Home Cosmetic Science Talk Formulating General Science Conversion of mix of different acids to conjugated base under certain pH

• # Conversion of mix of different acids to conjugated base under certain pH

Posted by on February 8, 2023 at 1:31 pm

Let’s say I have a mix of acids: 10g Salicylic (SA), 10g Glycolic (GA), 10g Lactic (LA).

Questions (the numbers might not be totally precise, but the questions are about knowledge in general):

1. pKa(SA) = 2.97 < pKa(GA) = 3.6 < pKa(LA) = 3.8. Does it mean SA is stronger than GA, GA is stronger than LA? In terms of chemistry pH is a strength of acid and pKa is relative to pH so it also shows the strength of acid, right?

2. Based on pKa we can estimate how much acid is converted to conjugated base under certain pH. Let’s say under pH 3.5 in mono acid solution we have ~23% unconverted SA left, ~69% GA left and ~70% LA left.

So if I have a solution of 10g lactic acid only under pH 3.5 I have 7g acid not converted. But what about mix of acids? If I have a mix above (10 SA, 10 GA, 10 LA) and I added Sodium Hydroxide to reach pH 3.5:

- 2.3g SA left, 6.9g GA and 7.0g LA + salicylate, lactate and glycolate?

- Or the whole SA will be converted to salicylate first because it is the strongest, and then part of GA will be converted with what’s left after SA?

replied 1 year, 3 months ago 3 Members · 4 Replies
• 4 Replies
• ### ketchito

Member
February 9, 2023 at 6:07 am

For the first question, pH only tells you the concentration of H ions in a solution, but it’s pKa the one that shows the strenght of your acid (its dissociation power in that same solvent). That varies when you change your solvent but for the sake of the argument, let’s assume it’s always water. So, the lower the pKa, the stronger the acid.

I’ll comment on the rest later, since I’m still in bed and only part of my brain woke up 😅

• ### fareloz

Member
February 9, 2023 at 4:47 pm

I am eager to hear.the answer, this question bugs me the whole day and I can’t find easy answer on the Internet

• ### Pharma

Member
February 12, 2023 at 5:38 am

If you had an adic with pKa of 4 and a pH of 4, then, by definition, 50% of the acid is in acid form and 50% deprotonated/neutralised (salt form). Because pKa is a logarythmic scale, at a pH of 3, 90% would be acid and only 10% conjugated acid whilst at a pH of 5, 10% would still be acid and 90% conjugated base.

In your example, you can calculate the values for each acid in the blend separately (the different ratios don’t depend on how much base you added but only on the final pH, which in turn depends on the amount of base).

You can calculate pH from pKa values and amounts of acid and base added (using mol instead of grams) but with blends it’s easier to just measure pH and then calculate for each acid separately.

Unfortunately, the new forum formate doesn’t let me see your original post and I’m currently too lazy to check whether or not I missed something…😊

• This reply was modified 1 year, 3 months ago by  Pharma.
• ### fareloz

Member
February 13, 2023 at 5:05 am

Thank you both! Now it is clear to me