Acidity of IHSS Samples

Sample

Cat. No.

Carboxyl

Phenolic

Q₁

Log K₁

n₁

Q₂

Log K₂

n₂

RMSE

Standard HA

Suwannee River I *

1S101H

9.59

4.24

10.69

4.42

3.79

2.28

9.68

1.11

113

0.0673

Suwannee River II

2S101H

9.13

3.72

9.74

4.35

3.30

4.48

10.44

1.73

171

0.0815

Elliott Soil

1S102H

8.28

1.87

8.90

4.36

3.16

0.85

9.80

1.00

169

0.0943

Pahokee Peat

1S103H

9.01

1.91

9.64

4.22

3.20

0.94

9.86

1.00

145

0.0865

Leonardite

1S104H

7.46

2.31

8.17

4.59

3.32

1.13

9.72

1.31

106

0.0532

Standard FA

Suwannee River I

1S101F

11.44

2.91

12.00

3.80

3.21

1.48

9.52

1.00

109

0.1053

Suwannee River II

2S101F

11.17

2.84

11.66

3.76

3.24

2.05

9.84

1.45

123

0.1072

Elliott Soil I *

1S102F

Elliott Soil II

2S102F

13.24

2.27

14.12

3.67

3.62

0.74

9.53

1.00

111

0.1375

Pahokee Peat I *

1S103F

13.34

2.32

14.22

3.99

3.33

0.76

9.57

1.00

117

0.1028

Pahokee Peat II

2S103F

Reference HA

Suwannee River *

1R101H

Pahokee Peat

1R103H

8.87

2.05

9.54

4.26

3.25

1.01

9.85

1.00

117

0.0773

Nordic Lake

1R105H

9.06

3.23

10.32

4.32

4.22

1.64

9.89

1.11

105

0.0740

Summit Hill Soil

1R106H

7.14

2.42

7.53

4.47

3.12

1.31

9.23

1.57

106

0.0480

Waskish Peat

1R107H

Reference FA

Suwannee River

1R101F

12.23

3.11

12.94

3.81

3.36

1.60

9.62

1.00

115

0.1312

Pahokee Peat *

1R103F

15.24

1.78

16.25

3.73

3.54

1.31

10.74

1.00

129

0.1586

Nordic Lake

1R105F

11.16

3.18

12.15

3.79

3.95

1.49

9.67

1.00

107

0.1121

Waskish Peat

1R107F

Aquatic NOM

Suwannee River

1R101N

9.85

3.94

10.57

3.94

3.60

2.61

9.74

1.19

112

0.0725

Nordic Lake

1R108N

Bulk Materials

Elliott Soil

BS102M

Pahokee Peat

BS103P

Leonardite

BS104L

Source: School of Earth & Atmospheric Sciences, Georgia Institute of Technology, Atlanta, GA, U.S.A.

Reference: J. D. Ritchie and E. M. Perdue, Geochim. Cosmochim. Acta, 67, 85-96 (2003).

Ten-ml solutions containing 0.36 ± 0.01 g L^-1 (on a dry ash-free basis) of an IHSS sample in 0.1 M NaCl were titrated from their initial pH values of 3.0-3.3 to final pH values of 10.5-10.7 in 25-35 minutes, using 6-8 mL increments of carbonate-free 0.100 M NaOH. All titrations were conducted under a N₂ atmosphere in a thermostated water bath at 25.00 ± 0.02 Celsius. At least three titrations were obtained for each sample. The stock solutions of soil, peat, leonardite, and Summit Hill humic acids also contained 0.002 M NaOH, which was needed to effectively disperse/dissolve those samples, and an equivalent quantity of HCl that was added just before a titration was started.

Carboxyl is the charge density (meq/g C) at pH 8.0; Phenolic is two times the change in charge density (meq/g C) between pH 8.0 and pH 10.0.

The overall charge density (in meq/g C) of a humic substance increases systematically with pH (or with the concentration of H⁺). The equation on the right, known historically as a modified Henderson-Hasselbalch equation for two classes of binding sites, was used to fit titration data and obtain the set of model fitting parameters in this

table. The fitting parameters of the model were obtained by a nonlinear least-squares fit of the model to aggregated sets of replicate titration data. Q₁ and Q₂ are the maximum charge densities of the two classes of binding sites, Log K₁ and Log K₂ are the mean log K values for proton binding by the two classes of sites, and n₁ and n₂ are empirical parameters that control the width (in log K) of a class of proton binding sites. N is the number of fitted titration data points, and RMSE is the root mean-square error for fitting this model to the data (see Reference). This model, also known in more recent literature as a Langmuir-Freundlich equation, is the primary building block of the NICA class of models.

An * means that a sample is no longer available, and nd means that an item was not determined.

Acidic Functional Groups of IHSS Samples