Hello everyone! 🙂
I have received many questions about pH and therefore I have decided to make a brief post about it. In fact, to learn how to formulate and be able to make safe cosmetics at home, it is really essential that you understand what pH is and that you have a way to measure it.
Well then! Let’s dive in:
“pH” stands for “power of hydrogen”, the capital “H” is because the symbol of the element hydrogen in the periodic table is “H” (always capital! Now it should be easier to remember how it is properly written 🙂 ).
“pH is a logarithmic measure of hydrogen ion concentration in an acqueous solution” (don’t you dare stop reading!!! 😀 )
pH = -log[H+]
Try to think of it this way: by measuring the concentration of something (the hydrogen ion: H+) in an acqueous solution (water, for instance) we get to know how acidic or alkaline the solution is. 🙂
Solutions with a pH lower than 7 are “acidic”, while solutions with a pH higher than 7 are “basic” (or “alkaline”).
The range of pH is from 0 to 14 (actually it can go over this range but for the sake of our homemade cosmetics we won’t have to consider that), where pH 0 is “extremely acidic” and pH 14 is therefore “extremely basic”.
The pH of pure water at room temperature is 7 and this is what we consider “neutral” because it means that the concentration of hydrogen ions (H+) is precisely equal to the concentration of hydroxide ions (OH–) which are formed as dissociation of water (I am sure you all know the formula of water: H2O).
Notice that when other chemicals are mixed in water the pH may vary and this is exactly why on the label of bottled mineral water the pH is never precisely 7: the disparity depends on the minerals that are in the bottled water. 🙂
A few examples should help you get a sense of what is alkaline and what is acid:
The pH of lemon juice is around 2.5
Black coffee is around pH 5
A saturated solution of Sodium Bicarbonate in water has a pH of approximately 8.5
Sodium Hydroxide (which should be your best friend if you make soap) has a pH of 13.5 and this is why we must handle it with care.
Well then, now we know what the pH is, but why should we care?
- The skin point of view:
Our skin has physiologically a pH that tends to be between 4.5 and 5.5. When formulating especially leave-in cosmetics it is essential to get the pH of the cosmetic as close as possible to the pH of our skin, in order to not unbalance it.
Something that might be a surprise to some of you is that from the moment we are born our skin is covered by microorganisms that essentially “live with us” and help us fight against other external microorganisms (also pathogens). Unbalancing the pH of our skin might unbalance the life of these silent friends and leave our skin bare of its natural shield (this is actually a huge topic! Maybe I will make a post only about the skin in the future 🙂 ).
- The ingredients point of view:
All of the ingredients we use in our cosmetics have a specific range of pH where they “work the best”. You can look for this information in the MSDS (Material Safety Data Sheet) of the ingredient, which should be sent to you along with the ingredient when you purchase it (or you can request it to the reseller).
Different ingredients can have very different ideal pH ranges and this can make them incompatible.
Just to make it clear: pH incompatibility is not the only way ingredients can be incompatible, but we are talking about pH now so I will talk only about pH incompatibility.
Sometimes pH incompatibility simply means that one of the ingredients doesn’t work as well as it would if it was in its preferred pH range. This is the case of an “internet favourite”: mixing L-Ascorbic Acid – aka Vitamin C – with Sodium Hyaluronate gel: because of the low pH, the HA breaks down forming only in very low percentage low-molecular-weight HA. Nothing dangerous forms but you have just wasted most of your expensive HA.
Sometimes, however, things can get worse. There are in fact many ingredients that can become even dangerous if their pH range is not respected. It is the case, for example, of niacinamide, which is effective in a pH range of 6 to 7.5. At lower or higher pH it tends to separate and form nicotinic acid (niacin) which has a flushing effect and, depending on its concentration, can burn the skin (just as an example, on the MSDS of niacinamide you can read: Materials to avoid : acids , alkali , Oxidizing agents).
So this is why you should ideally respect the suggested pH range of every ingredient in a formulation.
- The practical point of view:
no matter how many times you have repeated the same recipe, it is always good practice to eventually check the pH of your cosmetic. You might have mistakenly added too much of one ingredient, you might have changed supplier… but even if nothing of the sort happened, just repeat with me: it is always important to double-check the pH of your cosmetic. If, once checked, it results a little out of range, you can adjust it in few easy steps: adding few drops of an acid or of a base (I have already talked about this in “how to formulate”).
Measuring the pH is pretty simple.
You can use either strips or a pH meter.
Even if a pH meter might be preferred, there are many different kinds of pH strips which can be used: I don’t suggest you to buy the simple yellow strips as they are not very precise; those like in the picture below are more accurate.
Hope this post was helpful and convinced all of you to buy your pH strips! 😀
Have a great day! 😉