_ _ _____________________________________________________ Al Purpose and scope._______________________________________________ 2 Acknowledgments. ________________________________________________ 2 Previous studies.___________________________________________________ 2 Dissolved and suspended species___.______ _______________________ 3 Nature of precipitated solids, _-----__-__-________-_---___-______ 7 Structures of aqueous aluminum species_____________________________ 8 Hydrolysis experiments.--..--_____________-_____._____--_____--_-__ 11 Behavior of anionic aluminum species.___________________________ 15 Behavior of cationic aluminum species.___________________________ 20 Potentiometric measurements._-___-_-_-_-___-__-___________ 20 Measurement and significance of aluminum concentrations______ 23 Filtration experiments.____----_____--___-_____--__-_-______ 24 Characteristics of filterable material--_____-_____--_.__-______ 26 Structure of aluminum hydroxide polymers. __________________ 26 Mechanisms of polymerization.____-___________--__---______ 32 Verification of polymeric structure.__________________________ 33 Electron microscopy. ___--____--____--____-___._--__-__ 33 Reaction-rate studies___________________________________ 33 Interpretation of results of rate studies.____----____--____ 41 Direct determination of unpolymerized aluminum. ______ _.__ 43 Replacement of hydroxide by fluoride ions__-_____-___-_-_____ 45 Calculation of aluminum hydroxide solubility.________________ 46 Calculation of *Ki____---__--_____--____-____--_____---___48 Factors controlling pH_____________________________________ 50 Summary and conclusions. _________________________________________ 51 Application to natural conditions._______________________________ 52 Literature cited._______ _____________-________----_____._-----___53

Fluoride reacts with silicic acid to form SiF 2− 6 . A fluoride electrode was used to obtain an equilibrium constant of 10 30.18 for the reaction: Si(OH) 0 6 + 6 F − +4 H + = SiF 2− 6 + 4 H 2 O at 25°C Although there may be some experimental evidence for existence of traces of species containing less than six F − ions per silicon ( n = 6), the species SiF 2− 6 predominates for n values from about 0.1 to 6. Silicic-acid complexing with fluoride is important only in solutions which have rather low pH and low concentrations of other cations which compete with silicon for fluoride. Computations for cold volcanic condensates from Hawaii indicate that for some samples much of the silicon is complexed by fluoride as SiF 2− 6 . However, in most cooled acidic natural water samples Al and Fe are more important than Si in complexing fluoride.

The chemistry of water of Aqua de Ney, a cold spring of unusual character located in Siskiyou County, Calif., has been re-examined as part of a study of the relation of water chemistry to rock environment. The water has a pH of 11·6 and a silica content of 4000 parts per million (p.p.m.), the highest values known to occur in natural ground waters.