Table 1.

Protonation Constants of macropa²⁻, macropaquin²⁻, and macroquin–SO₃⁴⁻ and Thermodynamic Stability Constants of Their Alkaline Earth Complexes Determined by pH-Potentiometry (25 °C and I = 0.1 M KCl).^a

	macropa2−	macropaquin2−	macroquin–SO34−	DOTA4–b	DTPA5–c
log Ka1	7.41(1) (7.41)d	10.33(4)	9.34(4)	11.14	10.34
log Ka2	6.899(3) (6.85)	7.15(3)	9.43(1)	9.69	8.59
log Ka3	3.23(1) (3.32)	6.97(2)	6.75(4)	4.85	4.25
log Ka4	2.45(5) (2.36)	3.24(4)	6.62(4)	3.95	2.71
log Ka5	(1.69)				2.18
log KCaL	5.79(1) [5.25]e	5.90(4)	6.04(8)	16.37	11.77
log KCaHL		8.59(2)	8.60(4)	3.60	6.10
log KSrL	9.442(4) [9.57]	9.19(5)	8.62(2)	14.38	9.68
log KSrHL	3.35(8) [4.16]	8.92(2)	8.34(4)	4.52	5.4
log KSrH2L			6.920(3)
log KBaL	11.11(4)	10.87(2)	10.44(6)	11.75	8.78
log KBaHL	3.76(2)	9.76(2)	9.24(7)		5.34
log KBaH2L	2.49(7)	3.28(2)	7.80(2)
log KʹCaf	5.42	3.94	3.19	10.34	7.63
log KʹSrf	9.07	7.54	5.64	8.35	5.53
log KʹBaf	10.74	10.05	8.76	5.72	4.63
pCag	6.54	6.04	6.01	11.29	8.59
pSrg	10.02	8.50	6.70	9.30	6.61
pBag	11.69	11.01	9.72	6.76	6.15

^aData reported previously for DOTA⁴⁻ and DTPA⁵⁻ are provided for comparison.

^bRef 21, I = 0.1 M KCl.

^cProtonation constants and log K_CaL from Ref 42, I = 0.1 M KCl. Other values from Ref 15.

^dParenthetic values from Ref 27, I = 0.1 M KCl.

^eBracketed values from Ref 29, I = 0.1 M KNO₃.

^fConditional stability constant at pH 7.4, 25 °C, and I = 0.1 M KCl.

^gCalculated from −log [M²⁺]_free ([M²⁺] = 10⁻⁶ M; [L] = 10⁻⁵ M; pH 7.4; 25 °C; I = 0.1 M KCl).