text: The reaction is used in a chemical test for the detection of aldehydes in combination with ferric chloride. In this test a few drops of aldehyde containing specimen is dissolved in ethanol, the sulfonamide is added together with some sodium hydroxide solution and then the solution is acidified to Congo red. An added drop of ferric chloride will turn the solution an intense red when aldehyde is present. The sulfonamide can be prepared by reaction of hydroxylamine and benzenesulfonyl chloride in ethanol with potassium metal.