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Measuring pH of o/w emulsions - pH increase post emulsion.
So I was making a lactic acid lotion, and the water phase reached pH 3 after adding 4% lactic acid to it.
Then I combined the water phase with a ~20% total oil phase with Hemp Seed Oil, Castor Oil and Emulsifying Wax and the pH, as measured by pH strips shot up to 5. I had to add up to a total of 10% lactic acid for the pH to reach about 3.5.Are test strips unsuitable for emulsions? Should I have stuck with the original 4%? Please advice.EDIT: I added 4% Zinc Oxide to the emulsion, which could have caused the pH hike“it was found, for example, that magnesium and aluminium stearate as well as various types of micronized zinc oxide which are dissolved or dispersed in the oil phase can shift the pH of the aqueous phase during manufacture from an initial acidic value through neutral to pH 8-10. Time-related and temperature-related changes in pH in formulations containing thermally unstable additives such as dihydroxyacetone or urea can be easily monitored.”
http://www.ncbi.nlm.nih.gov/pubmed/19245446 -
3 Replies
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Zinc Oxide will react under both acidic and alkaline conditions. The use of coated versions can limit this a little, but will likely only slow the reaction.
The following is a bit of a simplification but under acidic conditions.
ZnO: + 2H+ -> Zn2+ + H20
Under basic conditions.
ZnO + 2OH- + H2O-> Zn(OH)42-
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test strips are not a good way to measure pH quantitatively in any type of product - you’d be better off using a pH meter with an electrode
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What do you want the pH to be? Doesn’t really matter what the native pH is, just adjust it to what you need.
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