Effect of pH on the measurement of biogenic silica

For investigations of the biogenic silica content of plankton, sediment trap, and sediment samples, alkaline extraction is the most common procedure. Different extraction techniques, with respect to the pH of the leaching agent and the method applied to correct for the release of SiO2 by non-biogenic silica phases as clays or feldspars, are applied. By sequential extraction of marine sediment samples and clay standards with solutions of pH 8 to 14, we investigated the sensitivity of these leaching techniques to the pH of the leaching solution. In addition to the dissolved silica concentration, the temporal increase of the dissolved aluminum concentration of the leaching agent was measured. For sediments of low biogenic silica content, the results are considerably affected by the pH of the leaching agent and relative deviations of more than 300% were observed. Correction by the calculated SiO2/Al2O3 ratios of non-biogenic silica phases had no significant effect on the observed pH dependence. Measurements of samples which exceed 10% biogenic silica are only negligibly affected by the pH of the leaching solution. Based on the pH dependence and investigations of a pure Miocene diatom sample, sequential alkaline extraction with a pH of 12.5 is recommended for the determination of biogenic silica.