with examples of substances.
Note. Adapted from General Chemistry (p. 705), by Petrucci, R.H.; Harwood, W.S.; Herring, F.G. 2011. Pearson. Madrid, 2011.
Why don't we talk about pH in fatty substances?
The explanation is simple: the key lies in the amphiprotic capacity of water and its polarity. Amphiprotic capacity is defined as the ability to accept or give protons. In other words, a water molecule can act as both a base and an acid, carrying out acid-base reactions, hydrolysis or dissociation of other molecules. Fatty substances, on the other hand, are generally non-polar and do not have this amphiprotic capacity, so they are not involved in such reactions. Therefore, it makes no sense to measure the pH of a solution of oils, since it is the water molecules that cause pH changes.
If there is no water, there is no pH.
If my cosmetic product is a solid, how can I measure its pH? If it contains water-soluble compounds in its composition, it makes sense to measure the pH. To do this we must dissolve the solid in water and measure the pH of the solution.
The skin is the main organ in charge of protecting our body. It defends us against external factors: solar aggressions, climate, pollution and infections. The epidermis is the outermost layer and to protect us against these aggressions it has the hydrolipidic film. This film, also called hydrolipidic film or epicutaneous emulsion, is composed of an aqueous part, known as protective acid mantle (pH= 4-7), and a lipidic part.
In order for the skin to do its job properly, it must be in a good condition and this is the importance of pH and its relevance in cosmetics. All the products that we apply on our body must have an optimal pH.
As we will see below, depending on the area to be applied, we must consider its optimum pH of care.
We should know that due to skin type (normal, dry, combination and oily), skin conditions (sensitivity, dryness, redness, imperfections), skin aging, ethnicity and sex of the person or exogenous factors (diet, stress, medication) the pH may vary.
The stability and efficacy of some of the ingredients of our compositions and their safety are also evaluated.
Sudden changes in pH can upset the balance of the skin and this can lead to adverse reactions. When the pH of the acid mantle is altered, the barrier function is altered as its composition changes and so does the microbiome. The skin is weakened, its properties are negatively modified and it no longer fulfills its functions in the correct way.
Depending on the pH deviation we have two possibilities:
As we have already mentioned, pH is determined by the amount of H+ and H3O+ ions we have in solution. We have two ways of measuring it that differ in the precision that they provide us:
Each part of the body requires different pH formulas:
Once the pH has been determined, perhaps it does not meet the optimum pH for the product, then...
Substances that modify the pH are acids or bases. According to the Brønsted-Lowry theory, an acid is defined as a proton giver and a base as a proton acceptor. To be understood, an acid, such as citric acid, partially dissociates in water, contributing protons to the medium and thus lowering the pH of the solution. On the other hand, a base, such as arginine, when partially dissociated will accept free protons from the medium, lowering its concentration and increasing the pH of the solution.
Knowing now the theory of acids and bases and understanding how pH works and its importance, we can think about the pH adjustment of a cosmetic.
Before adjusting the pH we must determine the pH of our product correctly and always under agitation, ensuring homogeneity. This step is important because we need to know the initial pH to be able to adjust with a suitable base or acid, if necessary.
With the pH value of our cosmetic product and knowing the pH we need, we have two possibilities:
Each formulator chooses the acid or base to adjust the pH according to different parameters, prioritizing those substances that are already present in the formulation.
To adjust the pH it is important to establish a measurement protocol so that it is always the same and measurements can be comparative.
It is advisable to carry out the measurements with the pH meter in series. During the measurement process under stirring, the acid or base should be added dropwise and slowly, waiting for the value to stabilize.
Achieving the optimum pH for each formulation guarantees a stable, safe and effective product.
Find out more about Verónica Díez Ojeda, R&D Formulation Technician at MuttuLab, author of this article.
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