pH slightly dropping

Good morning everyone,

I have a water-based microemulsion for aerosol air fresheners. My formula contains a solubilizer, fragrance oils, surfactants, preservative and buffer system. My stability test requires that my formulas stays in the oven at 110oF (43.3oC) for 3 MO. Along the checkpoints, I measure the pH among other variables. 
After 3 months, two of my formulas (BI and VR) seen in Plot 1 slightly drop the pH from 5 to 4.7 (VR) and 4.8 (BI).

So, then I try to increase the overall concentration of buffer that was originally 0.5% by weight to see if the pH could be controlled better. However, even after increasing the buffer concentration to up to 2.5% By weight, the pH slightly dropped once again (Plots 2 and 3).

My question are:
1) Is this pH drop somewhat expected? I mean, do you think it is ok?
2) Is there anything else I could try to control this pH drop?

Thank you in advance,



Plots 2 and 3


  • So your pH is really staying within a 0.5 range for the entire 3 months, to me that's acceptable for an air freshener. Do you notice any scent change?

    If you put together specifications for your formula that pH can be given as the range you saw throughout testing. 
  • Hi Evchem, thank you for your response.
    In fact, these two samples present some fragrance note change. Some change is always expected in aged samples, but some sour, vinegary notes that these fragrances presented after 3 months in the oven are not acceptable. It is very interesting that this small variation in pH affects the fragrance note.
     I need to improve this formula in order to get rid off these changes.
  • ozgirlozgirl Member, PCF student
    If your fragrance is changing (oxidising?) this could be why your pH has dropped slightly.

    Generally I would consider a drop in pH of <0.5 acceptable but the fact that you are noticing a change in odour suggests that one of your fragrance components is not stable.

  • PerryPerry Administrator, Professional Chemist
    Yes, it's most likely a change in fragrance causing the problem. You'll need to work with the fragrance house to help fix the problem.
  • Thank you ozgirl and Perry! :)
  • FekherFekher Member, Professional Chemist
    @erickafalves actually my interpretarion matching the decreasing of ph and changing fragrance note that your perfumes in product are not stable enough so as probable solution i suggest to add more preservative or change it  that's may solve your problem.
  • Herbal and citrus notes can go off relatively quickly!
    Dr. Catherine Pratt
    (B.Sc with HONS I , Ph.D Analytical/Organic Chem and Microbiology), Cosmetic Chemistry IPCS)
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