To a beaker containing N-phenylanthranilic acid (0.913g, 4.2 mmol), a thin layer of polyphosphoric acid was added*. The reaction mixture was heated to 110°C with constant stirring for 1 hour**. Upon cooling, the solution was poured over ice***, and the precipitate was isolated at the pump to yield 10H-Acridine-9-one (0.718g, 87.6%) as a golden solid.

* A 600 ml beaker was used due to the very dense and gloopy nature of polyphosphoric acid. A minimum amount of acid was used to cover surface of fine solid and this is always in excess due to the nature of the reaction. It should be noted that if the N-phenylanthranilic acid is wet from the previous synthesis, steam will be generated as the polyphosphoric acid is heated.  The large sided beaker helps to control this.    

** The termperature was controlled using a sensor on the hotplate. Constant stirring was maintained using a stirrer bar and controlled via the hot plate. The reaction did not require dry/inert conditions (as water is generated as a by-product) and took place in open air.  
*** Other methods of purification could involve piyring the reaction slowly into a stirring solution of hot water (80°C) rather then over ice.  Although this works, its can generate a vigourous reaction and serious consideration needs to be given to going down this route based on safety aspects.