I am an architect, not a chemist, but I do believe that the "citric acid anhydrous" you are referring to is not the same the same thing. The material that MB uses is a FORMULATION designed to work in the auto radiator.
Is the citric acid anhydrous designed for use in an auto radiator? If not, then I would not take the risk. By the way, anhydrous means "not water-based", or lacks water. In Latin, the use of the prefix "an" usually means "not", and "hydrous" means "water". Beyond knowing that, I don't know what the effect of using that in your system is, but I would ask more questions before using it....Andras