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Quest for the Missing Salicylic Acid
Hello,
I am not a formulator, but I work in process validation for a contract manufacturer, and sometimes I get drawn into issues for newly formulated products. In this case, we have pilot batches for two newly-developed products (skin cleansers). Both are about 63% water and 7% glycerin, and both use Sodium Hydroxide as a PH adjuster. One contains 2% (17kg) Salicylic acid, and the initial batch assay puts it about 1kg short (batch size is 850kg). The second product is worse. It contains 0.5% (4kg) salicylic acid, and the assay is 1.4 kg short (batch size is 800kg). We have taken all the normal investigational steps (raw materials reconciliation, checked the filters, etc.), and we cannot find this missing material.
Does anyone have any experience with a loss such as this? So far, this remains a mystery.
Thanks in advance for any help.
Mike
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6 Replies
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How are you measuring this loss?
Remember that adjusting pH with sodium hydroxide will remove free salicylic acid.
What is the function of this product? Usually a formulation containing salicylic acid requires the free acid, not its sodium salt.
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This is our method
“The analysis of Salicylic Acid is achieved by using reversed phase HPLC with UV
detection at 305 nm with a C18 column. The quantitation of Salicylic Acid in the
sample is achieved by the comparison of the peak area of Salicylic Acid in the sample
chromatogram to that of a reference standard of a known concentration using external
standardization.”
Prior to these issues, a lab batch was manufactured in which the Salicylic acid assay was within specification.Facial cleansers.
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Deleted repeated comment
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Are you checking for Sodium Salicylate?
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I’ve had this argument with analysts before - I put the correct amount of the ingredient in the batch, I made the batch and no residue was left over, but the assay shows less of the ingredient than I put in.
My feeling has always been - if the ingredient isn’t volatile, and there’s no evidence of it crystalizing or dropping out, then the analysis must be wrong - ingredients don’t just vanish.
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I agree with Bob and John. I think some of your salicylic acid is converted to sodium salicylate by the addition of sodium hydroxide.
You can validate this by testing your production batch samples for salicylic acid and sodium salicylate. Compare the results and see if the results would add up.
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