The Gallic acid reagent is used as a simple spot-test to presumptively identify drug precursor chemicals. It is composed of a mixture of gallic acid and concentrated sulfuric acid.
0.05 g of gallic acid is used for every 10 mls of sulphuric acid. The same ratio of gallic acid n-propyl ester in sulfuric acid can also be used.
Because of its short shelf life (changing to pale violet color) it is sometimes prepared by dissolving gallic acid into ethanol and adding the sulfuric acid at the time of testing from a separate bottle. In this case 100 mL ethanol is used and one drop of sulfuric acid is used per drop of gallic acid in ethanol.
References
Gallic acid reagent Wikipedia(Text) CC BY-SA