In alkaline soln. at room temp., ascorbic acid rapidly reduces o-dinitrobenzene to the violet-colored nitroxylic ion; cysteine, glutathione, uric acid, or creatinine do not react and reducing sugars require 10 to 120 mins., to allow for enolisation. Selectivity is lost by heating the test mixture. Boiled in aq. soln. buffered to pH 4, dehydroascorbic acid forms a green pigment extractable by amyl alc. This test is not given by reduced ascorbic acid, reducing sugars, amino acids, urinary solutes, or other familiar biological compounds. In alkaline solns. at pH > 9 kept from exposure to air by a layer of toluene, dehydroascorbic acid develops a yellow color that slowly changes to a stable carmine. The chemistry of the reactions is discussed.