Since writing this post on the influence of pH on exfoliating acid skincare ingredients, I’ve had a lot of questions on how much active free acid there is in specific formulations. The calculation is a bit fiddly to do every time, so I’ve developed an easy spreadsheet to help you predict the free acid content for a product, if you know the amount of the acid ingredient in it, and its pH. This might also be useful if you’re into making your own acid products, or messing around with the strengths of pre-made products.
Refresher on Acids and pH
pH is a measure of how acidic or basic something is – a lower pH is more acidic, which means there’s a higher concentration of H+ ions.
AHAs and BHAs (alpha and beta hydroxy acids) are common exfoliating ingredients. They’re acids, which means they can exist in two forms:
- as the free acid, which means it has no charge
- as the ionised or dissociated form, which means it’s lost a H+ and it has a negative charge (this is also called its conjugate base)
The two forms can interconvert. Here’s what they look like for glycolic acid:
The ionised form has a difficult time getting through skin’s oily lipid layer and into the skin where it can act. The free acid form is a lot more oil-soluble, so it penetrates a lot more easily. (In some circumstances, if the ionised acid is non-polar enough, it can still penetrate through skin.)
As pH decreases, more of the free acid form will be present, so more exfoliant will be absorbed through the skin. At high pH, more of the ionised form will be present, so it won’t get through skin as well. Hence low pH is better for exfoliation, but it also means there will be more irritation. Different acids will have different proportions of the free acid and ionised forms at different pHs, reflected in a property called pKa.
How to Use the Free Acid Calculator
You need to know:
- the concentration of the acid ingredient in your product (either in the product information, or you can ask the manufacturer)
- the pH of the final product (again, you can ask the manufacturer or test your product using pH strips or a pH meter)
- the name of the acid (straight from the ingredients list)
Enter this information into the yellow boxes on the left of the sheet (select the name of the acid from the dropdown menu).
Once you’ve entered the information, the overall percentage of free acid in the product will be calculated and show up in the green box on the right.
Here’s how it looks for a product containing 7% glycolic acid, at a pH of 2 – the calculated overall free acid content is 6.9%.
Easy! The free acid calculator has data for calculating the free acid content for the following acids:
- Glycolic
- Lactic
- Malic
- Mandelic
- Salicylic
- Citric
- TCA
- Ascorbic
If you’d like me to add more acids to the calculator, please let me know in the comments!
VERY IMPORTANT NOTE: There isn’t much point in using this to work out what a product can do for you – the free acid value isn’t going to translate into specific skincare effects! Pharmacology isn’t that straightforward. Everything else in the product – the solvent, the emulsifier – and how you use the product and how much you use is going to have a big effect. There’s also the fact that a product with a higher amount of total acid with the same free acid value as a product with a lower amount of total acid will probably have a depot effect, which will change how it’ll work on your skin. I go into this in more detail in this video.
The Mathematics Behind the Free Acid Calculator
Here are the nitty gritty details behind the calculations if you’re interested. It’s a variation of the Ka calculation that’s taught in first year university chemistry. If you see any errors, please let me know! You can see the formulae used in the spreadsheet if you click on the boxes. The greyed out area underneath has the juicy bits, and the pKa values and molar masses of each acid are shown on the right (they’re sourced from Wikipedia).
Assumptions:
- Product is primarily aqueous
- Product has a density of around 1 g mL-1
- Further dissociations are negligible
P.S. If this is all a bit too advanced, you can get the lowdown on all the basics of exfoliation in my free downloadable guide – find it here!
You’re so resourceful! Thank you so much for sharing, and giving me more motivation to keep learning about cosmetic chemistry!
Thank you! I’m glad you like it 🙂
This is awesome thanks!!
Wow! This is awesome. Now we will be aware of how many ph level a product has. Thanks for bringing it up!
Jess recently posted…Where Can I Find LifeCell Eye Cream?
Thank you so much for this!
You are amazng!!
Thank you! *blushes*
I agree. You are amazing.
I am not amazing or smart. Could you please answer a dopey question for me. I have a cream that has ammonium lactate. That means it’s already buffered, right? Will it break down into free acid? I think it’s 12%, with pH between 4.5and 5.5 (that’s what the PI says).
Would it be better to just get a Ponds formulation that has just plain lactic acid listed as an ingredient?
Ammonium lactate is made up of ammonium and lactate ions, so you can treat it a bit like lactic acid – you have to do a small conversion though! 12% ammonium lactate translates to 10% lactic acid, then you can plug it into the calculator with the pH.
Thank you SO, SO much. How can afford to be SO generous with your time?!
Again, thank you.
xoxo
Very helpful post, but what about betaine salicylate? Thank you!
It’s pretty much the same as salicylic acid, but at a lower concentration (assuming it dissociates from the betaine in solution). It’s roughly half, so for example if you have 4% betaine salicylate you should be able to use the calculator for salicylic acid at 2%.
Hello, could you explain in depth how your calculator works?
As in how to use it, or what the calculations behind it are?
Which is the pH of salicylic acid 2% ?
It can be anything… the pH can be adjusted (and is almost always adjusted) before the formulation is finished.
The free acid calculator is read only. Is it possible to get access to the calculator? I am trying to determine a concentration and pH of salicylic acid that will exfoliate well and still be tolerable.
I was using lactic acid in combination but it seems it may be too strong in free acid content at a pH of 3.3-3.5
Thanks for any input
Try making a copy and saving it to your drive – in the “File” menu, click on “Make a Copy”.
Can you add values for lactobionic acid and gluconolactone?
Thank you 🙂
If I bought a high-percentage salicylic acid product with a pH of 6, and simply add enough citric acid to lower the pH to around 3.5 or 4, will I still get the benefits of the salicylic acid? Or does salicylic acid always have to be kept at the appropriately low pH at all stages of manufacturing for it to be effective?
Salicylic acid seems to work at reasonably high pHs, but if you’d like to make sure, I’d recommend that you adjust the pH right before you use it to avoid destabilising the formula. It doesn’t need to be kept at low pH.
This is awesome. Thanks so much Michelle. May I ask, does this work with PHAs? I would be interested to know if it’s possible to calculate how much free acid is present in a formulation of 5% gluconic acid at pH 4.5.
It should work, but I think PHAs don’t need to be in free acid form to work…