Selenous acid protecting and changing the color of steel. Application: Selenous acid most commonly used as cold bluing process of steel. Mixture of selenous acid, copper(II) nitrate, and nitric acid changes the color of the steel from silver-grey to blue-grey or black. In place of copper nitrate and nitric acid BLUING process can be processed by use of copper sulfate and phosphoric acid with Selenious Acid.Dose: Note: Mecke reagent is a mixture of selenous acid and concentrated sulfuric acid. Mecke reagent is used as a simple spot-test to presumptively identify alkaloids as well as other compounds. Testing procedure : Simply dripped Mecke Reagent onto the substance being tested, If the Substance contains any nitrogen item ALKALOIDS spot will be develope. Mecke reagent can be produce by addition of 100 mL of concentrated (95–98%) sulfuric acid to 1 g of selenious acid.