Home Cosmetic Science Talk Formulating General Adjusting pH salicylic acid

  • Adjusting pH salicylic acid

    Posted by doreen on June 2, 2017 at 8:24 am

    Why do I see the addition of NaOH if the end result has a low pH?
    Example: BHA exfoliant with 1% salicylic acid, dissolved in pentylene or butylene glycol, end pH is 3.5. pH adjusters are NaOH and citric acid:

    Aqua, Glycerin, Pentylene Glycol, PEG-40 Hydrogenated Castor Oil, Citric Acid, Salicylic Acid, Methyl Gluceth-20, Dipotassium Glycyrrhizate, Hydroxyethylcellulose, Evodia Rutaecarpa Fruit Extract, Allantoin, Sodium Hydroxide, Ethylhexylglycerin, Butylene Glycol, Disodium EDTA.

    About the disodium EDTA: where in the process do you need to add it?

    belassi replied 6 years, 9 months ago 3 Members · 18 Replies
  • 18 Replies
  • markbroussard

    Member
    June 2, 2017 at 9:46 am

    @Doreen81:

    The end pH is adjusted to 3.5 for a couple of reasons …

    (1) Although SA is a beta acid, the regulations for alpha hydroxy acids is a pH or 3.5 or higher, so I would suspect the pH adjustment of this formulation to 3.5 is following that guideline.

    (2) At pH levels below 3.5, you often get SA precipitating out of solution.

    The LOI you listed seems a bit odd to me in that it contains more Citric Acid that SA and then an adjustment with Sodium Hydroxide.

    Methyl Gluceth-20 is a good solubilizer of SA … it would have been easier, in my opinion, to just use Methyl Gluceth-20 and 1% Sodium Citrate to dissolve the SA and perhaps Sodium Lactate to adjust the pH.  The Sodium Lactate provides moisturization … Citric Acid and Sodium Hydroxide do nothing for the skin other than adjust the pH.

    This formula can be made in one pot heated to 55C into which all ingredients are added, so you can just throw in the EDTA at any point in time, but I generally add the Chelants early in the process.

  • doreen

    Member
    June 2, 2017 at 10:24 am

    @MarkBroussard 

    The LOI you listed seems a bit odd to me in that it contains more Citric Acid that SA and then an adjustment with Sodium Hydroxide.

    Exactly! I just don’t get it.
    I would also prefer sodium lactate over sodium hydroxide.
    I can’t get my hands on Methyl-Gluceth 20, would propylene glycol also do as a SA solubilizer in a concentration of 1%?

    These are the ingredients I will be using (not in order btw):
    SA 1%, propylene glycol, PEG-40 HCO, glycerin, bisabolol, allantoin, citric acid, xanthan gum, sodium lactate, disodium EDTA.

    Propylene glycol will be >10%, disodium EDTA will be 0,1%. Would I need to add a preservative? Packaging will be airtight.

  • markbroussard

    Member
    June 2, 2017 at 10:58 am

    @Doreen81:

    Propylene Glycol (maybe 20%) + 1% Sodium Citrate will probably work to solubilize 1% SA.  

    SA in an airless container may not require additional preservation, but SA is not broad spectrum … I’d throw in some Sodium Benzoate to cover mold … or better yet, use Gluconodeltalactone + Sodium Benzoate as your preservative.

  • doreen

    Member
    June 2, 2017 at 11:57 am

    @MarkBroussard
    Would propylene glycol + 1% citric acid solution instead of sodium citrate, do aswell? And potassium sorbate instead of sodium benzoate?

  • markbroussard

    Member
    June 2, 2017 at 1:56 pm

    Nope

  • doreen

    Member
    June 2, 2017 at 3:04 pm

    Well that’s really shitty then. I can’t get sodium citrate, octyldodecanol nor other solvents for SA and the company I buy stuff from told me no buffers were needed with propyene glycol. Oh well, it’s my own mistake for being so gullible. 

    I’ll just disperse it in vaseline then :s :s

  • doreen

    Member
    June 2, 2017 at 4:34 pm

    @MarkBroussard
    New idea. 
    I’ll make the base product without SA adjust it to pH 3.5 add preservation etc.
    The days that I will be using it, I’ll make a premix with SA + propylene glycol and add it to the base.

    Is this an idea, or does it reappear out of solubilisation quicker than can be applied, or wouldn’t it solubilize at all without the buffer?

  • belassi

    Member
    June 2, 2017 at 5:49 pm

    Sodium citrate is easy to synthesise using sodium bicarbonate and citric acid. For every 2.1 g of citric acid, use 2.5 g of sodium bicarbonate, you’ll get 2.9 g of sodium citrate after evaporating the water.

  • doreen

    Member
    June 2, 2017 at 6:15 pm

    @Belassi,
    Thank you so much! :)  Which one do I need to dissolve first? Can I use demi water for this and how much? 

  • belassi

    Member
    June 2, 2017 at 6:52 pm

    Demi water??? Always use distilled or deionised water. It is just a simple replacement reaction.

  • doreen

    Member
    June 2, 2017 at 7:22 pm

    @Belassi ,
    Thanks anyway. I thought NaOH would be necessary, never thought of bicarb.

    Yes demi water. I boil it for about 20 mins. For own use.

    Demineralised water is made by passing the water through specially manufactured resins. The resin is designed to exchange ions in the water so that the final result leaves you with the chemical composition of pure water.

    I know distilled/aqua purificata is different, but I thought demi water is being deionized? ^ Source

  • belassi

    Member
    June 2, 2017 at 7:47 pm

    “Demi” is not a proper word. You’re using deionised water. No need to boil it.

  • doreen

    Member
    June 2, 2017 at 7:58 pm

    @Belassi,
    Ok. We always say demi water for demineralized here. ;) 

  • belassi

    Member
    June 2, 2017 at 8:02 pm

    Just dissolve the citric acid in as much water as it takes to dissolve it and then add the bicarbonate. After the reaction finishes you can either leave it as a solution or evaporate the water off. 

  • doreen

    Member
    June 2, 2017 at 8:14 pm

    @Belassi,
    Exactly. I’ll use it as a solution, so this can be done real quick.

    The equation says a 1:3 ratio btw? Or do I read it wrong, it’s late here, brain is foggy.
    C6H8O7 + 3NaHCO3


    Na3C6H5O7 + 3CO2(g) + 3H2O (with loss of heat)

    edit: it gives trisodiumcitrate. I’m not sure if my citric acid is anhydrous btw.

  • belassi

    Member
    June 2, 2017 at 10:16 pm

    Or you could titrate it against NaOH until you see the rapid pH change that indicates end of reaction. pH for citrate is 3.5? I’d have to check.

  • doreen

    Member
    June 4, 2017 at 11:01 am

    @Belassi, I don’t have NaOH, so I’ll do it the bicarb way. Actually, is it ok to rely on seeing no more CO2 bubbles as end of reaction, or is this too inaccurate?

    About not waiting for evaporation of the water and using the solution, what % of sodium citrate would I have if I would use the amounts you mentioned earlier, because the reaction gives extra H2O, do I have to keep that in mind if I calculate the concentration of the solution?

    A quick thought, please bear with me:
    For every 2.1 g of citric acid, use 2.5 g of sodium bicarbonate, you’ll get 2.9 g of sodium citrate after evaporating the water.
    I need to add it to the salicylic acid in propylene glycol. I thought of adding 50 ml sodium citrate 1% solution. So then I need:
    0,36 grams of citric acid and 0,43 grams of sodiumbicarbonate in 50 ml water to obtain 1% if I want to add no more than 50 ml 1% solution.
    Am I thinking right here, or is there a fallacy?

    Thanks!!!

  • belassi

    Member
    June 4, 2017 at 3:25 pm

    Calculation appears correct.

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