My day job and being a parent has kept me too busy to do much bubble-related work for quite a while. I have been conducted some little explorations in bubble juice concentrate brewing over the past year or two that have not made their way onto the wiki or into posts to SBF, the Soap Bubble Fanciers Yahoo Group, but some of these odds and ends seem worth mentioning. I get a lot inquiries from people that are running into problems with making concentrates. Hopefully, these little odds-and-ends will help. They will eventually make their way into a real article.
pH adjusters in concentrates. When I began making concentrates, in particular when I came up with eGoo, I (like everyone else) had an incomplete (or just plain wrong) idea of what baking soda/citric acid or baking powder did for bubble juice. At the time, it was generally believed that some byproduct of the reaction with water was benefitting the juice. It turns out that the benefit is primarily (possibly entirely) due to the pH shift caused by dissolved carbon dioxide. It turns out that unless the reaction happens when all of the water and detergent are present that the pH shift is not enough to really improve the mix much. So, baking powder in a concentrate has very limited influence on the resulting bubble juice. The same is true of baking soda+citric acid used when making the concentrate. These additives should be added when the juice is fully diluted. However, if you use an acid by itself (such as citric acid or vinegar), you can add enough to the concentrate so that the fully diluted juice will be in your target pH range (which depends on the particular detergent and polymers). Lesson: baking powder or baking soda+citric acid are best added after diluting. Use an acid by itself when making the concentrate if you don't want to add anything but water when diluting. You will need to be able to measure the pH of your diluted juice to determine the appropriate amount to add. If you are brewing concentrate for other people to use, make sure that the pH is reasonable regardless of the pH of the water added. Tap water pH can run anywhere from 6 to 9!
Competition for water. Bubble juice contains many ingredients that are fighting for water. The interactions can be quite complex. A stable uniform water/polymer solution might stratify when detergent or detergent plus another ingredient are added because they are all competing for available water molecules. PEO tends not to exhibit stratification because such minute quatities are used. Guar gum and HEC are much more sensitive. Guar gum, in particular, has a tendency to settle out when the detergent concentration is too high. Interestingly, it is the detergent concentration that seems more important (to an extent) than the guar gum concentration. Even fairly low guar gum concentrations can show stratification if the detergent concentration is high enough. Example: with 300 grams water and 180 grams of Dawn Pro, even only 1 gram of guar gum will settle out even though that is a pretty low guar gum concentration (0.3% as a function of the added water). 2% HEC (Natrosol 250 HHR) solutions are stable, but if you add enough detergent, the HEC will settle out as an insoluble gel disk. Baking soda and sodium citrate seem to increase guar gum's tendency to come out of suspension even when used with distilled water.
Experiments done by Thommy and myself in May/June of 2012 indicate that it does not require very much detergent to destabilize guar/water mixes. With 1 gram guar gum, 100 ml water, and 1 gram baking soda (with or without 0.5 grams citric acid), there will be no separation with only 20 ml of Dawn Pro (or Fairy). With 40 grams of detergent, separation happens fairly quickly. With somewhere around 28 grams of detergent, the separation is delayed. If the baking soda is omitted in these mixes, no separation occurs (at least not after 24 hours).