STUDIES ON SILICIC ACID GELS. VIII: The Time of Set of Gels Containing Strong Acids

Abstract

The time required for the setting of a sol of hydrated silica, giving what we call a silicic acid gel, has been found to depend primarily upon the concentration of silica, the temperature, and the hydrogen-ion concentration. A careful study of the relation between time of set, as we have defined it, and the temperature on the one hand and the hydrogen-ion concentration on the other, for gels produced from solutions of sodium silicate and acetic acid, has already been reported from this laboratory in previous papers of this series (3, 4, 5). In addition, the same study, extended to other weak acids,-citric, succinic, and tartaric,-was reported by Hurd (1).We have suspected that the weak acids, because of the buffering effect of the sodium salts formed during the reaction with the sodium silicate, give a practically constant hydrogen-ion concentration. Hence the reaction of setting, whose rate manifestly depends upon the hydrogen-ion concentration should, and does, proceed at a more uniform rate than it would in a solution whose hydrogen-ion concentration was not so constant.In order to investigate this phenomenon, we have made a study of the time of set of gel mixtures produced by mixing solutions of sodium silicate with solutions of the strong acids, nitric, hydrochloric, and sulfuric. The effect of temperature and of hydrogen-ion concentration upon the time of set of such mixtures is reported in this paper. EXPERIMENTALThe gel mixtures were produced in the same way as those reported in the former papers of this series. A large volume of sodium silicate solution was made by dilution of "E" brand silicate, produced by the Philadelphia Quartz Company, with recently boiled, distilled water. The large amount ensured uniform concentration throughout this investigation. This solution titrated 1.255 N, as equivalent in sodium hydroxide against 85

Studies on Silicic Acid Gels. X. The Time of Set of Gel Mixtures Containing High Concentrations of Mineral Acids.

“…In the least acid of these highly acid mixtures, a small pH drift was noted, always an increase. This disagrees with Batchelor's (1) results, but agrees with those of Hanks and Weintraub (2) and with former results from this laboratory (6). This small drift of pH agrees with the theory explained by Hurd, Frederick, and Haynes (6), namely, that during condensation the silicic acid becomes successively weaker.…”

Theories for the Mechanism of the Setting of Silicic Acid Gels.

“…The use of strong acids adds the complication of a change in pH of the mixture during setting. By making allowances for this (34) it is possible to show that the energy of activation is approximately the same when strong acids are used.…”

“…Measurements of the relation between time of set and the hydrogen-ion concentration in alkaline gel mixtures have proved very difficult, owing to the fact that the hydrogen-ion concentration changes as the gel mixture proceeds toward setting. The shift in the hydrogen-ion concentration in the poorly buffered solutions produced by mixing a solution of sodium silicate with insufficient hydrochloric acid to neutralize it is always toward lower hydrogen-ion concentrations (34). In other words, at first the monosilicic acid formed is enough of an acid to contribute a measurable concentration of hydrogen ions, but as condensation proceeds the polysilicic acids are less and less ionized, resulting in a shift toward lower hydrogen-ion concentration.…”

Studies of Silicic Acid Gels. IX. The Effect of a Change of pH upon the Time of Set of Some Acid Gels.

“…The technique employed has been reported several times in this Journal (5,2). A solution of sodium silicate equivalent to 1.26 N sodium hydroxide was obtained by dilution of "E" brand sodium silicate, prepared by the Philadelphia Quartz Company.…”