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For a while now, it has seemed more and more likely that the reason that baking powder or baking soda+citric acid improve bubble juice is that they shift the juice's pH into an optimal range where the surfactant seems to work best. The optimal pH depends on the detergent you are using and possibly other ingredients, too.

Last night, I mixed up a guar-based juice and used distilled vinegar instead of baking powder for the pH adjustment. This morning (60F and 70% humidity) I tried the juice with both 32" and 70" top-string wands, and it worked great.

The tricky part about using an acid directly in the juice (such as vinegar or citric acid) is that it is easy to add too much which would drop the pH TOO low. The juice becomes pretty useless if the juice drops far below 7 though I have not yet identify the optimal range.

My tap water has a pH that varies from 8.9-9.3. With 20:1 juice (20 parts water per part Dawn Pro), 1.75 teaspoons (8.75 ml) of vinegar per 1 liter of water lowered the pH of the finished juice to 7.1. 

My juice used 1000 grams water, 50 grams Dawn Pro, 15-20 ml isopropyl alcohol for slurrying and 1/4 teaspoon (1.25 ml) guar gum. I added the 1.75 teaspoons last. I actually added 1. 5 teaspoons and then another 1/4 teaspoon when I saw that the pH was a little higher than I wanted.

FOLLOW UP

I plan to do some follow-up tests to see

  • how much vingegar or citric acid are needed to achieve the target pH with various pHs of water
  • get a better sense of the true optimal range 
  • confirm my sense that you don't give up anything by directly adding acid (versus using the CO2 method of acidifying).
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