1,5-Diphenylcarbazone is used as an adsorption indicator in precipitation titration to determine the concentrations of Cl⁻, Br⁻, I⁻ and SCN⁻ (in the presence of HgCl₂). The titrant is AgNO₃. The titration endpoint is reached when the colour changes from red to blue-violet. The 1,5-diphenylcarbazone indicator solution is prepared by dissolving 0.1 g of 1,5-diphenylcarbazone in 100 mL of ethanol (96%).
It crystallises from EtOH (ca 5mL/g), and dry the orange-red needles at 50o. A commercial sample, nominally sym-diphenylcarbazone (m 154-156o) was a mixture of diphenylcarbazide and diphenylcarbazone. The former was removed by dissolving 5g of the crude material in 75mL of warm EtOH, then adding 25g Na2CO3 dissolved in 400mL of distilled water. The alkaline solution was cooled and extracted six times with 50mL portions of diethyl ether (discarded). Diphenylcarbazone was then precipitated by acidifying the alkaline solution with 3M HNO3 or glacial acetic acid. It was filtered off, air dried, and stored in the dark [Gerlach & Frazier Anal Chem 30 1142 1958]. Other impurities are phenylsemicarbazide and diphenylcarbodiazone. Impurities can be detected by chromatography [Willems et al. Anal Chim Acta 51 544 1970]. It is used for detection and estimation of Hg, Zn, Cd, Cr, Cu, Fe and Mo [Cheng et al. Handbook of Organic Analytical Reagents, Boca Baton 277 1982, Beilstein 16 H 24, 16 IV 17.]
