Abstract: The diazotization reaction taking place in the titration of sulphonamides with sodium nitrite in acid medium was studied potentiometrically. Various factors such as reaction velocity, acidity, concentration, and tempreture of the reaction mixture were observed. A new internal indicator—brilliant cresyl blue was found to fulfill the requirements for this titration with a clear end point changing from blue to red. It was tested against six commonly used sulphonamides giving satisfactory result in comparison with those by the official method. It is considered to be a suitable internal indicator for diazotization of aromatic primary amines.