CITRIC ACID

This organic acid gives the tang in some foods. It is used in the CWPBQ2 bleach to buffer the solution, and also alleviate the stain caused by the PBQ in the bleach. It can also be used to remove the stains on skin caused by Pyrogallol solutions simply by rubbing the affected areas with it.

Here is what the Photo-Lab Index from Morgan & Morgan has to say about it:

PLI Citric Acid