硅酸及其盐的研究(Ⅻ)—氟离子与硅酸的反应及其平衡常数

STUDIES ON SILICIC ACID AND ITS SALTS(XII)-REACTIONS OF FLUORIDE-ION WITH SILICIC ACID AND THEIR EQUILIBRIUM CONSTANTS

在酸性范围内,在不同F/SiO_(2)值及不同硅酸浓度的溶液中,用氟电极法测得氟和硅酸的结合量,并求得反应式:Si(OH)_(4)+HF■[Si(OH)_(3)F]+H_(2)O其平衡常数β_(1)=1.7×10^(2)。从而验证了在酸性含氟硅酸溶液中主要存在着这种初始反应。同时求算了六氟硅酸的总形成常数β_(6)=2×10^(12)。

Fluoride-ion combines with silicic acid only in acid solution,although it catalyses the polymerization of the silicic acid in neutral and alkaline solution.Fluoride-ion concentrations in silicic acid solution of different concentrations and varied F/SiO_(2) ratio were measured with fluoride-ion selective electrode.Results of measurements showed that a decrease of[H^(+)]was accompanied by an increase of fluoride concentration in solution,i.e.the amount of fluoride-ion combined with silicic acid decreased.At[H^(+)]<10^(-5)M,there was no combination at all.These findings are in agreement with the mechanism which was proposed previously concerning the effect of fluoride-ion on polymerization of silicic acid,In acid solution,when fluoride concentration is far less than that of silicic acid,the chief species of the latter is Si(OH)_(4) and that of fluoride is HF and their reaction may be represented as:The for mation constantβ1 of the product formed was found to be.β1=1.7×10^(3).At high acidity,[Si(OH)_(8)OH_(2)]+might be formed and its reac tion with HF is given as:When the ratio F/SiO_(2) was higher,the following reaction took place.Si(OH)_(4)+6HF=[SiF_(6)]^(2-)+2^(H+)+4H_(2)O.The gross constant,β_(6) was found to be:2×l0^(12).