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:
If you’d like me to add more acids to the calculator, please let me know in the comments!
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).
- 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!