Catalytic effect of fluoride on silica polymerization at neutral pH

The catalytic effect of fluoride on the polymerization of sodium silicate and phosphoric acid solutions at pH 7.15 was studied through a simple and inexpensive method: the observation of the meniscus of the resulting sol in a test tube. Reaction was found to be second-order regarding silicate concentration as reported previously. Novel results show that the reaction also follows a pseudo-second-order kinetics with respect to fluoride. A rate law for the catalyzed polymerization of the form r = K v [Si(OH) 4 ][HF] 2 was established. A mechanism for fluoride action on silica polymerization, where silicon increases its coordination number to five, is proposed.