Most surface waters have silica dissolved in it in the form of silicic acid H4SiO4 (pKa = 9.5) and its ionisation product H3SiO4- .
(a) Calculate the mole fraction of H3SiO4- and of H4SiO4 at pHs of 5.0, 6.0, 7.0, 8.0, 9.0, 10.0 and 11.0. The total silica concentration (H3SiO4- + H4SiO4) is 0.2 mM. Assume that activity = concentration.
(b) Draw a plot with mole fraction on the y-axis and pH on the x-axis. Use the pH range 55.0 to 11.0.
(c) On the plot, at what pH do the two mole fraction lines intersect? What is the significance of this pH?