EFFECT OF PH ON REACTION OF SODIUM-FLUORIDE WITH HYDROUS OXIDES OF SILICON, ALUMINUM, AND IRON, AND WITH POORLY ORDERED ALUMINOSILICATES

The release of hydroxyl ions from silica and ferric oxide gels on treatment with sodium fluoride solution is very small above pH values of 7.6 and 9, respectively. Hydroxyl release from alumina gel and poorly ordered aluminosilicates is appreciable at pH 9 and varies little with pH, but that from crystalline forms of alumina decreases with decrease of pH. For poorly ordered aluminosilicates, the ratio (OH'' released at pH 8.0):(OH'' released at pH 6.8) is directly proportional to the mole fraction Al/(Al + Si). The measurement of hydroxyl release at differing pH values may enable determination of hydrous alumina separately from the total poorly ordered inorganic gel material; the amount of hydroxyl released at high pH values is related to phosphate sorption capacity.