Citric Acid is found in small amounts in grapes and in larger amounts in most other fruits. Most of it will be lost in the fermentation. Adding 1 gram per 10 gallons will help prevent iron hazes (a malady that is no longer very common). It may be used as a substitute for tartaric acid to increase acidity, preferably added after fermentation has ended. 3.7 grams of citric acid per gallon will increase acidity about 0.1%.
It is also great for oak barrels