The TODB or Todb reagent is an in vitro chemical reagent that is essential in medical laboratory diagnosis. It is used to rapidly quantify salicylates in biological fluids, as well as to analyze different substrates and metabolic end products. In the presence of salicylic acid, ferric nitrate nonahydrate catalyzes the reaction with mercuric (II) chloride, producing a distinct purple color. The color intensity, as determined spectroscopically, is directly proportional to the analyte concentration. Furthermore, the TRINDER reagent is employed in the Trinder technique, in which hydrogen peroxide produced by the analyte's enzymatic reaction (for example, glucose oxidase oxidation) combines with phenol and 4-aminophenazone under the action of peroxidase to form a color complex and water.