Home Cosmetic Science Talk Formulating Is my calculation correct about neutralizing salicylic acid with sodium hydroxide?

  • Is my calculation correct about neutralizing salicylic acid with sodium hydroxide?

    Posted by abdullah on November 29, 2021 at 4:57 am

    Salicylic acid:
    Formula  C₇H₆O₃
    Molecular weight 138.121 g/mol

    Sodium hydroxide:
    Formula NaOH
    Molecular weight: 39.997 g/mol

    Sodium salicylate:
    Formula  C7H5NaO3
    Molecular weight: 160.11 g/mol

    Does one molecule of sodium hydroxide neutralize one molecule of salicylic acid to it’s sodium salt?

    If yes then would ~0.5g sodium hydroxide neutralize ~2g salicylic acid to its sodium salt according to these molecular weights?

    abdullah replied 2 years, 3 months ago 2 Members · 6 Replies
  • 6 Replies
  • pharma

    Member
    November 29, 2021 at 10:22 am

    Yes, it does. This results in 0.56 g sodium hydroxide for 2 g salicylic acid.

  • abdullah

    Member
    November 29, 2021 at 11:25 am

    @Pharma then what was the problem with my Formulation that salicylic acid came out of solution?

    I added 
    2% salicylic 
    5.8% sodium hydroxide(10% in water)
    0.1% tetrasodium EDTA 
    1% phenoxyethanol+ caprylyl Glycol
    Hot water

    Batch size 500g

    The process was weighting and adding everything and then adding hot water. Then mixing with stick blender for 2-3 minutes.

    When it was hot everything was solubilized.

    pH was about 4.8.

    Then i added some citric acid (50% solution) to bring the pH down to 3 but it directly produced white salt on top of solution. was this salicylic acid coming out of solution?
    This is the photo

    I then made another batch without adding citric acid. At first when it was hot everything was solubilized. Ofter one night crystals had precipitated in botom of bottles. Was that salicylic acid?

    The main question is why salicylic acid comes out of solution?

    And what confuses me is in the past i had made neutralized salicylic acids this way without any problem. Unfortunately i couldn’t find where had i written those formulas. But i remember i was using salicylic acid: sodium hydroxide at 1:0.29 ratio. But it is not working now 🤔

  • pharma

    Member
    November 29, 2021 at 2:31 pm
    At a pH of 3, nearly 50% of the salicylic acid is converted back to it’s free acid form… on one hand, you could just use half the amount of sodium hydroxide to achieve the same, on the other hand does it mean that you can, with luck, solublise 0.25% total salicylic acid at room temperature (equaling 1.25 g of free salicylic acid per litre… in other words, you can’t solubilise 2% SA in water at pH 3).
    Depending on manufacturing process and ingredients, it is sometimes (very unreliable and often non-replicable) possible to create supersaturated solutions which eventually crystallise out after some time (hours to months).
  • abdullah

    Member
    November 30, 2021 at 2:22 am

    @Pharma just to know, what would be the pH if we neutralize 2% salicylic acid with 0.56% sodium hydroxide?

    The one i made which had 0.1% tetrasodium EDTA too had pH of ~4.8 without any pH adjustment.

    Is sodium hydroxide flacks always 100% sodium hydroxide? 

  • pharma

    Member
    December 2, 2021 at 5:40 am
    If you hit a precise 1:1 molar ratio, pH would be neutral. Because one usually does not, pH can be higher or lower due to a small excess of acid or base.
    NaOH is hygroscopic, thoug in theory 100% pure, it tends to abstract water out of air and ‘gets diluted’ over time.
  • abdullah

    Member
    December 2, 2021 at 7:52 am

    Pharma said:

    If you hit a precise 1:1 molar ratio, pH would be neutral. Because one usually does not, pH can be higher or lower due to a small excess of acid or base.
    NaOH is hygroscopic, thoug in theory 100% pure, it tends to abstract water out of air and ‘gets diluted’ over time.

    Thanks

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