One method for the indirect determination of NO involves the spectrophotometric measurement of its stable decomposition products NO3− and NO2−. This method requires that NO3− first be reduced to NO2− and then NO2− determined by the Griess reaction as shown in Figure 10 Briefly, the Griess reaction is a two-step diazotization reaction in which the NO derived nitrosating agent, dinitrogen trioxide (N2O3) generated from the acid-catalyzed formation of nitrous acid from nitrite (or autoxidation of NO) reacts with sulfanilamide to produce a diazonium ion which is then coupled to N-(1-napthyl) ethylenediamine to form a chromophoricazo product that absorbs strongly at 540 nm (Bryan and Grisham, 2007). The nitrosating agent dinitrogen trioxide (N2O3) generated from acidified nitrite (or from the autoxidation of NO) reacts with sulfanilamide to yield a diazonium derivative. This reactive intermediate will interact with N-1-naphthylethelene diamine to yield a colored diazo product that absorbs strongly at 540 nm.