“…The zirconiumalizarin red S mixture utilizes color measurement at 520-525 mfi (28). The alizarin red S reaction product with fluoride has also been titrated with thorium nitrate, which depends upon the formation of the insoluble thorium fluoride (28). Two other zirconium lakes which have a low sensitivity to foreign ions and a high sensitivity to fluoride are the zirconium-eriochrome cyanine R (29) and zirconium-SPADNS [4,5dihydroxy-3-~p-sulfophenylazo)-2,inaphthalenedisulfonic acid, trisodiurn salt] (30).…”