When the ferric salts are used to precipitate FePO4, the PO^- ion must compete with the OH- ion. When the pH is high, there will be a large concentration of the OH-and the phosphate ion can lose the competition and fail to precipitate; Fe(OH)3 may precipitate instead of FePO4. Thus, to precipitate the phosphate ion, the concentration of OH- must be suppressed. This is done by adding more H +, which reacts with the OH-. If the OH- is "busy" satisfying the hydrogen ion, the Fe3+ ion is now available for reaction with phosphate to precipitate FePO4 instead of Fe(OH)3. The addition of the hydrogen depresses the pH and the optimum pH is found to be equal to or less than 3.
For the case of the AlPO4 precipitation, alum also produces the hydroxide Al(OH)3 analogous to the ferric salts producing Fe(OH)3. Thus, for the phosphate ion to outcompete the hydroxide ion, the pH must also be lowered in order to produce the desired precipitate of AlPO4, instead of Al(OH)3. Again, the pH is reduced by adding the hydrogen ion. The optimum pH range for AlPO4 precipitation, we found to be equal to or less than 5.
In the case of the use of lime, the OH - ion is a reactant for the production of the apatite, Ca5(PO4)3OH(s). This is the reason why the pH must be raised, which is found to be equal to or greater than 7. In addition, no other precipitate competes with the precipitation of the apatite. Thus, overall, of the three precipitants, lime is the best.
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