Table 3.

Stepwise stability constants (log K) of H₄pypa with Sc³⁺, Lu³⁺, Y³⁺ and La³⁺ ions.

Equilibrium reaction	Sc3+	Lu3+	Y3+	La3+
M3+ + L ⇆ ML	26.98(1)30	22.20(2)41	21.60(1) (a)	19.74(3)41
ML + H+ ⇆ MHL	3.55(2)30	3.60(6)41	2.14(5)(b) 2.29(c)	3.24(5)41
M(OH)L + H+ ⇆ ML	10.34(3)30	10.77(8)41	-	-
M2L2(OH) + H+ ⇆ M2L2	-	-	37.23(a)	34.40(6)41
pM(d)	27.130	22.641	22.0	19.941

^a)from direct UV-potentiometric titration method at 25 °C, I = 0.16 M NaCl;

^b)from EDTA potentiometric competition titration method at 25 °C, I = 0.16 M NaCl;

^c)from direct UV-acidic competition titration method at 25 °C, I ≠ 0.16 M NaCl;

^d)pM is defined as −log [M]_free at [L] = 10 μM, [M] = 1 μM and pH = 7.4.