Citric acid is one of the main components in bath bombs. It is the combination of the citric acid and sodium bicarbonate which causes the fizzing when added to water. While it comes in different sizes from different manufacturers, the size you really want to use is the Citric Acid Powdered Crystals. Knowing this, you will be able to make better bath bombs.
Citric acid comes from citrus fruits. It is a weak acid, and through some manufacturing processes, they are able to extract the citric acid from the source. Then, they begin to process the citric acid. This is when you can buy it at different sizes. We have tried many different micron sizes and have found the powder form works the best. The smaller size allows a much tighter compaction of the bath bomb, which will make it harder.
I don’t think the size actually matters in term of the fizzing aspect, just the compaction and how smooth the outside of the bath bomb is. It will also depend on the size of the sodium bicarbonate as well. If you use the Arm and Hammer baking soda, you will not need to worry about this part. We believe the Arm and Hammer baking soda combined with the citric acid powder form yields the best bath bombs.
If you can’t find the powder form, you can use the next size up. You will just need to test the amount of binding agent to use. It will not be the same as in this recipe.
I won’t go into the actual chemistry behind the chemical reactions here. It will be in another post. This is just an overview of why citric acid and to make sure you get the powder form.