檸檬酸的用途的原因

Citric acid is an organic acid when compare with inorganic acid such HCl , its acidity is very moderate . Since it occurs in fruits such lemon , orange ...& etc. It is eatable & good for our body . Since bacteria donot like acidic environment so vinegar or citric acid can be used to perserve food . As organic acids donot completely dissociate so once some hydrogen ions are neutralised , more hydrogen ions will be dissociated from the citric acid to replace hence it is used as a buffler . About the question #4, I am not sure because metal under even slightly acid environment , will also be oxidised . If you want better answer , put this question in "Chemistry" .