Neutral Solutions—Effect of Salts

Rate measurements on fine amorphous powder and colloids have been made by Doremus et al. (224) and by Friedberg (225). The effects of pH, temperature, and presence of salts were similar to those reported by others. More than 50 years ago, Dienert and Wandenbulcke (226) reported the basic facts that colloidal silica passed into solution as soluble silica, which was detectable colorimetrically with molybdic acid, and that alkalinity and salts were good catalysts for dissolution. They made an observation which apparently has never been followed up. They claimed that when salt is present, the dissolution rate is faster in a quartz container than in platinum and that in the absence of added salt, colloidal silica would pass into the soluble state when heated with water in quartz, but not in platinum. However, pH measurements were not made. 

In a study of the attack of water on glass, Wiegel (227) found that glass was more resistant to attack by water than by salt solution. Also when alkali vvas first extracted from glass by acid, it was then more resistant to attack by water, no doubt because there was no soluble electrolyte to build up in the water. Finally, he found that copper, zinc, tin, and aluminum all reduced the amount of alkali extractable from glass by water, while nickel, cobalt, and magnesium increa.sed it. 

It thus appears that in very pure water, in the absence of impurities which can leach from silica into water, silica is remarkably inert (167a), remaining undissolved for long periods of time, whereas it rapidly dissolves in seawater. Impurities have such marked effects in neutral solution that it is doubtful if reproducible rates of 

Van Lier (114) found that NaCl increased the rate of solution of quartz in water by factors of about 4, 14, and 67 in 10 , 10 , and 10 N solutions. 

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