Table 1.

Solubility products of phosphate and carbonate salts of metals commonly used for phosphopeptide enrichment.^{84, 85}

Metal	Phosphate salt	Ksp	[PO43−] (M)	Carbonate salt	Ksp	[CO32−] (M)	Selectivity
Calcium	Ca3(PO4)2	2 × 10−33	5 × 10−17	CaCO3	3 × 10−9	3 × 10−9	8 × 107
Iron (III)	FePO4•2H2O	10 × 10−16	10 × 10−15	FeCO3*	3 × 10−11*	3 × 10−11*	3 × 104*
Gallium (III)	GaPO4	1 × 10−21	1 × 10−21	Ga2(CO3)3	N/A	-	-
Barium	Ba3(PO4)2	3 × 10−23	6 × 10−12	BaCO3	2 × 10−9	2 × 10−9	8 × 1013

The calculated phosphate and carbonate concentrations listed are those required for precipitation with the corresponding metal if the metals are assumed to be at a normalized unit molarity of 1 M. Selectivity was calculated by dividing the minimum carbonate concentration for precipitation by the minimum phosphate concentration required for precipitation. All calculations are shown in the Supporting Material.

^*Since the K_sp for Fe₂(CO₃)3 was not available we used the K_sp of FeCO₃ as an estimate.