Table 1 Ligand and proton association constants at different ionic strength (mol dm−3, in NaCl), T = 298.1 K.

From: Synergistic use of siderophores and weak organic ligands during zinc transport in the rhizosphere controlled by pH and ion strength gradients

Ligand

Equilibrium

0.05

0.15

0.3

0.5

1

log β0

C

Citrate

H+ + Cit3- ⇌ HCit2-

5.69±0.01

5.44 ± 0.01

5.35 ± 0.01

5.26 ± 0.01

5.12 ± 0.01

6.198 ± .001

0.163 ± 0.001

2H+ + Cit3- ⇌ H2Cit-

10.18 ± 0.02

9.57 ± 0.02

9.45 ± 0.02

9.32 ± 0.02

9.13 ± 0.02

10.913 ± 0.005

0.254 ± 0.012

3H+ + Cit3- ⇌ H3Cit

13.00 ± 0.05

12.10 ± 0.03

11.94 ± 0.04

11.70 ± 0.03

11.53 ± 0.03

13.780 ± 0.006

0.142 ± 0.180

Zn2+ + H+ + Cit3− ⇌ [Zn(HCit)]

10.1 ± 0.1

8.52 ± 0.01

8.15 ± 0.02

7.83 ± 0.02

7.87 ± 0.02

10.640 ± 0.042

−0.147 ± 0.081

Zn2+ + Cit3− ⇌ ZnCit−

5.9 ± 0.1

4.885 ± 0.005

4.593 ± 0.006

4.310 ± 0.005

4.207 ± 0.005

6.583 ± 0.009

−0.057 ± 0.015

Zn2+ + 2Cit3− ⇌ [Zn(Cit)2]4−

nd

7.13 ± 0.04

6.85 ± 0.04

6.40 ± 0.06

6.88 ± 0.02

7.400 ± 0.07

0.284 ± 0.06

2Zn2+ + 2Cit3- + 2H2O ⇌ [Zn2(OH)2(Cit)2]4- + 2H+

−1.8 ± 0.2

−2.61 ± 0.01

−2.81 ± 0.02

−3.09 ± 0.02

−2.51 ± 0.02

−1.142 ± 0.019

0.902 ± 0.033

Zn2+ + Cit3− + 3H2O ⇌ [Zn(OH)3(Cit)]4- + 3H+

−21.9 ± 0.1

−22.46 ± 0.01

−22.92 ± 0.03

−22.63 ± 0.02

−23.05 ± 0.01

−21.601 ± 0.023

−0.248 ± 0.050

DFOB

H+ + DFOB3- ⇌ HDFOB2-

11.07 ± 0.07

10.74 ± 0.04

10.36 ± .02

10.35 ± 0.02

10.14 ± 0.06

11.491 ± 0.002

0.169 ± 0.003

2H+ + DFOB3- ⇌ H2DFOB-

20.94 ± 0.09

20.25 ± 0.06

19.84 ± 0.04

19.77 ± 0.04

19.77 ± 0.08

21.530 ± 0.006

0.173 ± 0.011

3H+ + DFOB3- ⇌ H3DFOB

30.05 ± 0.12

29.10 ± 0.09

28.61 ± 0.07

28.60 ± 0.08

28.57 ± 0.12

30.691 ± 0.012

0.194 ± 0.028

4H+ + DFOB3- ⇌ H4DFOB+

38.83 ± 0.14

37.46 ± 0.13

37.00 ± 0.10

36.97 ± 0.16

36.92 ± 0.14

39.250 ± 0.023

−0.079 ± 0.043

Zn2+ + DFOB3- ⇌ [Zn(DFOB)]-

11.67 ± 0.06

9.91 ± 0.02

9.97 ± 0.04

9.39 ± 0.05

9.32 ± 0.04

12.027 ± 0.039

−0.486 ± 0.077

Zn2+ + H+ + DFOB3- ⇌ [Zn(HDFOB)]

21.55 ± 0.11

19.85 ± 0.04

19.85 ± 0.06

19.34 ± 0.09

19.38 ± 0.06

22.105 ± 0.026

−0.075 ± 0.053

Zn2+ + 2H+ + DFOB3- ⇌ [Zn(H2DFOB)]+

29.30 ± 0.13

27.81 ± 0.05

27.42 ± 0.08

27.19 ± 0.12

27.25 ± 0.08

29.849 ± 0.028

0.009 ± 0.051

  1. The precisions  for the conditional (logβ) and intrinsic (logβ0) association constants are represented using experimental and model errors, respectively. The error for logβ is calculated from the titration fit using least quare optimisation within the Hyperquad program55. The error on the logβ0 is calculated using Monte Carlo simulations59.
  2. nd  not determined.
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