Carboxylic acids occur widely in nature. They are weak acids that will react with a base such as sodium bicarbonate (baking soda) to form carboxylate salts, carbon dioxide and water. It is the carbon dioxide that result in the bubbles.

Citric acid, a tricarboxylic acid shares the same chemical properties as tartaric acid. Like tartaric acid which is found in for example, grapes , citric acid is also found in food - citrus foods. Acetic acid (vinegar), one of the simplest carboxylic acid is another candidate. These would be safe to use in bath products.

http://en.wikipedia.org/wiki/Citric_acid

http://en.wikipedia.org/wiki/Tartaric_acid

http://en.wikipedia.org/wiki/Acetic_acid

Here is a recipe for foaming bath bombs using citric acid:

http://www.chemistrystore.com/Foaming_Bath_Bombs.htm

Alternatively, why not also consider the SALT form of tartaric acid rather than another acid? This would be potassium hydrogen tartrate more commonly known as cream of tartar.

http://en.wikipedia.org/wiki/Cream_of_tartar

Here is a solid bubble soap recipe that uses cream of tartar, sodium bicarbonate, surfactant plus some extra ingredients :

http://www.pvsoap.com/recipe_bubble_bath_drop.htm

Ah, I understand - you don't want fizzies. Acetic acid has only one carboxyl group and that is what you can try. But remember, vinegar is a dilute acetic acid solution though.

Then strongly consider EDTA (ethylenediaminetetraacetic acid): .Images available only to Customers.

It's used in alot of grooming products listed here

http://householdproducts.nlm.nih.gov/cgi-bin/household/brands?tbl=chem&id=938

Even for soap - this soap recipe (not the solid kind you want) uses EDTA :

http://www.ponapecoconut.com/international/bathsoap.htm

For a surfactant, use sodium lauryl sulphoacetate.

The free patents online website :

http://www.freepatentsonline.com/5955057.html

shows the proportional recipe you should be using to obtain a novel solid bubble bath soap - actual weights will depend on the batch quantity you wish to make:

1. 40-90% weight sodium bicarbonate

2. 5-50% weight of the organic acid

3. 0.5-15% weight of alcohol. (doesn't say which but I suggest isopropanol )

Plus other optional ingredients like oil, colouring agent and perfume to enhance the product.The method apparently offers fast mixing and manufacturing time. Try creating an account and see if you can get more information.

Another organic acids to try might be ascorbic acid (vitamin C) .

Good luck!

PS Here is a link which lists other natural carboxylic acids :

http://www.cem.msu.edu/%7Ereusch/VirtualText/crbacid1.htm

As you can see you have other natural alternatives besides ascorbic acid, like stearic acid and palmitic acid which have also been used in soap making.