Aluminium oxalate, Al₂(C₂O₄)₃

Aluminium oxalate, Al₂(C₂O₄)₃.H₂O, is precipitated when aqueous solutions of an aluminium salt and an alkali oxalate are mixed. It is soluble in oxalic acid.

When aluminium hydroxide is dissolved in hot, concentrated aqueous solutions of the alkali acid oxalates and the solutions allowed to cool, crystalline double or complex oxalates are obtained of the type M₃^IAl(C₂O₄)₃.xH₂O. The salts are very similar to the corresponding chromic, ferric, and cobaltic double oxalates, and salts having the same number of molecules of water of crystallisation are isomorphous. The salts M₃^IM^III.(C₂O₄)₃.3H₂O, for instance, are all monoclinic: -

salt	a	b	c	β
K.Al	0.9994	1	0.3951	92°51'
Rb.Al	1.0188	1	0.4049	95°2'
NH4.Al	0.9971	1	0.3915	92°26'
Tl.Al	0.9730	1	0.4128	93°39'
K.Cr	1.0060	1	1.3989	94°0'
Rb.Cr	1.0221	1	0.3963	92°25'
NH4.Cr	0.9830	1	0.3870	95°18'
K.Fe	0.9918	1	0.3896	94°15'
Rb.Fe	1.0106	1	0.4004	95°12'
NH4.Fe	0.9959	1	0.3932	92°15'
Tl.Fe	0.9601	1	0.4115	94°5'
NH4.Co	1.0017	1	0.3929	92°44'

The following are also isomorphous groups of salts:-

monoclinic	triclinic	monoclinic
Na3Al(C2O4)3.5H2O; Na3Cr(C2O4)3.5H2O; Na3Fe(C2O4)3.5H2O; Na3Co(C2O4)3.5H2O;	Li3Al(C2O4)3.6H2O; Li3Cr(C2O4)3.6 H2O; Li3Co(C2O4)3.6H2O;	Na3(NH4)3Al2(C2O4)6.7H2O; Na3Rb3Cr2(C2O4)6.7H2O; Na3(NH4)3Cr2(C2O4)6.7H2O; Na3(NH4)3Fe2(C2O4)6.7H2O