Formulating with Acids: pKa and pH
maria
Member
in Formulating
This is from the DECIEM website:
"pKa is the most important aspect to consider in formulating with acids. pKa implies acid availability. When pKa is close to pH, there is an ideal balance between salt and acidity, maximizing effectiveness of the acid and reducing irritation. Higher pH numbers in such a case would increase salt which counter-intuitively would make the formula even more irritating than if the formula was more acidic."
So, Glycolic Acid (pKa 3.6) is less irritating at pH 3.6 then at pH 4.6?
Thank you!
"pKa is the most important aspect to consider in formulating with acids. pKa implies acid availability. When pKa is close to pH, there is an ideal balance between salt and acidity, maximizing effectiveness of the acid and reducing irritation. Higher pH numbers in such a case would increase salt which counter-intuitively would make the formula even more irritating than if the formula was more acidic."
So, Glycolic Acid (pKa 3.6) is less irritating at pH 3.6 then at pH 4.6?
Thank you!
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that's a new one on me; have they cited any sources for this claim?
Sodium lactate has a high molecular weight. Therefore, it would stay on the skin and not penetrate which can be irritating.
Experts, do you agree? Thank you
While arbutin work good at 6 ph.
how to formulate of this combination?
The more salt you have in your acid, the less potent it is and potentially the more irritating it can be. This ‘irritation’ reaction should send a red flag to the professional aesthetician to check out the AHA product they are using to ensure the normal “erythema” end-point benchmark is attributed to acid activity or abnormal salt irritation. For this reason alone this fact is important to know your pKa and pH value to ensure your AHA peel is correctly formulated. If a manufacturer will not provide you the pH of the peel you use, change suppliers.
first of all, throughout the text, the term 'salt' is used in place of the more specific (and more generally recognised) term 'conjugate base'
"Lactic acid contains NMF"
NMF contains lactic acid, the converse is not true
"AHAs have no known systemic toxicity"
on the contary, using glycolic acid as an example, a dose of 600 mg/kg is teratogenic (causes birth defects), and the NOEL for teratogenicity is 150 mg/kg
"First and foremost, acid strength relates to the importance of pKa, which is a measure of acid strength and free acid availability"
on the contrary, it is the pH at which the free acid and its conjugate base are present in equimolar amounts; it is nothing to do with actual quantity of acid present
"The pKa is the logarithmic expression of the pH at which the acid possesses ‘free acid’ containing equal amounts of ion and salt."
1) pH is a logarithmic quantity by nature, and the fact it's logarithmic is irrelevant in this context
2) salts are ions by definition, so the phrase 'equal amounts of ion and salt' is nonsense
3) see above for a coherent, non-garbled explanation of pKa
"Acids, bases, and salts contain ions of the element hydrogen."
what is an "ion of the element hydrogen"?
"The lower the pH the more active the acid peeling solution."
not necessarily; if you put one drop of concentrated hydrochloric acid in a beaker of water, it will have a very low pH, but it will be a very weak acid
"The more salt you have in your acid, the less potent it is"
correct (notwithstanding the use of 'salt' to mean 'conjugate base')
"and potentially the more irritating it can be."
sources please?
"The pKa value tells you at what pH you have half acid and half salt. This translates that if you buffer an acid, you convert some of the acid to the salt form."
the first sentence is true, the second sentence makes no sense whatsoever
"Acid solutions are “buffered” not to make the acid stable but to raise the pH."
my head hurts
"The concentration is not as important as the pH and the pKa. I am asked all the time ‘what is the acid percentage concentration?’ Percentage of acid concentration is irrelevant"
if this is right, then the laws of thermodynamics are wrong
"and understanding this ‘acid chemistry’ explains why TCA, or trichloracetic acid (pKa of .26) will be far stronger in a five percent solution than lactic acid (pKa 3.86) or glycolic acid (pKa 3.83) at 30 percent. This is primarily because TCA is almost fully ionized and lactic and glycolic acids are not."
this is more to do with the chemistry of each acid than its strength; hydrochloric acid has a pKa of -7, and is therefore a much stronger acid than TCA, but it is much less corrosive to metals (and less toxic to life) because it has completely different chemistry
similarly, hydrofluoric acid has a pKa of around 3 (making it a weaker acid than TCA), but is a much more hazardous material, and is one of the few substances which can etch glass, due to its chemistry
god almighty, after reading that I need to lie down; the most depressing thing is that whoever wrote this probably got paid an ungodly sum for it
In an other blog i've found something that seams the opposit of what originaly stated by Deciem:
" 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."
Bill? Hope you are not mad at me throwing here whatever I find on the net
"in a perverse kind of way I actually enjoy tearing apart stuff like this"
And please continue doing so! Love it!
This has me confused? Sodium Lactate's skin conditioning/humectant properties are surely well recognised? ( and in fact the brand sells a NMF product with Sodium Lactate in it)
Plus, the idea that because a compound has high molecular weight it is categorically more likely to cause irritation?