(NH4)2SO4 is a soluble salt and is fully ionized in solution: (NH4)2SO4 => 2 NH4+ + SO42- NH4+ is a weak acid and ionizes further: NH4+<=> NH3 + H+ The H+ formed can react with C2O42- to form the weak acid oxalic acid (H2C2O4): C2O42- + 2 H+ <=> H2C2O4 Removal of C2O42- causes the equilibrium: ZnC2O4 <=> Zn2+ + C2O42- to shift to the right according to Le Chatelier\'s principle and more ZnC2O4 dissolves. Thus the solubility of ZnC2O4 increases in the presence of (NH4)2SO4. Solution (NH4)2SO4 is a soluble salt and is fully ionized in solution: (NH4)2SO4 => 2 NH4+ + SO42- NH4+ is a weak acid and ionizes further: NH4+<=> NH3 + H+ The H+ formed can react with C2O42- to form the weak acid oxalic acid (H2C2O4): C2O42- + 2 H+ <=> H2C2O4 Removal of C2O42- causes the equilibrium: ZnC2O4 <=> Zn2+ + C2O42- to shift to the right according to Le Chatelier\'s principle and more ZnC2O4 dissolves. Thus the solubility of ZnC2O4 increases in the presence of (NH4)2SO4..