“…The procedure of deBoer has been modified by Casares (7), Thompson and Taylor (8), Elvove (9), and the writer (10). In this method, use is made of the fact that, in an acid solution, fluorides cause a zirconium-alizarine mixture, or a solution of purpurin (1,2,4, trihydroxyanthraquinone) to change color from pink to yellow, the quantity of fluorine present being estimated by the amount of fading of the pink color.…”