Should cosmetics be diluted to test pH?
Belinda Carli, 17/09/2021
The short answer, regardless of what you read elsewhere, is no. This article will explain, in detail, why.
Do not dilute your cosmetics to test pH, unless it fits one of the few exceptions we will also provide you with below, along with the reason why. In every other case, do not dilute your cosmetics to test pH.
For a video explanation, please watch this video which explains why you should not dilute your cosmetics to test pH.
1. You need to know the exact pH of your product
The most important reason that you should not dilute your product to test pH, is that your customer is almost always not diluting the product before applying it - even shampoos or body wash are not diluted prior to application. They are washed off, but they are not diluted before application. This means you need to know what the pH of your product is to ensure it suits your consumer, not in a diluted form.
Knowing the exact pH of your product is also critical for other reasons, like:
- monitoring pH change over time – you cannot be sure of the pH or how it changes if you dilute it;
- compatibility with the preservative – some preservatives have very specific pH ranges over which they are effective, which you must make sure your product maintains, in order for it to be adequately preserved and safe for consumer use;
- compatibility with the active (or other functional ingredients, like gums, emulsifiers or surfactants) – some ingredients have very specific pH ranges to be biocompatible or bioavailable. In other words, if they are outside of their required pH range, they can become unstable or just not work.
2. Diluting the product CAN alter pH
There are some sources that suggest diluting your product to 10% will not alter the pH. This is not correct. It can. Sometimes it can alter it a little, sometimes it can alter it a lot. It is not predictable, and it can change over time, even for the same product, as the product ages.
In other words, you cannot ever be sure if diluting the product is having a big or small impact on the accuracy of your reading. As scientists, we must design experiments to be accurate and replicable – and diluting the pH does not conform to either of these requirements. Since it doesn’t give you a replicable or accurate reading, why do it? See also point 1 above.
3. Testing the pH is still easy, and possible, even in thick creams
Some articles suggest that it is hard to test pH in really thick creams. This is also incorrect. Let’s break this into three parts, as that is important in explaining why.
- A thick cream that has a water continuous phase, as in the case of oil-in-water (o/w) emulsions, will always measure pH easily, and readily. Even if its so thick it can’t move on its own, a pH probe can still easily measure the ion flow. The cream does not need to flow for the ions to flow – that is electrical conductivity, and happens readily where water is the continuous phase, even if the product is so thick it won’t move on its own. If you are using strips, scoop some product out the way, put the strip in, put the product back on top, and drag the strip out. Wipe it gently on the side of the jar to remove excess product, and you will still get an accurate result. With probes, sweep it through the product. It will give you an accurate reading easily.
- A thick cream that is actually water-in-oil (w/o) has an oil continuous phase. These are always hard to check pH and adjust it, even when they have a low viscosity, because water is trapped inside the micelles of the emulsion. This means the water is in discrete droplets, and not able to carry charge, like it does when it is an o/w emulsion. This difficulty in reading and adjusting pH has nothing to do with viscosity, it has everything to do with the water being trapped inside droplets of the dispersed phase. When working with w/o emulsions, check and adjust the pH of the water phase before you form the emulsion, as it is very difficult to test and adjust it accurately once the water is trapped inside. This also applies to ointments or any other products with a very small water input.
- Anhydrous products - those without any water, like balms - don’t have a pH. You need water to be present, and carry charge, for pH to be measured. This is why you need to treat o/w and w/o emulsions differently (refer to the explanations above). A semi-solid product that is without water does not need to be tested for pH, as it cannot carry charge, and therefore pH is irrelevant.
4. What are the exceptions?
A bar soap or bar product that gets mixed with water before use would be an exception to this rule. The reason for this is that you cannot test the pH in a product that is solid, and in these cases, the product is diluted before you use it. You cannot use a bar of soap without mixing it with water first, so a 10% dilution in this case would give you a good representation of the pH a consumer is likely to encounter during use. In this case, diluting the product to test pH is practical and acceptable.
What about a solid stick deodorant I hear you ask? A solid stick deodorant is not mixed with water before use, and does not contain water, so pH is not relevant.
Solid hair pomades? Also not mixed with water before use, and also typically without water or such a low input of water that the direction for anhydrous or w/o emulsions would apply.
What about powders? Again, is the product to be mixed with water before use? A mineral foundation is not to be mixed with water, so pH is not useful or relevant. A clay mask that comes dry to be mixed with water before application should be prepared the way it will be used and tested for pH in that way, to ensure consumer suitability.
We hope this has cleared up this very important discussion.
Just remember: why are you testing pH? Because you need to know the pH of water continuous phase products for very important reasons of safety, stability and performance. Diluting the product is not going to give you the pH of that product with 100% certainty, so why do it? The answer is simple: don’t.
If you are new to formulating, please enjoy our videos on:
- How to adjust pH
- Should cosmetics be diluted to test pH?
- How to prepare pH buffer solutions (toward the end of this video)
Happy formulating!
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