Rapid determination of the p-aminobenzoic acid excretion index in urine without use of radioactivity.

234 CLINICAL CHEMISTRY, Vol. 32, No. 1, 1986 basic chemical principles. Reviewing uric acid methodology, we discovered one incorrect and two unbalanced reaction equations for uric acid analysis by the uricas&peroxidase-phenol-4-aminoantipyrine method. Figure 1 illustrates the overall reaction. An investigator has incorrectly prothe formation of a dichloro-oquinoneimine dye from the reaction between 2,4-dichiorophenol and 4aminoantipyrine (1). Formation of this sterically hindered product would require the unlikely liberation of a hydride ion. Another investigator has suggested that the predominant product is a chloro-p-quinoneimine dye (2). The correct reaction is illustrated in Figure 1, reaction 2(a). Two unbalanced reaction equations have also appeared (3, 4). Balanced equations for the reaction of tribromophenol or 4-hydroxybenzoic acid with 4-aminoantipyrine are given in Figure 1, reaction 2(b-c). A major difference in these reactions is the amount of hydrogen peroxide consumed. Ifone assumes that the products have equivalent melar absorptivities, the stoichiometry suggests that the tribromophenol method (3) [Figure 1, reaction 2(b)] should be twice as sensitive as the 4hydroxybenzoic acid method (4) [Figure 1, reaction 2(c)]. This important point was not obvious from the unbalanced equations. The choice between similar methods that are being developed or applied should be based on their chemical pmciples. Authors should ensure that the chemistry involved is correctly represented, to facilitate such choices. Others who are interested in developing a more sensitive or selective phenol-4aminoantipyrine method should consult previous work (2) in order fully to understand the chemistry of this colorimetric method.