CEE 680

Fall 2011

Water Chemistry

Homework #9

Aluminum Solubility Continued

Below are the relevant chemical equilibria for amorphous aluminum hydroxide and aluminum phosphate. These will be used in the problems below.

Equilibria	Log K
AlOH3(s) (amorphous) + 3H+ = Al+3 + 3 H2O	10.8
Al+3 + H2O = AlOH+2 + H+	-4.97
Al+3 + 2H2O = Al(OH)2+ + 2H+	-9.3
Al+3 + 3H2O = Al(OH)3(aq) + 3H+	-15.0
Al+3 + 4H2O = Al(OH)4- + 4H+	-23.0
AlPO4•2H2O(s) (variscite) = Al+3 + PO4-3 + 2H2O	-21
H3PO4 = H+ + H2PO4-	-2.15
H2PO4- = H+ + HPO4-2	-7.2
HPO4-2 = H+ + PO4-3	-12.3

9.1 Solubility and Mixed Solids (4 points)

Prepare a solubility diagram for Aluminum in water. Assume that the hydroxide phase that forms is amorphous. Also assume that 0.5 mM total phosphate is present in the system. Present the diagram in the usual form (log C vs pH). Outline the zones of precipitation and mark the identity of the precipitates.

9.2. Predominance Diagram (4 points)

Prepare a predominance diagram based on the Aluminum Hydroxide / Aluminum Phosphate equilibria in the table above. Use a soluble aluminum concentration (Al_T of 1 mM). Again, you should outline the zones of precipitation and mark the identity of the precipitates. Outside of these zones, you should indicated the principal soluble species.

Assigned: 28 Nov 11

Due: 5 Dec 11