Preservative for buffer fluids

I'm running out of my preservative for pH meter buffers, Hydrion Color Key Preservative (isopropyl alcohol, polyethylene glycol, methyl p-hydroxy benzoate, thymolsulfonphtalein, water).
It can only be purchased along with a complete bufferset, so I wonder if I can preserve it with something else, without affecting the pH of the buffer?
I think the thymolsulfonphtalein is only there as pH indicator and the PEG (+ IPA) mainly  to enhance solubility in water? So that leaves methyl paraben and IPA for preservation?
The cheapest way for me would be adding Germaben II, but I don't know if the formaldehyde releaser and formaldehyde itself affect the pH somehow.

I use pH 4,7 and 10 buffers. pH 10 would be too high for Germaben II, but the shelf life of the pH 10 buffer is already shorter due to CO2 uptake, so I make a fresh batch every 1 to 2 weeks. I would mainly be have to focus on pH 4 and 7, especially pH 4 because I use this as storage solution.

Does anyone know about preservatives for buffer fluids?
Could Germaben II be an idea, or should I make a methyl paraben/IPA/water solution?
Please feel free to share your knowledge!

Comments

  • PharmaPharma Member, Pharmacist
    What about phenoxyethanol and a glycol (such as Lexgard HPO)? Alternatively, thiazolidinones should work. I wouldn't use formaldehyde releasers because they might (at least over time) affect a glass electrode.
    I guess that IPA is likely added at low amounts to increase solubility/fluidity and less for preservation because higher % alcohol will affect pH. The combo of IPA and PEG reduces water activity but is that enough? BTW phthaleine dyes show some antimicrobial activity ;) .
    As storage solution, KCl is recommended for glass electrodes.
    I never use the pH 10 solution... Unless you're into perm products, who cares whether or not the meter ain't accurate at an alkaline pH? :)
  • DoreenDoreen Member
    @Pharma
    Thank you so much for your answer! :)

    Totally forgot about the effect on the glass electrode! :flushed:
    Glad you brought that up!
    Yes I reckoned the IPA was mostly there for solubilizing the Thymol Blue, since it isn't soluble in water. I know phtaleine dyes have microbial activity, but I think here it is mainly used as pH indicator. The disadvantage is that it leaves stains on the pH meter. :confused:

    I have two Extech PH110 Refillable pH Meters, I use one for emulsions and the other for toners, exfoliants etc. (The probes have a flat surface, so they're very easy to clean, especially when used in emulsions.)
    Extech recommends to use the pH 4 fluid (Hydrion Buffer pH-4.00) as storage solution (potassium biphtalate). I use KCl for refilling. Very unlogical, actually. Would the KCl in the probe be contaminated now, you think? So far (since 2016) I haven't had problems with these meters.

    You're also right about the pH 10. When I started homecrafting, I was like, I want the meter to be accurate in a broad range, because I didn't know what I would be making, so I chose the regular 3 points calibration at 4, 7 and 10. Actually I only make slightly acidic to acidic stuff, pH 3.5 to 7, so it would be best if I had a pH 2 buffer and use 2, 4 and 7. The most ideal would be the meter that's used for emulsions on 4 and 7 and the meter for exfoliants/acidic toners on 2 and 4. Point is that I can't get pH 2 buffers here (for a reasonable price, that is).
    Despite all that, the readings are quite accurate and probably accurate enough for my homemade stuff, which I don't sell.

    About preservation. What do you think of phenoxy and parabens, like Phenonip P4? 
  • PharmaPharma Member, Pharmacist
    If a manufacturer recommends a product, then it's supposed to be the right one. Besides, it's all about potassium and that's in both liquids ;) .

    I've honestly never really thought about preservatives in calibration liquids... it should be water soluble (the more the better) and pH neutral. Hence, phenoy and parabens should work although I'd be careful with more lipophilic ones. Careful means: Clean your electrode well, for example with a solvent such as IPA (something you're likely doing anyway after measuring creams) and don't use them for storing the electrode. That's the beauty about KCl, it's just KCl and water.
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