Abstract
A potentiometric method employing quinhydrone and solid-state fluoride electrodes was used to observe the free hydrogen ion and the free fluoride ion concentrations in equilibrium experiments in aqueous solutions at 0 to 60/sup 0/C. In dilute silicic acid solutions the predominant reaction can be expressed as Si(OH)/sub 4/(aq) + 4H/sup +/ + 6F/sup -/ reversible SiF/sub 6//sup 2 -/ + 4H/sub 2/O. The logarithm of the equilibrium quotient has values of 31.610, 29.980, and 28.23 at 0, 25, and 60/sup 0/C in 1 m NaCl (molal units). There is evidence for the existence of small amounts of additional species with fewer fluorides under certain conditions. The absence of a regular sequence of species from the array Si(OH)/sub 4-x/ F/sub y//sup x-y/ is proven by an analysis of the results. The average number of fluoride ions complexed by millimolal silicic acid approaches 6.0 in 0.01 m fluoride as the pH is reduced to 3 or lower in titration experiments. Simultaneously the average number of hydrogen ions consumed approaches 4.0. 2 tables, 4 figures.
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