The pH Dependence on the Formation Process of Tungsten Oxide Gel

In order to investigate the pH-effect on gelation process of tungstic acid, the simultaneous measurements of transmitted-light intensities were carried out for specimens with several different-pH values (pH=1.3~1.6), prepared by adding different amounts of NaOH aqueous solution into ion-exchanged tungstic acid. The transmitted-light intensity decreased faster, as the pH-value of the specimen became lower. Moreover, it was found that the transmitted-light intensities of the different-pH specimens showed a unique time dependence when the observation time was scaled by T1/2 (the time when the transmitted-light intensity became the middle value of the initial and the final intensity). This result indicates that, though the light-scattering mechanism in tungstic acid might change during gelation process, the mechanism of gelation process itself is same irrespective of pH-values. In addition, the critical pH value for the gelation was extrapolated to be 1.71 from T1/2 vs. pH value curve.