pH of cosmetics: how to adjust and what buffer solutions are

The concept of pH is not considered something unknown now. You may have heard a lot about it from articles and videos about the effect of acidity on skin and hair, from marketing descriptions on product labels. But when creating your own cosmetics, you need to know not only the general concepts of acidity, but also what it affects, how and with what it is corrected, why it is important to maintain it. This is what we will figure out today.

What is pH and why is it so important

In simple terms, the pH level is a hydrogen indicator (that is why the pH of oils and oil products cannot be measured), which shows the concentration of positive (H+) or negative (OH-) ions in water or an aqueous solution. If positive H+ ions predominate in the solution, then the pH of the solution is acidic (and the more such ions there are, the lower the pH, i.e. the solution is more acidic). If negatively charged OH- ions predominate in the solution, then the pH of the solution is alkaline (accordingly, the more of them, the higher the pH, i.e. the alkali is stronger). The pH scale ranges from 0 to 14 and looks like this:

Why is pH so important when creating cosmetics

For cosmetics, pH is a key parameter, starting from the development phase and ending with the assessment of the formula's effect on the skin. We have already talked about the pH level of the skin and the required value of this indicator in cosmetics for each type in the article WHAT IS PH AND WHY IS IT IMPORTANT FOR THE SKIN: And today we will consider the influence of pH from the side of creating stable formulas and ways to adjust it.

To begin with, let's figure out what parameters pH affects when developing a cosmetic product:

1. Preservative efficiency. Each preservative has a working pH level, which must be strictly adhered to, because if it goes beyond this range, the preservative partially or completely loses its effectiveness and your cosmetic product will quickly deteriorate.
2. Emulsion stability. Most emulsifiers, in addition to HLB indices, working concentration, also have a pH range at which their maximum emulsifying capacity is achieved. And if you don't follow it, your emulsion will have the wrong texture or even separate.
3. Gelling ability. Most thickeners, such as various carbomers, form a stable transparent gel in a neutral or slightly alkaline environment. While, for example, guar gums, on the contrary, work better in an acidified environment. And if you don't adhere to the required acidity intervals, gelling will take a long time or will not occur at all.
4. Color stability. Sometimes we want to make the appearance of a product attractive by coloring it a certain color. But we are faced with a situation where the color is pale, too bright, or not at all the shade we tested or as the manufacturer stated. Or a perfectly colored product becomes completely colorless after a month (most often this happens with natural dyes). And all this is affected by the working pH range of the dyes, which can be violated during the manufacture of the product.
5. Deactivation of assets. Many modern assets are very picky about many conditions, including the level of acidity. Having crossed the permissible pH limits (which are usually specified in the technical documentation), the component may partially or completely lose its properties, and some may even transform into another chemical form, which not only does not help, but also harms the skin.

How to avoid mistakes when measuring

Considering the fairly wide range of parameters dependent on acidity, it is very important for us not to make mistakes when measuring this parameter, so we will analyze several basic rules and frequently asked questions.

The first thing we need to know is that pH is highly dependent on temperature. And if most modern laboratory pH meters have a temperature sensor that takes this dependence into account, then simple devices used by beginners do not have additional sensors. Therefore, it is important to always measure pH at the same temperature, the optimal range is considered to be 20-25 ° C. Also remember that the final acidity indicator of the product is affected by each component contained in it (after all, it has its own pH). And in order to get the correct result, you need to wait until all mixing and neutralization processes are complete. Therefore, you should not rush to take measurements after adding the last component, but rather wait another 10-20 minutes until all processes are completed. Following these simple rules will help to avoid situations where the measured acidity value after preparation changes significantly the next day.

Let's look at another common situation. Let's say the pH of our product is 8.0 units. But we added an active ingredient that requires a pH range of 4-6.5. Is it really possible that our component is now inactive and the entire product will have to be redone? We hasten to reassure you, nothing needs to be redone. In order to deactivate or turn an active ingredient into a harmful form, a long period of time and sometimes many additional conditions are required. Therefore, we simply adjust the acidity level to the required level and get the desired effect.

pH Adjustment

There are cases when you may need to adjust the pH. Then you need to use special components to lower or increase acidity, depending on the purpose.

Lowering the pH

If the pH of your product is too high, then acids are used to lower it. The most common pH regulators are citric or lactic acid. And beginners often have a question: in which cases should I use which acid? In fact, there are no rules on this matter. You can use the acid that is available and convenient to use. But lactic acid also works as a good active ingredient, so if you want to give the product additional beneficial properties, choose it.

Citric acid is sold in powder or crystal form. To use it as a pH regulator, prepare a solution by dissolving citric acid in water. It is best to use a 10% solution (1 g citric acid and 9 g water), but if you need to significantly lower the pH, you can use a 50% solution (5 g citric acid and 5 g water).

Lactic acid is sold as a solution, usually 80%, but sometimes more. It can also be diluted with water in a ratio of 1:1 or 1:2, or use a concentrated solution. If you are preparing small portions of cosmetics, then it is better to make less concentrated acid solutions to adjust the pH.

Increasing pH

Usually this happens less often, but there are cases when it is necessary to increase the pH level. For this, the following are most often used:

• sodium hydroxide (NaOH) or potassium hydroxide (KOH);
• Triethanolamine (TEA);
• Arginine.

The most effective and most common are alkalis (NaOH and KOH), which for some reason everyone is afraid of in cosmetics, saying that they are aggressive and harmful. But there is no need to be afraid, now we will explain why. The basis of pH adjustment is the simplest chemical reaction - neutralization. In simple words, acid ions H + bind with alkali ions OH-, forming a neutral water molecule H2O. Therefore, after neutralization, you no longer have aggressive alkali in the form in which you added it, especially considering those low concentrations.

The second most popular pH regulator is arginine. After all, this amino acid is, first of all, an excellent asset, which in addition to changing acidity, also has caring properties. Note that arginine is advisable to use when it is necessary to increase the pH by a small range, up to 1 unit. If you do, for example, peeling or toner with a large amount of acids, then it is better to choose an alkali solution to regulate acidity. Because arginine has much weaker neutralization properties and to raise the pH by 1 unit, it will have to be used much more than NaOH.

Buffer solutions in cosmetics

Separately, we can take out cosmetic components that require a clear level of acidity and quickly deteriorate if it is not maintained. There are few such components and most of them are various forms of vitamin C. In order to stabilize such assets, buffer solutions are used.

Buffer systems are solutions that maintain a constant level of acidity when adding a certain amount of acid or alkali, when diluting or concentrating the solution. Buffer systems exist in two types - a mixture of a weak acid and a salt associated with it, or a weak base and a salt associated with it. Each buffer solution has its own stable pH level and maintains it in the system with which the buffer was mixed. Let's look at some examples of buffer systems:

• acetate buffer (acetic acid + sodium acetate) - pH 3.7-5.4
• phosphate buffer (phosphoric acid + sodium phosphate) - pH 5.9-7.6
• citrate buffer (citric acid + sodium citrate) - pH 3.6 - 7.0
• ammonia buffer (ammonium hydroxide + ammonium chloride) - pH 8.4-10.3.

How to prepare a citrate buffer

The most common in cosmetics is the citrate buffer, because it is used to stabilize vitamin C. A new effective form of vitamin C has just appeared in the BEURRe store, so we suggest using it as an example to understand how to properly create and work with the buffer.

This form of vitamin C is most stable at a pH of 6.5. To prepare a citrate buffer with such acidity, it is necessary to adhere to the proportions:

• sodium citrate - 22.8%
• citric acid - 2.2%
• water - 75%
• NaOH 10% solution - to the required pH.

Preparation of the buffer: dissolve citric acid and sodium citrate crystals in water, finally bring the pH to 6.5 with a NaOH solution. Our citrate buffer is ready, it can be stored in a separate closed container for up to 1 month.

For example, we want to create 50 g of cream with ethyl ascorbic acid at a concentration of 1%. In order to properly stabilize vitamin C, it must be separately mixed with a buffer solution before introducing it into the main product. To do this, we take 0.5 g (1%) of vitamin C and dissolve it in 1.5 g of water (3%), taken from the total prescription amount. You can arbitrarily adjust the amount of water depending on the % of vitamin C introduction and the ease of dissolution. To this solution, add 0.5 g (1%) of a separately prepared citrate buffer, according to the scheme above. And then add this stabilized mixture of vitamin C with the buffer to the cooled emulsion. This sequence of work with vitamin C is maintained, regardless of the type of product you are making. That is, when making a tonic with vitamin C or a cleanser, where the main phase is water, we still dissolve it separately in a small amount of water, mix it with the buffer and only then add it to the product.

As we can see, the pH value affects many factors in the development and creation of cosmetics. However, do not worry. Study the information, follow the advice that we have provided you and create truly effective and high-quality cosmetics.

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