Journal of Nuclear and Radiochemical Sciences, Vol. 6, No. 1, pp. 25-29, 2005

Accepted
Potentiometric Study on the Proton Binding of Humic Substances
Jinzhou Du, Nobuaki Sato, and Osamu Tochiyama*
Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, 1,1 Katahira, 2-Chome,
Aobaku, Sendai 980-8577, Japan
Received: November 15, 2004; In Final Form: April 21, 2005
The acid-base properties of humic substances (HS) (humic acid (HA) and fulvic acid (FA)) extracted from weathered coal, peat and soils obtained from International Humic Substance Society (IHSS) and Chinese natural
samples were investigated by potentiometric titration in 0.050~1.00 M NaClO4 solution. For the purpose ofcomparison, similar titrations were carried out on the homogeneous polymeric acid, polyacrylic acid (PAA).
Titrations were conducted with 0.500 g/L HA or FA solution in the range of pH 3.0 ~ 11.2 under a nitrogen gas
atmosphere at 298 K. Based on the conceptual model proposed for the cation-humic interaction considering theheterogeneous composition and polyelectrolyte property of humic substances (HS), the apparent dissociation
constant expressed by pKi,app = pKi + mipcH − blog [Na+] (i = 1 for carboxylic and i = 2 for phenolic groups) for
Kapp = [H+][R−]/[HR] was applied to the titration results. In the expression, pKi , mi, and b are constant parameters
depending on the HS. The result indicated that the amount of total carboxylic groups per gram HS decreased in
the order of 7.2 meq/g for FA (coal) > 5.1 meq/g for FA (peat) > 4.4 meq/g for HA (peat) > 3.4 ~ 4.5 meq/g for
HA (soil), and that the value of pK1/(1 – m1), which corresponds to pK1, app at α (degree of dissociation) = 0.5,
increased in the order of 3.1 ~ 3.3 for FA (coal) < 3.5 for HA (peat) ≈ 3.6 for FA (peat) < 3.7 ~ 4.0 for HA (soil).
As compared with the fitted values of m1 = 0.45 ~ 0.55 for PAA, the values were larger for HS (0.54 ~ 0.70) indicating the heterogeneous distribution of the –COOH in HS. The fitted values of b were larger for PAA (0.30 ~0.39) than for HS (0.15 ~ 0.29) and higher for HA than for FA, reflecting the structure of the electrical double
layer formed around the macromolecule, which in turn depends on the density of the anionic sites, molecularweight and conformation of HS.

1. Introduction

cations with HS in our previous paper.23

Humic substances (HS) are complex mixtures of refractoryhigh-molecular weight organic materials formed by the decomposition and condensation of plant, animal and microbial materials. They are abundant and persistent in the biosphere andimmediate subsurface, being present in particulate anddissolved forms in soils, waters and sediment. Since they
contain functional groups such as carboxyls and phenols, theyextensively interact with cations (protons and metal ions) andcontrol the acidity of natural waters, speciation and the transport of metal ions.1−5 However, when we try to estimate theireffect on speciation, we are confronted with the problem that a
humic substance cannot be treated as a simple complexforming ligand.1−8 This is because a HS is not a single welldefined molecule but a mixture of various heterogeneous
macromolecules containing a variety of functional groups
resulting in polyelectrolyte and polyfunctional properties.
Among the interactions of HS with cations, the interaction
with protons (acid-base property) is one of the most importantand fundamental because the interaction of HS with metal ionsis strongly dependent on pH. Numerous attempts have been
carried out to model and describe cation-humic interactions,8−23
and in many of these cases, to apply the findings to protonhumic interaction.8, 9, 14−19, 22 The most advanced models are
Model V/VI 2, 8 and the NICA-Donnan model 9−14 where theheterogeneous site distribution in HS is described with acollection of discrete simple chemical reactions or with continuous functions. The effect of crowded negative charges on themacromolecules is described by considering the Donnan equilibrium. By considering these models, the following simplified equation was proposed to describe the interaction of*Corresponding author. E-mail: tochio@tagen.tohoku.ac.jp. FAX:
+81-22-217-5141.

log Kapp = log K + alog α − blog [Na+] − m log [Mz+], (1)where
Mz+ + zR−

Mz+ · zR−

Kapp =[Mz+ ·zR−][Mz+][R−](2)and where [Mz+ ·zR−] is the concentration of the cation bound to
the HS, [Mz+] is the concentration of the free cation, and [R−] =(C R − z[M z+ · zR − ]) α (C R is a total concentration of proton
exchanging sites and a is a degree of dissociation). Log K, m,a, and b are constant parameters depending on the cation andHS. [Na + ] is the concentration of the background cationcompeting with the cation of interest. The objective of thepresent study is to apply this model to the interaction of HS
with protons in order to test the applicability of the model andto discuss the characteristics of HS of various origins. For the
proton-binding of HS, eq 1 can be expressed by
pKapp = pK + mpcH − blog [Na+] ,(3)

where pcH = –log [H+] andHR

H + + R−

Kapp =[H+] [R−]
[HR]

(4)

Potentiometric titrations were conducted for humic (HA) andfulvic acids (FA) extracted from weathered coal, peat and soils
obtained from the International Humic Substance Society(IHSS) and from China as well as for polyacrylic acids in
0.050 – 1.00 M NaClO4 solution. The results were analyzed byapplying eq 3 to carboxylic and phenolic groups of these HS
samples, and the values of the obtained parameters— C W
R (theamount of functional groups per unit weight of HS), pK, m,© 2005 The Japan Society of Nuclear and Radiochemical Sciences
Published on Web 07/31/2005

26

J. Nucl. Radiochem. Sci., Vol. 6, No. 1, 2005

Duand b—were discussed in view of the heterogeneity and otherproperties of HS of various origins.

−degree of dissociation (α = [R ]CR). According to our previously proposed model, pKapp is described by
pKapp = pK+ mpcH − blog [Na+],

2. Experimental

where pcH = –log [H+] and pK, m and b are constant parameters for each PAA. pK is the pKapp at pcH = 0 and [Na+] = 1.0
M. Chemically, pK/(1 – m) is more meaningful since this value
gives pKapp (or pcH) at α = 0.5. Using the pKapp given above,the concentration of the dissociated carboxylate groups, [R−],is given by the following equation:[R−] = CRα =

12
11
10
9
8
7
6
5
4
3
2

3. Results and Discussion[H+] + CbVb /(V0 + Vb) − [OH−][R ] =
V0 /(V0 + Vb)−(5)where C b and V b are the concentration of NaOH and thevolume of added titrant, respectively, and V 0 is the initial
volume before the titration. From this relation, the apparent
dissociation constant is calculated as+

pKapp = −log−

[NaCl] /M = 1.0(left), 0.4, 0.1, 0.04, 0.025(right)

1

Proton dissociation of polyacrylic acid. The potentiometrictitration directly gives the relation of added amount of alkali(NaOH) and the resultant pH. This result can been reorganized
to give the total concentration of dissociated functional groups[R−] as a function of [H+] for each titration point by using thefollowing relation:+−[H ] [R ][H ] [R ]= −log[HR](CR − [R−])(6)

where CR is the total concentration of proton-releasing sites.
For HS and PAA, Kapp is not a constant but a function of the

CR
CR
=
, (8)
+
10 pKapp −pcH + 1 10 pK+(m−1)pcH−blog [Na ] + 1

where CR is also a constant parameter obtained from the titration result.
The titrations of different PAAs (MW from 2000k to 90000)
were carried out with a concentration of 0.500 g/L in 0.040 ~1.0 M NaClO4 solution and 0.025 ~ 1.0 M NaCl solution. The
results were reorganized to give the relation of [R−] calculated
by eq 5 versus pcH and were fitted to eq 8 by nonlinear leastsquares fittings, and the best set of the parameters pK, m, b,
and CR were obtained. The data (markers) and fitted curves(solid lines) for PAA of MW = 90000 in 0.025 ~ 1.0 M NaCl
are shown in Figure 1, and the resultant parameters for various
molecular weight PAAs are given in Table 1, where (R−) and−CW
R (expressed in meq/g-PAA) were converted from [R ] and
CR. Note that the five solid lines in Figure 1 were calculatedwith one set of parameters for MW = 90000 listed in Table 1.

(R-)/(meq/g)

Humic substances. Three samples were purchased fromthe International Humic Substance Society (IHSS): humic acidextracted from soil (Cat. No.1 S102H), abbreviated as HA(soil, IHSS), and humic and fulvic acids extracted from peat
(Cat. No.1 R107H and Cat. No.1 R107F), abb. as HA (peat,
IHSS) and FA (peat, IHSS), respectively. Other samples werekindly provided by Dr. Tao Zuyi, Lanzhou University (P.R.
China), which were extracted according to the IHSS procedure: fulvic acid extracted from coal (Gongxian, Henan
province, P.R. China), abb. as FA(coal, Gongxian); and humic
acids extracted from the soil of south of China (Daya Bay,
Guangdong province, P.R. China) and from the soil of northeast of China (Helongjiang province, P.R. China), abb. as
HA(soil, Daya Bay) and HA(soil, Helongjiang), respectively.
All three were characterized elsewhere,24−26 Polyacrylic acids(PAA, [–CH2CH(COOH)–]n) (MW = 2000, 5000 and 450000
Daltons, powder from Aldrich Chemical Co. and MW = 90000
Daltons, 25 wt% aqueous solution) from Polysciences wereused as received.
Potentiometric titration. Potentiometric titration was conducted with an automatic titrator (COM-1500, HIRANUMA,
Japan) equipped with 50-mL water-jacketed glass vessel at(298.2 ± 0.1) K under a nitrogen atmosphere. A combinationelectrode (TOA Dempa, GST-5421C) was used for pcH measurement and its reference electrode was filled with 4.0 M
NaCl (instead of KCl) to avoid precipitation of KClO 4 at thejunction of the electrode and the solution. The calibration of
the electrode for the conversion of the pH reading into pcH wasconducted as described previously.27 In the titration, the solution in the reaction vessel was bubbled with nitrogen gas that
had been passed through a dilute NaOH solution and NaClO4
solution in sequence. The CO2-free NaOH solution of 0.1044
mol/L was used as a titrant. De-ionized water (Milli-Q® Pureline, Yamato, Japan) was used to prepare the solution. The HSconcentration of all the stock solution was 1000 mg/L. FA
was directly dissolved in the solution. HA was first dissolved
into dilute NaOH solution (~pH 11.5) and placed under anitrogen atmosphere overnight. Then, an equal amount of
HClO 4 was added to this solution to compensate for theamount of previously added NaOH. Finally, de-ionized water
was used to dilute the solution to reach a fixed volume.

(7)

4

5

pcH

6

7

8

Figure 1. The application of the model to the proton dissociation ofpolyacrylic acid (MW = 90000) in 0.025, 0.040, 0.1, 0.40 and 1.00 M
NaCl. Solid lines are drawn by using pKapp = pK + mpcH − blog [Na+]with the parameters given in Table 1.

TABLE 1: Apparent Acid Dissociation Constants pKapp andSite Contents CW
R (meq/g PAA) of PAA Obtained by Fitting
the Titration Data to the Equation, pKapp= pK + mpcH −blog [Na+]
CWR

pK
1−m

m

b

CWR

pK
1−m

m

b

MW

NaClO4NaCl

450k

13.8(2) 4.78(4) 0.55(1) 0.35(1)13.7(1) 4.73(3) 0.53(1) 0.37(1)90k

12.8(1) 4.85(4) 0.48(1) 0.36(1)13.2(1) 4.78(2) 0.51(1) 0.39(1)5k

11.7(1) 4.76(4) 0.46(1) 0.34(1)12.0(1) 4.72(4) 0.47(1) 0.35(1)2k10.2(1) 4.74(4) 0.45(1) 0.30(1)10.3(1) 4.67(5) 0.46(1) 0.33(1)The digits in the parentheses following numerical values represent the estimated standard deviation (σ) of those values in terms of the final listed digits.

J. Nucl. Radiochem. Sci., Vol. 6, No. 1, 2005

Potentiometric Study on the Proton Binding of Humic Substances

To check the effect of the concentration of HS on the dissociation property of HS, titrations of 0.200, 0.500 and 1.00 g/L
solutions of FA(coal, Gongxian) were carried out in 0.050 ~1.0
M NaClO4 solution. Although the titrations were conducted to
the final pcH of 11.2, the data below pcH 10.5 were taken for
the analyses since the data obtained at high pcH contain large
experimental uncertainties. Only the obtained parameters are
given in Table 2; as shown, there are no significant differencesin the results. Thus, the titrations of all HS samples were carriedout using 0.500 g/L of HS in 0.050 ~1.0 M NaClO4 solution.
The results of the nonlinear fittings are given in Figure 3 andTABLE 2: Apparent Acid Dissociation Constants pKapp andSite Contents C WR (meq/g HS) of HS Obtained by Fitting the
Titration Data to the Equation, pKapp = pK + mpcH – blog [Na+]m1

FA(coal, Gongxian)

7.0(1) 3.16(1) 0.70(1) 0.17(1)0.6(1) 9.89(7) −0.06(1)FA(peat, IHSS)

5.1(1) 3.63(2) 0.66(1) 0.11(1)1.2(1)

HA(peat, IHSS)

4.4(1) 3.51(2) 0.67(1) 0.15(1)3.5(2) 10.4(1) 0.68(1)HA(soil, Helongjiang) 4.5(1) 3.68(2) 0.60(1) 0.29(1)1.3(1) 8.76(1) 0.50(2)4.2(1) 3.81(1) 0.64(1) 0.22(0)1.2(1) 9.17(1) 0.42(0)HA(soil, Daya Bay)

3.4(1) 4.02(5) 0.54(2) 0.23(2)1.4(1)

8

8

FA(peat, IHSS)

FA(coal, Gongxian)6(R-)/(meq/g)

6

4

2

8

4

2

0

0
3

4

5

6

7

8

9

10

3

4

5

6

pcH

7

8

9

10

7

8

9

10

7

8

9

10

pcH
8

HA(peat, IHSS)

HA(soil, Helongjiang)

6+

10 pK2+(m2-1)pcH−blog [Na ]+1

8.8(4) 0.55(5)For HS, two groups of pK app are assumed (obtained for 0.500 g/L HS in
NaClO4 unless otherwise denoted).
The digits in the parentheses following numerical values represent the estimatedstandard deviation (σ) of those values in terms of the final listed digits.

6(R-)/(meq/g)+

9.2(6) 0.34(6)HA(soil, IHSS)(R-)/(meq/g)+

10 pK1+(m1-1)pcH−blog [Na ]+1m2

1.5(1) 10.3(1) 0.13(1)0.8(1) 9.8(1) 0.06(1)

(9)

CR,2

pK2
1 − m27.1(1) 3.34(1) 0.70(1) 0.15(1)7.1(1) 3.07(1) 0.70(1) 0.20(1)

8

CR,1

C WR,2b

FA(coal, Gongxian)
0.250 g/L
1.000 g/L

[R−] = CR,1α1 + CR,2α2=

pK1
1 − m1

C WR,1(R-)/(meq/g)

No significant differences in the fitted parameters were observed
between NaClO4 and NaCl solution, and the model curves fit
the data fairly well. This suggests that, in spite of its homogeneous structure, the energy of deprotonation of the carboxylic
functional group smoothly changes with the degree of dissociation due to its polyelectrolyte nature; in other words, thedissociation of the proton from a certain –COOH is affected by
the deprotonated or protonated conditions of its neighboringcarboxylate groups. Obtained values of pK/(1 – m) are only
slightly larger than those of monomeric acids (CH 3COOH:
4.60 ± 0.01, C 2 H 5 COOH: 4.73 ± 0.01, C 3 H 7 COOH: 4.68 ±0.012 at I = 1.0 M NaClO4),28 indicating that the value of 4.7 ~4.8 corresponds to the dissociation constant of isolated –COOH
in CnH2n+1COOH for a high n at I = 1.0 M. Table 1 also showsthat m and b slightly increase with the molecular weight ofPAA. This is probably due to the increased interaction of thefunctional groups in larger molecular weight polymers. Figure
1 shows that the dependences of the experimental data onlog [Na+] are smaller than the curves predicted by the model at
low degrees of dissociation and are larger at high degrees ofdissociation. This is reasonable because the formation of the
double layer depends on the charge density of the macromolecule, and the model does not take into account this effect.
Proton dissociation of HS. The titrations of various HS
were carried out in the same manner as those for PAA. In
Figure 2, the results for the titration of various HS in 0.1 M
NaClO4 are shown in the form of (R−) as a function of pcH.
Each of the curves showed a dependence on NaClO4 concentration. All the titration results obtained for various concentrations of NaClO 4 were reorganized in the form of (R − ) as afunction of pcH and analyzed. In the case of HS, two groupsof pK app were assumed, each of which follows eq 7. Twogroups of pKapp were assumed because two kinds of functionalgroups (carboxylic and phenolic groups) are considered to becontained in HS, and their dissociation properties are affectedboth by the polyelectrolyte nature and the heterogeneous distribution of the functional groups. The parameter b is taken as
common to two acidic groups since this parameter is related tothe charge and charge density of the macromolecule and notdependent on the kind of functional group. Thus, the concentration of the dissociated functional (carboxylate and phenolate) groups, [R−], is given by the following equation:

27

4

2

4

2

7
0

6

0
3

4

5

6

7

8

9

10

3

4

5

6

pcH
8

8

HA(soil, IHSS)

HA(soil, Daya Bay)

4
3

FA(coal, Gongxian)FA(peat, IHSS)
HA(peat, IHSS)
HA(soil, Helongjiang)
HA(soil, IHSS)
HA(soil, Daya Bay)

2
1
0
3

4

5

6

7

8

9

10

pcH

Figure 2. Plots of dissociated HS (meq/g HS) vs. pcH in 0.10 M
NaClO4 solution for FA(coal, Gongxian)(△); FA(peat, IHSS)(○);
HA(peat, IHSS)(●); HA(soil, Helongjiang)(□); HA(soil, IHSS)(■)
and HA(soil, Daya Bay)(◇).

6(R-)/(meq/g)

6(R-)/(meq/g)-(R )/(meq/g)

5

pcH

4

2

4

2

0

0
3

4

5

6

7

pcH

8

9

10

3

4

5

6

pcH

Figure 3. Plots of dissociated HS (meq/g HS) vs. pcH: 0.050 (■);
0.10(□); 0.40(●) and 1.00(○) for [NaClO4]/M; markers are calculated
by eq 5 and solid lines are calculated by eq 8 using the parameters
given in Table 2. All the HS concentrations used were 0.500 g/L.

28

Du

J. Nucl. Radiochem. Sci., Vol. 6, No. 1, 2005

values of b are larger for HA than for FA in spite of the fact that
HAs have smaller C WR,1 values. This is because HAs containlarger amounts of large-molecular-weight components, andthe structure of the double layer around the macromolecule isdetermined not only by the amount of charged sites per unit
weight of HS but also by its molecular weight and molecularconformation. As for the phenol groups in HS, the C W
R,2 values
are usually small except in the case of HA(peat, IHSS). The
values of pK2 /(1 – m2) are close to the pK of phenol (9.82 ±0.02* at I = 1.0 M).28 However, as shown in the results on
FA(coal, Gongxian), the experimental uncertainties are fairly
large and make further discussion difficult.

pK1 /(1-m1 )

4.5

4.0

FA(peat)
HA
3.5

FA(coal)

3.0

m1

0.8

4. Conclusions

FA(coal)0.6

FA(peat)

HA
0.4
0.2
2

4

6

8

W

C R,1 /(meq/g)
Figure 4. Plots of the fitted heterogeneity parameters m1 and themean dissociation constants pK1/(1–m1) against the estimated carboxyl
contents of HS.

the obtained parameters are listed in Table 2. The results ofthe fitting shown in Figure 3 indicate that the model equation
can be successfully used to reproduce the titration results ofvarious HS samples. The fitted parameters in Table 2 indicatethat the amount of total carboxylic groups C W
R,1 decreases in theorder of 7.2 meq/g for FA (coal) > 5.1 meq/g for FA (peat) >4.4 meq/g for HA (peat) > 3.4 ~ 4.5 meq/g for HA (soil), andthat the value of pK1/(1 – m1), which corresponds to pK1,app at α
(degree of dissociation) = 0.5, increases in the order of 3.1 ~ 3.3
for FA (coal) < 3.5 for HA (peat) ≈ 3.6 for FA (peat) < 3.7 ~ 4.0
for HA (soil). The larger C WR,1 for FA than for HA is considered to be due to the fractionation procedure, which separates
FA from HA by solubility in acidic solution. The values of
pK1/(1–m1) are smaller than those of PAA and isolated –COOH.
This clearly indicates that –COOH groups are heterogeneously
distributed in the HS macromolecule; that is, there should besome pairs with shorter distances to give smaller dissociation
constants, or there should be some –COOH groups which arestrongly affected by the neighboring atoms or structures suchas aromatic rings. The value of m1 is considered to reflect theheterogeneity of HS since m gives the change of pKapp for theunit change of pcH; i.e., m = ∂pKapp /∂pcH. As compared with
PAA, the larger values of m1 for HS probably result from theheterogeneous distribution of –COOH. Roughly, m1 is larger
for FA than for HA and larger for HA from peat than for HA
from soil. It is interesting to note that it seems that the heterogeneity parameters m 1 and the mean dissociation constantpK1/(1 – m1) are more or less linearly related to the estimatedcarboxyl contents CR,1 of HS as shown in Figure 3; which inturn indicates that the distribution of –COOH is more heterogeneous for the HS containing a larger amount of acidic functional groups. This can be explained by the fact that, whenfulvic acids and humic acids are fractionated, fulvic acids are
the fraction that is soluble in acids and bases, and humic acids
are the fraction that is soluble in bases but insoluble in acid.
Therefore, the humic substances containing a larger number ofacidic groups and more-acidic groups are fractioned into afulvic fraction. Thus, the fraction containing a larger amountof functional groups usually contains a larger amount of moreacidic groups. The parameter b gives the change of pKapp forthe unit change of log [Na + ], b = –∂pK app /∂ log [Na + ]. The
fitted values for HS are smaller than that for PAA probably
due to the larger C W
R for PAA than HS. However, for HS, theEquation 1 proposed for the cation-humate interaction constant
was successfully applied to the proton dissociation of various
humic substances in the form of eq 8. With the parameters listed
in Table 2, the dissociated amount of functional groups can bewell reproduced as a function of pcH and log [Na+]. Obtained
parameter values of CWR reflect the fractionation procedure forfulvic and humic acids. The values of pK/(1 – m) indicate that
there should be heterogeneous distribution of the functionalgroups in the humic macromolecule. The obtained values of b
are considered to be determined by the molecular weights andthe molecular conformations of the humic substances.
Acknowledgment. The present work was supported by aresearch grant from the Japanese Society for the Promotion of
Science.
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