4、 Aluminum system When pH<4.0, Al ions exist in Al(H, o)'+ form 2A(OH2)3+20H→A2(OH)2(H2O)4+4H2O 4A(OH2)6+6OH一[A14(OH)6(OH2)2+12H2O When pH>5.0,Al ions become Al(OH)3 precipitation, it will lose its buffering capacity 5、 Organic acid system4、Aluminum system When pH<4.0, Al ions exist in Al(H2O)6 3+ form 2 2 4 2 3 3 2 2 2 8 4 Al(OH ) OH Al (OH) (H O) H2 O + − + + + Al OH OH Al OH OH H2 O 6 4 6 2 12 3 2 6 4 ( ) + 6 [ ( ) ( ) ] +12 + + When pH>5.0 , Al ions become Al(OH)3 precipitation，it will lose it’s buffering capacity. 5、Organic acid system