Again, as in previous chapters, the best place to investigate for determining the modes of removal is the table of solubility products constants as shown in Table 14.1. A precipitation product that has the lowest Ksp means that the substance is the most insoluble. As shown in the table, the phosphate ion can be precipitated using a calcium precipitant producing either Ca5(PO4)3(OH)(s) or Ca3(PO4)2. Of these two precipitates, Ca5(PO4)3(OH)(s) has the smaller Ksp of 10 ; thus, it will be used as the criterion for the precipitation of phosphates. Ca5(PO3)3(OH)(s) is also called calcium hydroxy apatite.
As shown in the table, the other mode of precipitation possible is through precipitating the phosphate ion as AlPO4(s) and FePO4. The precipitant normally used in this instances are alum and the ferric salts (ferric chloride and ferric sulfate), respectively.
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