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Home Cosmetic Science Talk Formulating Advanced Questions Math equation for pH adjusting?

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  • Math equation for pH adjusting?

    Posted by siouxchem on February 1, 2022 at 3:28 pm

    Would someone be able to help me out with a math formulating question?
    With formulas that we adjust the pH in, is there a specific equation we can use in order to figure out how much pH adjuster to put in with the entire formula? Rather than adjusting the pH at the end of formulation and diluting the active ingredients. Making the pH adjuster included to the totals (100%) of the formula.
    For a more detailed example, say I have 2% of an active ingredient in a formula. At the end I use ‘x’ grams adjuster 
    (50/50 CA or TEA for example; anything), which ends up being 0.4% of the total formula to get the desired pH.  
    But now my active ingredient is diluted and can no longer be said it is at 2%. What equation can I use in the future so I know how much pH adjuster to use from the very start?

    Fekher replied 1 year, 4 months ago 4 Members · 4 Replies
  • 4 Replies
  • OldPerry

    Professional Chemist / Formulator
    February 1, 2022 at 3:36 pm

    There is not really an equation you can use.  But what you can do is to use the principle of “q.s.”   

    So, in your formula your active would be used at 2%. Say you have water at 97% and then your acid would be listed as q.s.

    You would figure out over time how much acid is really needed to get the pH that you desire. Sometimes it might be 0.5% which would bring your total formula to 99.5% (97 + 2 + .5).  In this case you would also top it off with 0.5% water to get to 100%.

    There might be another case where you need 0.8% acid to bring it to your desired pH. Then your formula adds to 99.8% and you’d have to add 0.2% water to get everything to even out.

    As long as you leave a volume for the acid suitable for adjustments (1% should be enough) then you can always keep your active ingredient to it’s desired %

  • siouxchem

    February 1, 2022 at 4:26 pm

    @Perry This is so helpful! So basically just a small water discount in the beginning and replace what is left over at the end to make 100% after adjusting pH. 
    Thank you much for the detailed and quick response. It is very appreciated!  :)

  • CedarS

    February 1, 2022 at 6:14 pm

    From experience on working on a scale up currently, you really can’t always say ‘use x’ because sometimes your raw materials will vary. I had a surfactant that would pass when it came in at 8.8 but then a batch came in at 6.5 (still within CoA specs) and we had to adjust based on that, which was a change. 

    So it varies! 

  • Fekher

    Professional Chemist / Formulator
    February 2, 2022 at 3:48 pm

    I agree totally with @CedarS so @siouxchem
    May Math equation  pH can help for prediction a near value(and may not also because it contains a lot of suppositions) however it will be more accurate and even easier to do the desired experience may a suggestion to have just your adequate borders for having a start point is seeing the molecular mass for every reactif and their coefficients in the reaction equation.