In this post on bath bombs, Emily ask: Can you use bio oil instead of vegetable or soy bean oil?
Are you referring to this product, Bio-Oil? Yes, you could use any type of oil soluble ingredient or oil in a bath bomb. But I'm wondering why use this ingredient? It's really really expensive, and it won't do anything for our skin as it's so diluted in that giant amount of water.
If you use a 100 gram bath bomb, you are using maybe 7 grams of oil in 80 litres of bath water, which is very very little.
In this same post, Ale said: Can I use ascorbic acid instead of citric acid?
No, for quite a few reasons. The main one for me would be that ascorbic acid - Vitamin C - is extremely expensive in this application and would offer nothing to your skin while you're sitting in the bath.
As an aside, I've seen people suggesting you use cream of tartar instead of citric acid in a bath bomb. My suggestion is that you don't. Citric acid is inexpensive and gets the job of fizzing and melting away done very nicely. Cream of tartar is expensive and wouldn't do a better job. You could add a little cream of tartar to the bath bomb if you wanted to, but using it as the citric acid in a recipe is costly and unnecessary.