Barritt’s A reagent consists of alpha-naphthol in ethanol. It is used with Barritt’s B reagent (40% KOH) to detect the presence of 2,3-butanediol (so-called "neutral fermentation products") produced from glucose in MRVP broth.
After inoculating MRVP broth and incubating overnight, five drops of each reagent are added to the culture. Barritt’s A should be added first. This is followed by addition of five drips of Barritt's B. Development of a red color indicates the presence of 2,3-butanediol.