Question
Consider the equilibrium of an aqueous solution containing solid manganese dioxide as indicated in the following equation. MnO-_4 (aq) + 4H_jO^+ (aq) + 3e- MnO_2 (s) + 6H_2O How arc the solution's pH and pE affected by the individual addition of each of the following: MnO_2 CO_2(g), Fe^2+ (aq), and CaCO_3? A potential measurement in a paddy field yields a reading of -278 mV versus the saturated calomel reference electrode. What is the pE value for this soil? Iron toxicity has occasionally been observed in rice plants grown in lowlands (submerged soils). Use the simple pE/pH diagram for iron in Fig. 10.P.1 to explain how this can happen.
Explanation / Answer
5) Given potential measurement = -278 mV = -0.278 V
The potential E recalculated using the potential of saturated calomel electrode of value +0.242 V is:
E = -0.278 + 0.242 = -0.036V
pE = E/0.0592 = -0.036/0.0592 = -0.608