7 the total solubility curve follows the line

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+1, and at pH > 11.7, the total solubility curve follows the line representing the concentration of H 2 SiO 4 2- . Near each of the pK values (i.e., the crossover points), significant and nearly equal concentrations of two species are present, so the total solubility curve rises above the lines for the individual species. Also shown on this plot is the total solubility curve for amorphous silica (dotted green line). This curve has exactly the same shape as that for quartz, but is displaced upward by 1.3 log units (log 20 = 1.3), which is reflective of the fact that amorphous silica is 20 times more soluble than quartz. The plot illustrates that, over the pH range of most natural waters, silica solubility is independent of pH. However, as pH rises above 9, the solubility of
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SILICA SOLUBILITY - V An alternate way to understand quartz solubility is to start with: SiO 2 (quartz) + 2H 2 O(l) H 4 SiO 4 0 Now adding the two reactions: SiO 2 (quartz) + 2H 2 O(l) H 4 SiO 4 0 K qtz H 4 SiO 4 0 H 3 SiO 4 - + H + K 1 SiO 2 (quartz) + 2H 2 O(l) H 3 SiO 4 - + H + K 4 10 0 4 4 SiO H qtz M K 4 log log 0 4 4 SiO H qtz M K  H SiO H qtz a M K K K 4 3 9 . 13 9 . 9 4 1 10 10 10
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SILICA SOLUBILITY - VI Taking the log of both sides and rearranging we get: Finally adding the three reactions: SiO 2 (quartz) + 2H 2 O(l) H 4 SiO 4 0 K qtz H 4 SiO 4 0 H 3 SiO 4 - + H + K 1 H 3 SiO 4 - H 2 SiO 4 2- + H + K 2 SiO 2 (quartz) + 2H 2 O(l) H 2 SiO 4 2- + 2H + K pH M SiO H 9 . 13 log 4 3     2 6 . 25 7 . 11 9 . 9 4 2 1 2 4 2 10 10 10 10 H SiO H qtz a M K K K K pH M SiO H 2 6 . 25 log 2 4 2
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pH 2 4 6 8 10 12 14 log activity of dissolved silica species -5 -4 -3 -2 -1 0 Amorphous silica H 4 SiO 4 0 H 2 SiO 4 2- H 3 SiO 4 - Total Quartz Activities of dissolved silica species in equilibrium with quartz and amorphous silica at 25°C. Note that silica solubility is pH-independent at pH < 9, but increases dramatically with increasing pH at pH >9.
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This plot illustrates the principles discussed in the previous slide. The light red lines show the concentrations of the various dissolved silica species. The concentration of H 4 SiO 4 0 is represented by the horizontal line. The concentration of H 3 SiO 4 - is represented by the straight line with slope +1, and that of H 2 SiO 4 2- by the straight line with slope +2. The points where the lines for the concentrations of two successive species cross occur at the pK values for silicic acid. For example, the lines representing the species H 4 SiO 4 0 and H 3 SiO 4 - cross at pH = pK 1 = 9.9, and the lines for the species H 3 SiO 4 - and H 2 SiO 4 2- cross at pH = pK 2 = 11.7. The heavy dark red curve represents the logarithm of the sum of the concentrations of all the species, that is, the total solubility of quartz. At pH < 9.9, H 4 SiO 4 0 accounts for almost all the dissolved silica, so the curve representing the total solubility is nearly coincident with the line representing the concentration of H 4 SiO 4 0 . Similarly, at 9.9 < pH < 11.7, the predominant species is H 3 SiO 4 - , so the total solubility curve has a slope near +1, and at pH > 11.7, the total solubility curve follows the
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