NaOH过量：Al2(SO4)3+8NaOH=2NaAlO2+4H2O+3Na2SO4
Al3+ +4OH- =AlO2-+2H2O
Al2(SO4)3过量：Al2(SO4)3+6NaOH=2Al(OH)3+3Na2SO4
Al3+ +3OH- =Al(OH)3
(Al(OH)3都要加沉淀符号）