Measuring product pH

Measuring the pH of a product is very important in some procedures for creating natural cosmetics. Let’s find out together how to do it.

You have certainly already come across various terms related to pH when it comes to cosmetic products. Either the given product had a pH close to that of the skin, or it was presented as pH neutral. But what does pH actually mean and what are the procedures for measuring the pH of products? Why is it important at all? You will find out in the following article.

What pH is and what it tells us

Put simply, the pH value tells us how acidic or alkaline a product is. You have certainly already encountered the concept of acidity; acetic acid or citric acid are commonly used in cooking and their sour taste is easy to recognise.

The concept of alkalinity (basicity) is the opposite of acidity. Alkaline substances that you can commonly encounter include, for example, sodium bicarbonate (baking soda) or ordinary solid soap. In the middle between alkaline and acidic substances there are neutral ones, i.e. those that have a neutral pH. An example of such a substance is demineralised water (ordinary tap water contains dissolved minerals or chlorine that affect its pH).

It is best to imagine pH as a scale from 0 to 14. From a value of 0 to 7 the acidity decreases (that is, substances with pH 0 are extremely acidic and corrosive and those with a pH close to 7 are only slightly acidic). A pH of 7.0 represents a neutral pH, and from pH 7 to 14 the alkalinity increases (substances with pH 14 are extremely alkaline and corrosive).

For a simplified understanding of pH it is enough to know that this value tells us about the acidity or alkalinity of a product. However, for those who would like to know more: pH actually expresses the magnitude of the negative common logarithm (base 10) of the activity (or concentration) of H+ ions, that is, those which acids release in solution.

At pH 1 their concentration is 10^-1 mol/l, at pH 7 it is 10^-7 mol/l, which shows that acidity decreases. You do not need to deal with this when simply making cosmetics at home. If, however, you want to produce on a larger scale or you are more advanced in formulating cosmetic products, more detailed knowledge of pH will certainly come in handy.

Why is it important to measure the pH of a product?

Now that we know what pH is, you are certainly interested in what this value is used for. pH has a wide range of applications in cosmetics, not only because of marketing claims on products, but above all for safety reasons.

pH and safety risks

If you applied a product with a too high or too low pH to your skin, it would result in a chemical burn, which, depending on the area of damaged skin, is a serious injury. If you have not yet encountered a chemical burn, to some extent it can be compared to a thermal burn – the skin is intensely red and burns, in more severe chemical burns blisters appear and scars may form (of course, this depends very much on the type of chemical; a burn does not always have to manifest exactly in this way).

The physiological pH of the skin is in the range of 4.5 – 5.5. Therefore, if you have sensitive skin, it is good to target your preparations to this range. When making cosmetics at home, you should avoid products with a lower pH in order not to damage your skin unnecessarily.

There are also products that are slightly more acidic, with a pH around 4, but these are suitable only for occasional application (such as chemical peels) and are often available only in beauty salons, where they are applied by professionals. From this it also follows that pH measurement plays an important role when making homemade cosmetics.

Overly alkaline products are also not suitable for the skin. Ideally, when formulating, you should keep the pH of the product up to a value of 7.5, or up to 9 if it is solid or liquid soap made from hydroxide.

Efficacy of preservatives at different pH values

Another reason why it is useful to measure the pH of products is to choose a suitable preservative. For each preservative, you will find the manufacturer’s information about which pH range this preservative is effective in. If your product does not meet this range, the preservative will not work in it or its efficacy will be reduced, which has a significant impact on the stability of your products and their shelf life.

Since almost all natural preservatives have limited efficacy linked to the final pH of your product, if you want to formulate stable products, make sure that you know this value for your preservative and that the final pH of the product corresponds to it.

One example is the preservative Cosgard, which works in the range of pH 2 – 7, with its highest efficacy below pH 5.5. Therefore, this preservative is suitable for more acidic products, such as various vitamin serums or products with AHA acids. On the other hand, Euxyl PE 9010 is effective up to pH 12, which makes it a good choice for beginners in cosmetic making, although it is a synthetic preservative.

pH stability in the product

The pH value also affects some properties of the product, such as its colour, viscosity (in simple terms, thickness) and other physical and chemical properties, which should not change during storage of the product.

At the same time, sudden changes in pH in the product can also be a signal that significant microbial contamination has occurred. Therefore, a non-fluctuating pH value of the product during its storage is a prerequisite for good stability and safety of the finished product.

Efficacy of active ingredients in relation to pH

Some active ingredients added to cosmetics are effective only at certain pH values. For example, ascorbic acid (vitamin C) will be ineffective in products that are too alkaline, because it will be neutralised. At the same time, a pure, more concentrated aqueous solution of vitamin C has a relatively low pH that is unsuitable for the skin, so again it is appropriate to slightly increase this pH by adding a base to the physiological pH value of the skin.

Retinol (a form of vitamin A) is most effective at a pH of 5.5 to 6. Alpha hydroxy acids (AHA acids) such as lactic acid or glycolic acid, which are often used for chemical peels, are relatively strong even among organic acids. Their 10% solutions would be too acidic and would damage the skin excessively, so when making cosmetics with them it is necessary to measure the pH and, if needed, adjust it to a physiological value.

It is true that by doing so you neutralise some of the acid, which may seem uneconomical. On the other hand, if you wanted to add only a small amount of acid, just enough to reach a pH of 4.5, you would not be able to weigh such an amount on ordinary scales – it is simply too small, because these are strong acids.

Measuring the pH of a product — how to do it?

There are several methods for measuring the pH of a product, from less accurate ones to the most accurate. We will introduce some of them to you. It is important to realise, however, that pH measurement always takes place in a solution that is homogeneous (i.e. well mixed, and therefore has the same composition everywhere and there are no pieces of substances floating in it).

Therefore, if you want to measure the pH of a thicker serum or a face cream, you must prepare a 10% solution of it in demineralised water (e.g. 5 g of product and 45 g of water, or 10 g of product and 90 g of water). Make sure to mix the solution thoroughly, otherwise the pH measurement will not be accurate. Dilution in demineralised water will have only a negligible effect on the final pH.

If you want to speed up pH measurement, there are also special pH meters calibrated for use in very viscous solutions (i.e. creams, serums) without the need to dilute them.

Litmus paper for measuring pH

This is the simplest way to carry out pH measurement. Litmus paper strips are impregnated with the acid–base indicator litmus, which changes colour depending on the pH of the solution. These paper strips are sold together with a colour scale, usually from pH 1 to 12, or 14.

The procedure for measuring with a strip is simple:

1. Apply the solution to the strip

   Drop one drop of your product solution onto the strip. Alternatively, you can dip its end into the solution, although the dye from the paper may slightly colour your solution.

2. Read the value immediately

   Immediately read the value from the scale based on the colour — the colour can change quickly.

Sometimes it may happen that you cannot decide between two colours on the scale. In that case, you can use more accurate indicator papers, which have four different pads (pieces of paper) with indicator on them. Each takes on a different colour at a given pH, so you can more easily read the value from the attached scale, because you have up to 4 reference points.

Litmus paper strips therefore represent a simple and quick way to roughly check the pH of a product. This is not, however, a very accurate pH measurement. It is also important to store the strips away from moisture, light and heat.

pH meters

pH meters are electronic devices that, depending on their accuracy, allow pH measurement to as many as two decimal places. Inside, they contain an ion-selective glass electrode that is sensitive specifically to the H+ ions that affect pH.

The results obtained with pH meters are more accurate; on the other hand, they are more demanding to handle and they are more expensive. pH meters must be calibrated at least once a month by immersing them in calibration solutions with a defined pH value (these can be purchased).

The electrodes in the pH meter must be immersed in a buffer solution during storage, and after each use it is necessary to rinse them by immersing the pH meter in a container of demineralised water. The electrodes are also sensitive to scratching and damage, so never touch them or wipe them.

Depending on the type of pH meter you choose, you can measure pH even in samples without prior dilution (always follow the manufacturer’s instructions). If you want to adjust the pH of your products, pH meters are essential for precise work.

Measuring the pH of soap

Soap is a specific product that requires pH measurement, because soap that is too alkaline can easily irritate or damage the skin. For liquid soaps, the procedure for pH measurement is the same as for creams or other more viscous products.

If you measure the pH of a solid soap, either prepare a 10% solution of it in the way described above, or, for a quick pH measurement with lower accuracy, use litmus or phenolphthalein paper. Simply press the strip onto the wet surface of the soap and read the value from the colour scale.

Adjusting the pH of a product

If you have successfully measured the pH of your product but it is too high or too low for the specific product, there are ways to change it. This is the pH adjustment of products. This pH adjustment is carried out before you add an ingredient that requires a certain pH of the medium, such as a preservative.

Of course, when adjusting pH you must also take into account the overall composition of the product. Before changing the pH, make sure that you do not have other ingredients in the product that might not be effective at the new pH.

For example, if you want to use a preservative that works in an acidic environment, but you are making a facial cleansing foam containing liquid soap, which is naturally alkaline and thus also increases the overall pH of the product. In that case, you have to change the preservative, because soap will always be alkaline (if it had a pH other than alkaline, it would no longer be soap). In the case of such incompatibility of raw materials, you need to modify the formula and often replace the given raw material with another one.

Calculating pH for advanced users

In this case it is more of a method for connoisseurs, because the greater the number of components in the product that affect its acidity or alkalinity, the more complex the calculation. For simpler products such as toners with AHA acids or similar products, however, this method can be suitable for an approximate determination of pH. Using the calculation, you can know in advance approximately how much base or acid you will need to adjust the pH value.

Thanks to this, you do not have to add the pH-adjusting solution drop by drop from the very beginning, but instead you add a larger amount that is slightly lower than the calculated value, and then you add drop by drop while stirring and continuously measuring the pH. For these types of calculations, the Henderson–Hasselbalch equation and the dissociation constants of acids are most commonly used.

Lowering the pH of a product

If you want your product to be more acidic, you will lower its pH. You can lower the pH value using acid solutions. Most often, citric acid is used for this purpose, as it is inexpensive, readily available and safe.

Alternatively lactic acid, which is also safe when diluted, but in concentrated form can cause chemical burns. Its advantage is that you are already working directly with a solution that only needs to be diluted — unlike crystalline citric acid, you do not have to dissolve it in demineralised water.

1. Prepare an acid solution

   To use any of the acids mentioned, you first need to prepare a 10% solution of them in demineralised water.

2. Add it drop by drop

   You can then add the solution drop by drop to your product while stirring thoroughly. Add really small amounts and mix the product thoroughly, because sometimes even a few drops of acid can cause a significant drop in pH.

3. Measure the pH as you go

   Each time, take a small amount of the product and prepare a 10% solution of it for pH measurement using a pH meter, as mentioned above.

   Litmus paper strips are not very suitable for this purpose due to their lower accuracy. It is, however, possible to find indicator papers that, for example, work only in the range of pH 4 – 7 and therefore have a scale with smaller increments, but often even that will not be accurate enough.

Raising the pH of a product

The opposite of lowering acidity is increasing alkalinity, or in other words, raising the pH of a product using bases. Most often, a 10% solution of sodium hydroxide is used for this purpose.

1. Prepare the solution in a resistant container

   Sodium hydroxide is a solid substance sold in the form of granules. When they dissolve in water, a large amount of heat is released, so always use plastic or stainless-steel containers for preparing the solution.

   The heat generated could cause glass to crack, and sodium hydroxide is corrosive to other common kitchen metals (apart from stainless steel). An exception for glass is chemical glassware (most often borosilicate glass), from which beakers and other laboratory tools are made, which should withstand thermal stress (always check, however, that they are certified).

2. Add and measure as you go

   Add the hydroxide and then measure the pH in the same way as in the case of acidification, until you reach the desired pH.

An alternative to sodium hydroxide can be sodium hydrogen carbonate (baking soda).

Measuring pH — conclusion

With the help of this article, measuring pH does not have to be such a science at all. Choose the method that suits you best and get started. It only takes a moment and you have the result straight away.

• Safe for the skin

  A product with a too high or too low pH can cause a chemical burn — checking the pH prevents that.

• Preservation that works

  Every preservative works only within a certain pH range — outside it, its efficacy drops and the shelf life of the product suffers.

• A stable product

  Fluctuating pH during storage signals impaired stability or microbial contamination.

• Effective active ingredients

  Vitamin C, retinol and AHA acids only work at the correct pH values.

We believe that this article has helped you to better understand the whole topic of pH and its measurement. We will be happy about that.

We wish you a pleasant, creative day. Until saponification, friends.

Source of information: https://skinchakra.eu/blog/archives/497-How-to-adjust-the-pH-of-your-cosmetic-products.html

Frequently asked questions about measuring pH