Continuing on my pH/hydroxy acid kick (check out last week’s Stridex pH test here), today’s Fact-check is on the influence of pH and alpha and beta hydroxy acids. Hold onto your hats for this one – it gets a bit technical – but hopefully I can translate it to something understandable. Further questions are always welcome and I will try to answer them as best I can!
A brief recap of what these things are:
pH is a measure of a substance’s acidity or alkalinity (also called basicity) – it gives you information about the concentration of H+ floating around in something. Low pH means more acidic, high pH means more alkaline.
pH values for some common substances:
1.4 – stomach acid
2.3 – lemon
2.6 – fizzy drinks
2.9 – vinegar
4.5 – orange juice
7.0 – pure water
7.4 – blood
9.0 – many soaps
10.5 – milk of magnesia
11.5 – ammonia cleaning solution
13.5 – oven cleaner
AHAs and BHAs are alpha and beta hydroxy acids, respectively. These are chemicals which are found in many plants naturally, and have an exfoliating effect on the skin. AHAs are less oil soluble than BHAs, so they can’t penetrate clogged pores as well, but AHAs have an additional moisturising effect. More on AHAs and BHAs here.
How are pH and AHAs/BHAs related?
AHAs and BHAs are acids, which means H+ ions (the things that make acids acidic) can come off them. For glycolic acid, this looks like:
For every acid, there’s a measurement called a pKa which tells you whether the acid prefers to be one uncharged thing (on the left hand side, also known as the free acid) or a negatively charged thing and a H+ ion (on the right hand side, also known as the dissociated or ionised form), and by how much.
(Even water (H2O) falls apart to a tiny extent to give H+ – hence why the pH of water is at a medium pH 7.)
The pKas of various AHAs and BHAs are:1
Glycolic acid: 3.83
Lactic acid: 3.86
Madelic acid: 3.41
Malic acid: 3.40
Salicylic acid: 2.97
The most important factor determining how much a particular acid falls apart is the pH of the solution it’s in. It just so happens that when the pH is equal to the pKa, it’s at 50/50 – half the acid is in free acid form, and half is dissociated.
So if you have a tube of 8% glycolic acid at pH 3.83, for example, you have a tube that has 4% glycolic acid as a free acid, and 4% as the charged, dissociated H+ and glycolate.
How does this make a difference on my face?
The fact that AHAs and BHAs behave differently at different pHs is very important in skincare because the stratum corneum is oily. If you’ve been following this blog, you’d have heard me say many times that oily things like dissolving uncharged, oily things, and don’t like dissolving charged, polar, watery things. To pass through the oily stratum corneum and into the skin to do their work, the hydroxy acids need to be in their uncharged, free acid form…which we can control by playing around with pH.2
If you’re mathematically minded, or you’ve suffered through a year of university chemistry, you’ll be able to calculate exactly what percentage of each AHA or BHA is in free acid form at a given pH. I plugged in a bunch of values into a spreadsheet to show the effect of pH on % free acid for the three most popular AHAs/BHAs, glycolic, lactic and salicylic acid:
(These are approximate – the %s will vary with concentration as well. I actually showed one of my high school students this spreadsheet in response to one of his questions… he rightfully made fun of me for spending my free time making pKa spreadsheets. The pain of being an unrepentant nerd.)
Now, if you multiply the percentage of free acid by the concentration of AHA or BHA present in the bottle, you can work out how much is ready to pass through your stratum corneum. For instance, at pH 2.5, a 8% glycolic acid treatment will have 7.64% glycolic acid in its active form (multiply . To get the same amount of active glycolic acid in a glycolic acid treatment at pH 4.5, you would need a product with 43% glycolic acid!3
As you can see, the lower the pH, the more free acid we have. So why not make all products at pH 0? Because it would burn our faces off. And low pHs all burn our faces off to varying degrees – depending on how hardy your skin is, different people will be able to tolerate different pHs, and you’ll know from the stinging, redness, itchiness, dryness and peeling whether your skin can handle it.
For a more exact calculation, you can use this free acid calculator.
But wait… there’s more!
OK, so you’ve applied your pH 3.83 8% glycolic acid to your face, and the 4% that’s uncharged has all absorbed (theoretically that is, nothing ever all absorbs!) – what happens to the other 4% sitting on top of your skin? Does that just stay charged and shut out forever?
The answer is no. The acid likes to be a certain percent free acid, so now that the free acid’s gone, that essentially leaves space for some of the dissociated ions to become uncharged free acid. This can then go through your skin. So theoretically, at any pH, ALL of the HA could turn into the uncharged free acid and will be absorbed.
This is why some HA products at higher pHs (sometimes labelled as “buffered products”) are said to be time-release – instead of being able to absorb all the free acid straight away, some of the HA sits on top of the skin and slowly turns into absorbable free acid over time. This has the advantage of decreased irritation – both from too much HA, and from the low pH.
Even though theoretically all of the HA will eventually be absorbed, the HA that’s in your skin is also being removed and broken down in your body. If the pH is too high, the HA will be absorbed too slowly, and will end up too dilute in your skin to have an effect (if you pour a bottle of Coke into a swimming pool and drink all the water, you won’t taste the Coke even though you’ve drunken a whole bottle of it).
For an idea of how much pH affects absorption, one study (PDF link) compares the absorption of 5% glycolic acid and 5% lactic acid, and pHs of 3 and 7. At pH 3, total absorption of a 5% glycolic acid cream after 24 hours was 27% as opposed to 3.5 % at pH 7; with 5% lactic acid, absorption dropped from 30% to 10% when pH changed from 3 to 7. However, another study found that when the pH of a 10% lactic acid exfoliant was increased above pH 4, there were negligible effects on skin turnover.
– pH changes the amount of HA in your product that will be absorbed into your skin.
– Lower pHs increase the amount of HA that can be absorbed, but also come with increased irritation and without time-release properties.
– A product that has a pH higher than the recommended range will still work, just not as well (and possibly so poorly that it won’t have any effect). A product with a higher concentration (%) of AHA or BHA can get away with a slightly higher pH and still be effective.
To achieve a happy medium, the rules of thumb for choosing an AHA or BHA exfoliant are:
– For AHAs, you should look for a product with a pH of less than 4, and a concentration of 4-10%
– For BHAs, you should look for a product with a pH of less than 3.5, and a concentration of 1-2%
1. Some of these chemicals have more than one H+ which can come off, so if you Google their pKas you’ll find more than one value – the values I’ve quoted here are for the hydroxy acid H+.
2. Some charged species can pass through skin via special mechanisms, but this doesn’t really happen for AHAs and BHAs.
3. You can compare different products containing the same HA, but you can’t compare products containing different HAs – each HA has a different potency, so you might need less of a HA to do the same job as another HA.
N Leveque, S Makki, J. Hadgraft and P. Humbert, Comparison of Franz cells and microdialysis for assessing salicylic acid penetration through human skin, Int J Pharm 2004, 269, 323.
SE Wolverton, α-Hydroxy acids. In Comprehensive Dermatologic Drug Therapy, 3rd ed., Elsevier, 2012, 570.
MK Kraeling and RL Bronaugh, In vitro percutaneous absorption of alpha hydroxy acids in human skin, J Soc Cosmet Chem 1997, 48, 187.