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
