pH – it’s a safety thing.
If you make skin care products that contain water you need a pH meter and you need to use it. You simply can’t assure your products safety without it.
pH is a measure of hydrogen ions and hydrogen ions can tell us if a product is acidic, neutral, basic or somewhere in between – you don’t need to really know what, how or where or when but it might help you appreciate the situation a little more if I explain the WHY.
Why test pH?
With more and more people formulating with natural derived ingredients, especially when it comes to preservatives you MUST keep an eye on your products pH. Many (not all but many) of the natural preservative options are based on acid technology and work best in an acidified environment where the pH is below 6 (and often below 5.5). You simply can’t feel the difference between a product with a pH of 6.5 and pH 5 without a pH meter. Those indicator papers won’t cut it either so don’t even go there. However, you will notice the difference if your product starts growing mould or falling apart because it has become micro soup!
Organic formulations are even more at risk as they are more likely to be using a higher percentage of microbe-friendly additives – plant-based actives, vegetable oils that might oxidize, herbs and more and so for these at least pH measurement and monitoring (during stability testing) is an absolute must.
A non-pH-adjusted cosmetic cream usually has a pH of somewhere in the region of 7 ish as demineralised water is around that pH. Some emulsifiers push the pH up a little and some actives (especially vitamin C) bring it down. Strangely enough ingredients like hyaluronic acid don’t often change the pH very much as most hyaluronic acid is actually sodium hyaluronate and not really an acid at all.
To adjust the pH of a cream after testing you can use Sodium Hydroxide solution (lye to soapers) to increase the pH and make the product more alkali – a few drops usually do the trick in a 500g batch so you don’t need much. To make a product more acidic (as is often the case) citric acid solution does the trick. I usually make 20% solutions as that is strong enough without being over the top. These are made by adding 20g of the sodium hydroxide flake or citric acid (mono or anhydrous) into 80g of water and stirring until a solution is made.
Adjusting the final pH of a formula is something that is done once the product is cooled down to below 40C. pH is temperature dependent but not dramatically (for the purposes of this application anyway) and as you aren’t going to get that much benefit from waiting the hours that it can take for a bulk cream to go from 40C to 20C it is OK to adjust at 40C. If you can get your product down to 40C relatively quickly without icing it (icing isn’t a great idea as it results in a lumpy product, cool water baths are better or even transferring the bulk into a plastic rather than metal container – less conductive) add the preservative after adjusting the pH and then test again before making any final adjustments. I say this because when you start off with pH adjusting you can go mental and over-shoot your mark a bit. Once you get into the hang of it you can make your own method to suit yourself and your equipment. By the way, just in case you are wondering preservatives that are pH sensitive are usually OK to be outside of their comfort zone for a little bit (say, while manufacturing) but ideally need to be brought to their happy place within hours rather than days, weeks or never.
Of course preservative stability and efficacy isn’t the only thing that is affected by pH but it is quite a major one and as such I would encourage you to get out your meter (or treat yourself to one) and start using it.
After all, we all talk about safe cosmetics but how many of us really know what that means…………