ph changing in stability test


I am working on a cream and I have noticed that the pH changes over time in the stability test, more noticeable in accelerated conditions.

Ingredients: Methyl salicylate, menthol, sodium cetostearyl sulfate and Cetostearyl alcool, xanthan gum, glycerine, water

I have noticed in the preparation process, if I melt the oil phase at 60-70 C the pH is around 6.5 and it drops to 6 after a month is the stability in accelerated conditions
If I melt the oil phase at 50-60 C the pH is about 7.5 but it drops to 7 in normal conditions and 6.1 in accelerated conditions (1 month). 

What might be the cause? 

Thank you


  • Breakdown of menthyl salicylate to acid??
  • at a guess, the fact there are no salts that can act to stabilise the pH; something as simple as disodium EDTA can be very beneficial in this regard

    however, my bigger concern is that there appears to be no preservative
    UK based formulation chemist. Strongest subjects: hair styling, hair bleaches, hair dyes (oxidative and non-oxidative) I know some stuff about: EU regulations, emulsions (O/W and W/O), toothpaste, mouthwash, shampoos, other toiletries
  • @Chemist77, good point. I thought methyl salicylate was stable. 

    @Bill_Toge, I was thinking of adding something to stabilise the pH. Thank you for the suggestion. Regarding the preservatives, I have seen several similar products in the market and none of them had preservatives. I guess having a high % of methyl salicylate and menthol was enough to avoid preservatives. 

  • @em88 it is purely a guess, since pH is involved and the only logical point is presence of a component responsible for pH drop. 
  • Has anyone else faced this?
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