- Email: [email protected]
Factors controlling stability of Al-humate
Geoderma, 26 (1981) 1--12 Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands
1
FACTORS CONTROLLING STABILITY OF A1-HUMATE
SHIGEMITSU ARAI and KYOICHI KUMADA
Faculty of Agriculture, Nagoya University, Nagoya (Japan) (Received July 10, 1979; accepted after revision March 24, 1981)
ABSTRACT Arai, S. and Kumada, K., 1981. Factors controlling stability of Al-humate. Geoderma, 26: 1--12. S o m e of the factors controlling Al bumate bonding, a major aspect of organo-mineral interactions in soils,were studied. The methods used were potentiometric titration and stability measurement. The samples consisted of 4 humic acids plus 2 polymeric and several monomeric carboxylic acids as model compounds. Humic acids as well as alginic and polyacrylic acids were shown to form complexes with AI. The stability of Al humate complexes was shown to be controlled by polymerization of carboxyl-containing units and by the arrangement of carboxyl groups in the units. A contribution by salicylate type units was doubtful.
INTRODUCTION
One of the main aspects of organo-mineral interactions in softs is complex formation between soil organic matter and metal ions. Many investigations were devoted ~o the nature of these complexes and it is therefore possible nowadays to refer to the formation constants of metal complexes with soil organic matter {Coleman et al., 1956; Himes and Barber, 1957; Gcering and Hodgson, 1969; Schnitzer and Khan, 1972; Langford and Khan, 1975; Stevenson, 1976; Rosell et al., 1977; Bresnahan et al., 1978; Takamatsu and Yoshida, 1978, etc.) or to data on the characteristics of this bonding (Schnitzer and Skinner, 1965; Yariv et al., 1966; Schnitzer and Khan, 1972; Gamble et al., 1976; Gamble et al., 1977; Senesi et al., 1977; McBride, 1978; Bresnahan et al., 1978, etc.). More information has also been obtained on the chemical structure of soil organic matter which is intimately related to complex formation (Beckwith, 1959; Wood et al., 1961; Wagner and Stevenson, 1965; Gamble, 1970; Van Dijk, 1971, etc.). Nevertheless, there are many problems still remaining to be solved. Although aluminum is universal in softs and is closely related to soil organic matter ac,cumulation {Greenland, 1971), the relationships between aluminum and soil organic matter have not been fully investigated. This is due to the diversity of aluminum compounds in soils and to the complexity of soil organic matter.
0 0 1 6 - 7 0 6 1 / 8 1 / 0 0 0 0 - - 0 0 0 0 / $ 02.50 © 1981 Elsevier Scientific Publishing Company
Thus, the purpose of this study was to ascertain the nature of Al-humate formation, to determine conditional stability constants and to examine the factors affecting complex formation, such as chemical structure and polymerization. For the study, high and low molecular weight model compounds and humic acids were used. MATERIALS AND METHODS Materials
Four humic acid samples which belong to the A, B, Rp and l) types, according to Kumada's classification (Kumada et al., 1967}, were used. These were Shitara humic acid extracted from a so-called "kuroboku soil" (Umbrept)-A type, Kuragari sample from an acid brown forest soil (Umbrept)-B type, Sanage sample from a grey lowland soil (Aquent ?)-Rp type and Tsubame sample from a buried horizon-P type. These were the same samples as those covered by the authors in a previous report (Arai and Kumada, 1977). The humic acids were extracted with 0.1N NaOH, refined by repeated acidification and dissolution and stored as dried powders. Prior to determining pH and stability constants, the samples were redissolved in 0.1N NaOH and refined through ion exchange resins (IR 120 and IRA 400). The ash contents (dry weight basis) were as follows: Shitara 3.02%, Kuragari 3.66%, Sanage 0.79% and Tsubame 2.62%. Polyacrylic acid and alginic acid samples were prepared from sodium polyacrylate (average degree of polymerization, 3--4 × 104, Nakarai Chemicals Ltd.) and alginic acid (Nakarai) after being dissolved in water and 0.1N NaOH solutions, respectively, and passed through anion and cation exchangers as was done with the humic acids. Concentrations (N) were determined conductometrically. The other carboxylic acids were: acetic acid (Wako Pure Chemical Industries, Ltd., JIS special grade), malonic acid (Wako, Wako special grade), succinic acid (Kanto Chemicals Co., Inc., guaranteed grade}, glutaric acid (Nakarai, extra pure grade), adipic acid {Wako, JIS special grade), 1,2,3-propanetricarboxylic acid (Wako, Wako special grade), 1,3,5pentanetricarboxylic acid (Nakarai, extra pure grade), benzoic acid (Wako, JIS special grade) and o-phthalic acid (Tokyo Chemical Industry Co., Ltd. guaranteed grade). Their concentrations were determined volumetrically. P o t e n t i o m e t r i c titration curves
An automatic titrator RAT-11S produced by Hiranuma Sangyo Co., Ltd. equipped with a reference electrode K-421N and a glass electrode MG-511 produced by Denki Kagaku Keiki Co., Ltd. was used. The pH values were measured without any supporting electrolyte. The interval of titration was set at 30 sec but, if equilibrium was not attained during that period, the interval was extended automatically. Calibration was done with use of standard pH buffers, i.e. K-phthalate (pH 7) and K-oxalate (pH 4), according to Japanese Industrial Standard (JIS K8802-1958).
4.60 4.86 4.64 4.77 4.86 5.35 4.89 4.72 2.89 2.91 4.76 4.88 log ~ , s 5.0946 5.1863 4.8461 4.8831 4.5272 4.5317
Shitara
acid
4.44 5.25 2.89 3.24 2.42 3.20 4.00 4.00 1.18 1.29 0.90 1.02 log 13.6 2.5673 2.5849 3.9444 3.9363 ---
n .2
0.60 0.58 --1.65 --1.64 --1.91 --1.91
0.33 0.34 0.27 0.27 0.27 0.25 0.37 0.36 0.57 0.53 0.23 0.25
log b = p ( [ H A ] / [ H ] ) ; i = o . 5 .3
1.67 1.77 3.97 3.92 3.69 3.70
2.91 2.92 2.92 2.92 2.88 2.90 2.85 2.85 2.80 2.84 3.15 3.14
---
--
----
0.3 0.3 0.2 0.2 0.1 0.1 0.2 0.2 0.4 0.5 0.1 0.1
pHi- =0.5 a ~ = 0 . 5 * '
× 1 0 - 3 : 7 . 6 0 8 × 1 0 - ' = 13.1
× 1 0 - 3 : 7 . 6 0 8 × 10-4 = 1 3 . 0
1 . 9 7 6 × 1 0 - 2 : 1 . 5 2 1 × 10 -3 = 1 3 . 0
9.95
9.87
0 . 5 7 5 x 1 0 - a : 3 . 0 4 3 x 10 -4 = 1.9
1 . 0 5 5 × 1 0 - 3 : 4 . 5 6 5 × 10 .4 = 2.3
1 . 0 9 3 × 1 0 - 3 : 4 . 5 6 5 x 10 -4 = 2.4
1 . 0 9 4 × 10 - 3 : 4 . 5 6 5 × 10-4 = 2.4
1 . 1 0 5 × 1 0 - 3 : 4 . 5 6 5 x 10-4 = 2.4
1 . 0 9 8 × 1 0 - s : 4 . 5 6 5 × 10-4 = 2.4
T A ( e q u i v . / l ) : A l ( m o l e / l ) .5
N o t e : t o t a l v o l u m e 30 ml a n d t i t r a t e d b y 0 . 1 N N a O H ( S h i t a r a - - p o l y a c r y l i c a c i d ) o r 1N N a O H ( m a l o n i c - - a c e t i c acid). ,1 a n d ,2 C o n s t a n t s in m o d i f i e d H e n d e r s o n - - H a s s e l b a c h e q u a t i o n , p H = p K a - - n l o g ( 1 - - ~ ) / , ~ . ,3 H A ; u n d i s s o c i a t e d acid g r o u p . ~; a v e r a g e n u m b e r o f acid g r o u p b o a n d to m e t a l ion. "4 ~ ; d e g r e e o f d i s s o c i a t i o n . , s T A ; t o t a l c o n c e n t r a t i o n o f acid. ,6 ~1 a n d 132; ( i o n i z a t i o n c o n s t a n t ) -l.
Acetic
S u c c i n i c acid
M a l o n i c acid
P o l y ~ c r y l i c acid
Alginic acid
Tsubame
Sanage
Kuragari
p K a *~
Sample
C o m p l e x a t i o n c o n s t a n t s o f AI w i t h h u m i c a n d t h e o t h e r a c i d s in 1M N a C I O ,
TABLE I
50
4.515 4 . 5 1 5 av. 4 . 5 1 5
4 . 9 7 7 av. 4 . 9 7 6 4.973 4.982 4 . 9 9 0 av. 4 . 9 8 6
Glutaric acid Adipic acid
1,2,3-Pro- 5.744 p a n e t r i c a r - 5 . 7 4 5 av. 5 . 7 4 4
4.606
4.603 av. 4 . 6 0 5
2.676 av. 2 . 6 7 2 2.668
3.743
3.750
av. 3 . 7 4 6
3.440 3 . 4 3 3 av. 3 . 4 3 7
---_
---
-------_
l o g 53*~
Note. Total volume 30 ml and titrated by 1N NaOH. ,1 ~1--~3 ( i o n i z a t i o n c o n s t a n t ) -i. ,2 L = dissociated acid group. ,3 T L = t o t a l c o n c e n t r a t i o n o f acid.
1,3,5-Pen- 5.186 5.184 t a n e t r i c a r - 5 . 1 8 2 av" boxylic acid
boxylic acid
4 . 1 3 0 av. 4 . 1 3 1 4.133 4.238 4 . 2 3 3 av. 4 . 2 3 5
4.911 av. 4 . 9 0 8 4.906
----2.976 av. 2 . 9 8 0 2.983 3.986 3 . 9 8 4 av. 3 . 9 8 5
0-Phthalic acid
av. 5 . 1 5 8
av. 5 . 0 1 7
av. 3 . 9 7 2
av. 4 . 5 3 7
4.538 4.537 3.972 3.972 5.018 5.016 5.160 5.155
Acetic acid Benzoic acid Malonic acid Succinic acid
l o g ~2*~
l o g ~*~
Acid
,.
4.53
4.52
av. 4 . 5 3
5.09 5 . 1 3 av. 5 . 1 1
3 . 8 5 av. 3 . 8 5 3.85 3.79 3 . 7 9 av. 3 . 7 9
4.07 av. 4 . 0 6 4.05
av. 3 . ~ 5
av. 6.8
av. 2 . 7 0
av. 2 . 6 5
4.00
4.01
3.97 3.96
3.92 3.92 3.98 3.98
3.17 3.21
3.66 3.68 3.22 3.11 1.76 1.80 3.91 3.91
k=PLn=o.~2pH~=o. 5
2.66 2.63 2.77 2.83 6.7 6.8 3.94 3.96
log
C o m p l e x a t i o n c o n s t a n t s o f AI w i t h s e v e r a l l o w m o l e c u l a r c o m p o u n d s in 0 . 1 M NaCIO4
T A B L E II
3 . 2 9 × 1 0 - 3 : 7 . 6 1 × 1 0 -4 = 4 . 3
4.3
6.5
4 . 9 7 × 1 0 - 3 : 7 . 6 1 × 10-* L 3 . 3 0 × 1 0 - 3 : 7 . 6 1 × 1 0 -4 =
6.5
4 . 9 8 × 1 0 -3:7.b3 × 1 0 -4 =
6.5
6.4
4 . 9 5 x 1 0 - 3 : 7 . 6 1 x 1 0 -4 = 5 . 9 8 × 1 0 - 3 : 9 . 1 3 x 1 0 -4 =
6.4
4 . 9 5 × 1 0 - 3 : 7 . 1 6 x 10 -4 =
1 . 2 9 x 1 0 - 2 : 9 . 1 3 x 1 0 -4 = 1 4
1 . 9 9 x 1 0 - 2 : 1 . 5 2 x 1 0 -3 = 1 3
TL(mole/l):Al(mole/l),3
Conditional stability c o n s t a n t s
The potential difference (E, mV) was measured in 1M NaC104 solution (humic acids etc.) or 0.1M NaClO4 (low molecular weight organic acids). The value was then converted to the hydrogen ion concentration using a calibration curve prepared from an HCIO4 standard solution at the same NaC104 level. In these measurements, the KC1 solution of the reference electrode was replaced by NaC1 solution to avoid interference due to the precipitation of KC104. The ionic strength of a polymeric acid system, u = 1, was selected after a preliminary test so that gelation was almost complete without A13÷. Titration was begun after allowing the mixed solution to stand for 2--3 h. The NaOH standard solution was added using a microsyringe. Enough time was allowed for equilibrium to be attained and, in most cases, the interval was 0.5-10 min. Each determination was made in a range of pH (---log [H] ) less than 4 to minimize the effect of the hydrolysis of A1 ions, because the hydrolysis constant of A1, Q,1, is about 10 -s (Baes and Mesmer, 1976). To keep the pH low, a suitable amount of perchloric acid was added if necessary. The ratio of ligand to metal (mole or equivalent to mole) was kept as high as possible. These ratios are shown in Table I and II. Calculation
Conditional stability constants (b) of high molecular-weight polymeric acids, including humic acids, and some low molecular-weight acids were determined with a modification of Bjerrum's method (Gregor et al., 1955) by calculating p ( [ H A ] / [ H ] ) = log ka.le = log b, against n, where [HA] is the concentration of the undissociated acid group, ka the dissociation constant, k the formation constant and n the average number of ligands to the total concentra tion of metal ions. Constants for low molecular-weight acids were obtained by Bjerrum's method. In these cases, the formation of mixed complexes, such as M L p X q , where M is the metal, L the ligand and X hydronium or hydroxyl ion and p and q are constants, was not taken into consideration for the first approximation. m
RESULTS
The p H titration curves
Fig. 1 shows the pH titration curves of Shitara humic acid, polyacrylic acid, alginic acid and acetic acid, with and without A1 ions. In these figures, the dotted lines are hypothetical curves obtained by additions of titration curves of the organic acids to those of A1 with a correction as described below. It is shown that the titration curve for an acid HA and a strong base B follows the equation, t = C B / C H A = ( K w / [ H ] - - [ H ] ) / C H A + (1 + [ H ] K H A ) - I , where t is the degree of neutralization, C B and CHA the cor4centrations of the added
I0
I0
(b)
9
9
7
7
6
6
5
5
" o
.........
; .........
;,
"
'# ~ "
"o'.,
.........
0'2 ........
'o'3
/
8 7
6
6
.," ~
+ f " ll" l
" .~
5
"
.
0 O.INHCI ml
.~'
,o (d9
~7
5
÷
o ......
,o9
8
÷ ..'Ib'"
I O.INNaOH ml
.
.
.
0 O.INHCI ml
.
.
.
.
.
.
.
.
|
I
.
.
.
.
.
.
.
.
.
!
.
.
.
.
.
2
0.1N NaOH ml
Fig. 1. The potentiometric titration curves of Shitara humic acid (a), polyacrylic acid ( b ) , alginic acid (c) and acetic acid (d) with and without Al. Total volume 25 ml; o - a c i d , A = A I , ® = acid with Al and + = quasi-theoretical values assuming the simple mixing of acid and A l .
base and the acid, respectively, Kw the ionic product of water ([H] .[OH] ), and KHA the dissociation constant of the acid (Tanaka and Nakagawa, 1966). When two titration curves are added together, the first term in the above equation will be summed up also so that correction of this term is necessary. The effect of this correction was almost negligible between pH 4 and 10. In a mixed solution of acetic and oxalic acids, the curve thus corrected agreed well with the measured one (not shown in a figure). The titration curves for humic acid and other acids with AI crossed the dotted curves at about pH 5, i.e. excess proton was set free in the acidic range whereas the hydroxyl ion increased more than expected if the hydrolysis of the Al ion took place at a pH of more than about 5 (Fig. 1). This phenomenon seems due to coordination of the ligands of acids with cations instead of water. The same results were obtained regardless of the type of the humic acid or the ratio of humic acid to Al. Similar observations were made by earlier Kawaguchi and Kyuma (1959) and Yoshida and Nakao (1971). The effect of acetic acid on Al titration was similar to that of humic acid although
the effect of the former was slight (Fig. l(d)). Thus, coordination of the carboxyl group with the A1 ion appears c o m m o n to carboxylic acids. Conditional stability constants (b, or K,) Modified Bjerrum plots for humic and other acids are shown in Fig. 2 and the values of p ( [ H A ] / [ H ] ) at n = 0.5 are tabulated in Table I. If successive formation constants are separated far enough, p ( [ H A ] / [ H ] ) at n = 0.5 equals log b, - log [MA] [ H ] / [ M ] [HA] (Gregor et al., 1955). But, as seen in Fig. 2, the separation was not large enough, although circumstances differed from sample to sample. Thus, the p ( [ H A ] / [ H ] ) value should be taken as an index describing the relative tendency of complex formation at a condition of n =0.5. m
w
L~
o
~
o.,
It
o
"~
0.6
!
o
E
I
ii I
-0.2
°'"
'
I
-0.1
0
I
I
I
I
I
..I
0.1
0.2
0.3
0.4
0.5
0.6
Fig. 2. Modified Blerrum plot of AI humates, polyaerylate, alginate, malonate, sueeinate and acetate.
Conditional ,c~ability measurements are reported to be theoretically incomplete for polydisperse systems, because stability can be influenced by experimental conditions such as degree of dissociation, cation concentration etc. (Gamble et al., 1970; Stevenson, 1976; Saar and Weber, 1979). The effect of sample concentration was relatively small in Cu 2÷ fulvate system (Gamble et al., 19"/0) or in the solution of low Cd 2÷ to fulvic acid ratio (Saar and Weber, 19"/9). In our measurements, as indicated in Table I, the ratio of A1 to humie or other polymeric acids (ca. 1 mole: 2.4 equiv.) or the degree of dissociation (~ 0.1--0.3) was low and pH values (< 4) remained within a limited range. Therefore, the values of conditional stability could be used for relative comparison. For humic acids, the n values in the modified Henderson-Hasselbalch equation were higher than the ones usually observed, viz. 1--2 (Table I). This may
be due to the fact that different types of acid groups occur together even though the equation seems applicable within a restricted range of ~ (0.1--0.4). The values of p ( [ H A ] / [ H ] } at n = 0.5 were slightly greater in Shitara (A t y p e ) and Tsubame (P) than in Kuragari (B) and Sanage (Rp). These results appear reasonable because the amounts of acid functional groups and strongly dissociating groups in these humic acids are in t h e order of A > B , P> Rp (Arai and Kumada, 1977). The degree of dissociation ~ at n = 0.5 differed between Shitara (0.3) and Tsubame (0.2) ~.nd b e t w e e n Kuragari (0.2) and Sanage (0.1) (Table I). Consequently, if it were possible to compare t h e m at identical conditions, the difference among the p ( [ H A ] / [ H ] ) values o f various types of humic acid would become clearer. The values of p ( [ H A ] / [ H ] ) at n = 0.5 for the humic acids were between those for alginic and polyacrylic acids, though there remained some uncertainty about the value of polyacrylic acid because the pH at n = 0.5 was so low that the constants Ka and n in the modified Henderson-Hasselbalch equation might also include some uncertainty. The values of p C [ H A l / [ H ] ) at n = 0.5 of humic and other polymeric acids were less than t h a t of malonic acid and greater than those of acetic and succinic acids. Bjerrum plots of low molecular weight carboxylic acids are shown in Fig. 3, and the pL values at n = 0.5 are listed in Table II. The c o m p o u n d s used ranged from mono- to tri-basic acids. Among polybasic acids, the number of carbon atoms between two carboxyl groups ranged from 1 to 4. Two aromatic acids, benzoic and phthalic acids, were also included. The value n = 0.5 was attained at pH less than 4 in each case. The pL at n = 0.5 would be log k, if the successive complexations were sufficiently separated and mixed complexes were absent. ~_~
.o
~_ o
~
=o
_
.-
~o o~e
;~oo:~e
\
C:].~
0
2.5
I
I
I
I
i
I
I
:
I
30
3.0
4.0
4.5
5,0
5,0
6.0
6,5
7.0
pL F i g . 3. Bjei~rum plot of AI c o m p l e x with several low m o l e c u l a r w e i g h t c a r b o x y l i c
acids.
The pL value at ~ = 0.5 was greater in polybasic acid than in a monobasic acid, greater in a tribasic acid than in a dibasic acid and decreased with the increase in the number of carbon atoms b e t w e e n two functional groups. The decrease in pL value was large between malonic acid and succinic acid but was slight among succinic, glutaric and adipic acids (Table II). The tendencies ob-
served among acetate, malonate, succinate etc were similar to those observed for Zn, Pb and Cu complexes (Yasuda et al., 1960). DISCUSSION The effect of the hydrolysis of A1 ion will be considered first. The hydrolysis constant Q,1 = [MOH]/[M] is about 10 -s, so the formation constant of hydroxide KMO n -=[MOH]/[M] [OH] - Q I ~ / K w amounts to 109. This value is far greater than the stability of Al acetate, about 103, which was calculated ignoring hydrolysis. Thus it may be possible for a mixed complex with hydroxyl groups to occur even though the concentration of acetate ion, ca. 10 -3 mole/l was far greater than that of hydroxyl, ca. 10 -'° in the system. The hydrolysis, however, did not appear to affect the determination of pH nor the concentration of the dissociated ligand. On the other hand, there is a possibility that some mixed complexes with protons may occur in the acidic systems described in the preceding section. On these premises, the stability of the A1 complex will be discussed by comparing the values of p L or p{[HA]/[H] ) at n = 0.5 with each other. The examination of potentiometric titration curves suggests that the carboxyl group can be coordinated with the A1 ion regardless of its dissociation constant {Fig. 1). Moreover, the stability of an A1 complex with a low molecular-weight acid was shown to increase with decreasing distance between two adjacent carboxylic groups in a molecule or with increasing basicity {Table II). According to these results, complex formation of A1 with a polymeric carboxylic acid of high molecular weight can be ascribed partly to the inherent ability of carboxyl group to bind A1 and partly to intensified binding caused by certain arrangements of ligands or by the increased basicity in the polymer. The bonding between A1 and carboxyl may not be a primary bond but an electrostatic bond because the A1 ion is a typical "hard" acid {Pearson, 1963). Recent studies, however, have indicated various types of bonding in metal complexes with humic substances. Ferric ions {"hard" acids) could form complexes of inner spheric character {Gamble et al., 1976; Senesi et al., 1977). Yariv et al. {1966) suggested a water bridge combining organic molecules with exchangeable cations on montmorillonite. This mechanism was supported by (3amble et al. (1977) for an Mn2÷--fulvate complex, where the water of hydration was kept unexchanged with the ligand (Gamble et al., 1976). Mn2+--humate was also shown to be an outer sphere complex (McBride, 1978). Mn 2÷ ion is of a "borderline" nature between " h a r d " and "soft" acids. The cupric ion (a "borderline" acid) was considered to form a covalent complex (Gamble et al., 1976) or electrostatic and partly covalent complex with humic substances (McBride, 1978). Concerning the additional electrostatic field effect or polymeric effect, Gamble {1973) concluded that Na ÷ and K ÷ ions are held by these forces in the fulvate system. Gregor et al. (1955) analysed the complex formation of Cu 2÷ with various carboxylic acids ranging from acetic to polyacrylic acids
10 and they explained polymeric effects as due to the stabilization of 8-membered chelates by the powerful field effects of the polyelectrolyte chains. Similarly, some contribution of electrostatic field effects to the stabilization of A1 complexes seems probable, because the stability of A1 complexes tended to increase in the compounds having easily disssociating ligands, e.g. alginic acid>humic acids>polyacrylic acids, or malonic acids>other low molecular weight acids (Tables I and II). In the acid mediums used in this experiment, the reacting functional group might be easily dissociated (Type I according to Gamble, 1970, or S cluster of Arai and Kumada, 1977), whereas weakly acidic groups might react only with difficulty. Among the former are the ortho-carboxy-phenol(salicylate) and ortho
11
tures or dissociations could contribute in different ways to the formation of A1 complexes, at least in fulvic and humic acids. ACKNOWLEDGEMENTS
We are indebted to Dr. N. Nakasuka for several helpful discussions and to Mr. Stephen Duah-Yentumi for reading through the English manuscript and wish to thank Mrs. Iwatsuki for technical assistance. The research was supported in part by a "Grant-in-Aid" from the Ministry for E d u c a t i o n Japan, on soil organic matter {subject No. 347096).
REFERENCES
Arai, S. and Kumada, K., 1977. An interpretation of the conductometric titration curve of humic acid. Geoderma, 19: 21--35. Babko, H. and Rychkova, T.N., 1949. Salicylate complex of aluminum. Zhur. Obschei Khim., (J. Gen. Chem.), 18: 1617--1725. C.A., 43: 2536d (1949). Baes, C.F. Jr. and Mesmer, R.E., 1976. The Hydrolysis of Cations. Wiley, New York, N.Y., pp. 112--123. Beckwith, R.S., 1959. Titration curves of soil organic matter. Nature, 184 : 745. Bloom, P.R. and McBride, M.B., 1979. Metal ion binding and exchange with hydrogen ions in acid washed peat. Soil Sci. Soc. Am. J., 43: 687--692. Bresnahan, W.T., Grant, C.L. and Weber, J.H., 1978. Stability constants for the complexation of copper(II) ions with water and soil fulvic acids measured by an ion selective electrode. Anal. Chem., 50: 1675--1679. Coleman, N.T., McClung, A.C. and Moore, D.P., 1956. Formation constants for Cu(II)peat complexes. Science, 123: 330--331. Gamble, D.S., 1970. Titration curves of fulvic acid: the analytical chemistry of a weak acid polyelectrolyte. Can. J. Chem., 48: 2662--2669. Gamble, D.S., 1973. Na ÷ and K+ binding by fulvic acid. Can. J. Chem., 51: 3217--3222. Gamble, D.S., Schnitzer, M. and Hoffman, I., 1970. Cu:÷-fulvic acid chelation equilibrium in 0.1 M KCI at 25.0°C. Can. J. Chem., 48: 3197--3204. Gamble, D.S., Langford, C.H. and Tong, J.P.K., 1976. The structure and equilibria of a manganese(II) complex of fulvic acid studied by ion exchange and nuclear resonance. Can. J. Chem., 54: 1239--1245. Gamble, D.S., Schmitzer, M. and Skinner, D.S., 1977. MnII--fulvic acid complexing equilibrium measurements by electron spin resonance spectroscopy. Can. J. Soil Sci., 57: 47--53. Geering, H.R. and Hodgeson, J.F., 1969. Micronutrient cation complexes in soil solution, III. Characterization of soil solutior ligands and their complexes with Zn ~* and Cu ~*. Soil Sci. Soc. Am. Proc., 33: 54--59. Greenland, D.J., 1971. Interactions between humic and fulvic acids and clays. Soil Sci., 111 : 34--41. Gregor, H.P., Luttinger, L.B. and Loebl, E.M., 1955. Metal--polyelectrolyte complexes, I. The polyacrylic acid---copper complex. J. Phys. Chem., 59: 34--39. Himes, F.L. and Barber, S.A., 1957. Chelating ability of soil organic matter. Soil Sci. Soc. Am. Proc., 21: 363--373. Kawaguchi, K. and Kyuma, K., 1959. On the complex formation between soil humus and polyvalent cations. Soil Plant Food, 5: 54---63.
12 Kumada, K., Sato, O., Ohsumi, Y. and Ohta, S., 1967. Humus composition of mountain soils in central Japan with special reference to the distribution of P type humic acid. Soil Sci. Plant Nutr., 13: 151--158. Langford, C.H. and Khan, T.R., 1975. Kinetics and equilibrium of binding of Fe 3÷ by a fulvic acid: a study by stopped flow methods. Can. J. Chem., 53: 2979--2984. McBride, M.B., 1978. Transition metal bonding in h~,mic acid: an ESR study. Soil Sci., 126: 200--209. Pearson, R.G., 1963. Hard and soft acids and bases. J. Am. Chem. Soc., 85: 3533--3539. Rosell, R.A., Miglierina, A.M. and De Novilla, L.Q., 1977. Stability constants of some complexes of Argentine humic acids and micronutrients. Soil O~ganic Matter Studies, II: 15--21. Saar, R.A. and Weber, J.H., 1979. Complexation of cadmium(II) with water- and soilderived fulvic acids: effect of pH and fulvic acid concentration. Can. J. Chem., 57 : 1263-1268. Schnitzer, M. and Desjardins, J.G., 1962. Molec,:lar and equivalent weights of the organic matter of a podzol. Soil Sci. Soc. Am. Proc., 26: 362--365. Schnitzer, M. and Skinner, S.I.M., 1965. Organo-metallic interactions in soils, 4. Carboxyl and hydroxyl groups in organic matter and metal retention. Soil Sci., 99: 278--284. Schnitzer, M. and Khan, S.U., 1972. Humic substances in the Environment. Dekker, New York, N.Y., pp. 203--251. Senesi, N., Griffith, S.M. and Schnitzer, M., 1977. Binding of Fe 3÷ by humic materials. Geochim. Cosmochim. Acta, 41: 969--976. Stevenson, F.J., 1976. Stability constants of Cu ~÷, Pb ~÷, and Cd ~÷ complexes with humic acids. Soil Sci. Soc. Am. J., 40: 665--672. Takamatsu, T. and Yoshida, T., 1978. Determination of stability constants of metal-humic acid complexes by potentiometric titration and ion-selective electrodes. Soil Sci., 125 : 377--386. Tanaka, M. and Nakagawa, M, 1966. San-enki heiko to chuwa tekitei (acid-base equilibrium and neutralization analysis). In: M. Kotake et al. (Editors), Jikken Kagaku Koza (experimental chemistry). Maruzen, Tokyo, II-7, pp. 1--14 (in Japanese). Van Dijk, H., 1971. Cation binding of humic acids. Geoderma, 5: 53--67. Wagner, G.H. and Stevenson, F.J., 1965. Structural arrangement of functional groups in soil humic acids as revealec by infrared analysis. Soil Sci. Soc. Am. Proc., 29: 43--48. Wood, J.C., Moschopedis, S.E. and Den Hertog, W., 1961. Studies in humic acid chemistry. II. Humic anhydrides. Fuel, 40: 491--502. Yariv, S., Russel, J.D. and Farmer, V.C., 1966. Infrared study of the adsorption of benzoic acid and nitrobenzene in montmorillonite. Israel J. Chem., 4: 201--213. Yasuda, M., Yamasaki, K. and Ohtaki, H., 1960. Stability of complexes of several carboxylic acids formed with bivalent metals. Bull. Chem. Soc. Jpn., 33: 1067--1070. Yoshida, M. and Nakao, Y., 1971. Behavior of aluminum--humic acid complexes as revealed by titration curves. J. Sci. Soil Manure, Jpn., 42 : 333--337 (in Japanese with English abstract).
1
FACTORS CONTROLLING STABILITY OF A1-HUMATE
SHIGEMITSU ARAI and KYOICHI KUMADA
Faculty of Agriculture, Nagoya University, Nagoya (Japan) (Received July 10, 1979; accepted after revision March 24, 1981)
ABSTRACT Arai, S. and Kumada, K., 1981. Factors controlling stability of Al-humate. Geoderma, 26: 1--12. S o m e of the factors controlling Al bumate bonding, a major aspect of organo-mineral interactions in soils,were studied. The methods used were potentiometric titration and stability measurement. The samples consisted of 4 humic acids plus 2 polymeric and several monomeric carboxylic acids as model compounds. Humic acids as well as alginic and polyacrylic acids were shown to form complexes with AI. The stability of Al humate complexes was shown to be controlled by polymerization of carboxyl-containing units and by the arrangement of carboxyl groups in the units. A contribution by salicylate type units was doubtful.
INTRODUCTION
One of the main aspects of organo-mineral interactions in softs is complex formation between soil organic matter and metal ions. Many investigations were devoted ~o the nature of these complexes and it is therefore possible nowadays to refer to the formation constants of metal complexes with soil organic matter {Coleman et al., 1956; Himes and Barber, 1957; Gcering and Hodgson, 1969; Schnitzer and Khan, 1972; Langford and Khan, 1975; Stevenson, 1976; Rosell et al., 1977; Bresnahan et al., 1978; Takamatsu and Yoshida, 1978, etc.) or to data on the characteristics of this bonding (Schnitzer and Skinner, 1965; Yariv et al., 1966; Schnitzer and Khan, 1972; Gamble et al., 1976; Gamble et al., 1977; Senesi et al., 1977; McBride, 1978; Bresnahan et al., 1978, etc.). More information has also been obtained on the chemical structure of soil organic matter which is intimately related to complex formation (Beckwith, 1959; Wood et al., 1961; Wagner and Stevenson, 1965; Gamble, 1970; Van Dijk, 1971, etc.). Nevertheless, there are many problems still remaining to be solved. Although aluminum is universal in softs and is closely related to soil organic matter ac,cumulation {Greenland, 1971), the relationships between aluminum and soil organic matter have not been fully investigated. This is due to the diversity of aluminum compounds in soils and to the complexity of soil organic matter.
0 0 1 6 - 7 0 6 1 / 8 1 / 0 0 0 0 - - 0 0 0 0 / $ 02.50 © 1981 Elsevier Scientific Publishing Company
Thus, the purpose of this study was to ascertain the nature of Al-humate formation, to determine conditional stability constants and to examine the factors affecting complex formation, such as chemical structure and polymerization. For the study, high and low molecular weight model compounds and humic acids were used. MATERIALS AND METHODS Materials
Four humic acid samples which belong to the A, B, Rp and l) types, according to Kumada's classification (Kumada et al., 1967}, were used. These were Shitara humic acid extracted from a so-called "kuroboku soil" (Umbrept)-A type, Kuragari sample from an acid brown forest soil (Umbrept)-B type, Sanage sample from a grey lowland soil (Aquent ?)-Rp type and Tsubame sample from a buried horizon-P type. These were the same samples as those covered by the authors in a previous report (Arai and Kumada, 1977). The humic acids were extracted with 0.1N NaOH, refined by repeated acidification and dissolution and stored as dried powders. Prior to determining pH and stability constants, the samples were redissolved in 0.1N NaOH and refined through ion exchange resins (IR 120 and IRA 400). The ash contents (dry weight basis) were as follows: Shitara 3.02%, Kuragari 3.66%, Sanage 0.79% and Tsubame 2.62%. Polyacrylic acid and alginic acid samples were prepared from sodium polyacrylate (average degree of polymerization, 3--4 × 104, Nakarai Chemicals Ltd.) and alginic acid (Nakarai) after being dissolved in water and 0.1N NaOH solutions, respectively, and passed through anion and cation exchangers as was done with the humic acids. Concentrations (N) were determined conductometrically. The other carboxylic acids were: acetic acid (Wako Pure Chemical Industries, Ltd., JIS special grade), malonic acid (Wako, Wako special grade), succinic acid (Kanto Chemicals Co., Inc., guaranteed grade}, glutaric acid (Nakarai, extra pure grade), adipic acid {Wako, JIS special grade), 1,2,3-propanetricarboxylic acid (Wako, Wako special grade), 1,3,5pentanetricarboxylic acid (Nakarai, extra pure grade), benzoic acid (Wako, JIS special grade) and o-phthalic acid (Tokyo Chemical Industry Co., Ltd. guaranteed grade). Their concentrations were determined volumetrically. P o t e n t i o m e t r i c titration curves
An automatic titrator RAT-11S produced by Hiranuma Sangyo Co., Ltd. equipped with a reference electrode K-421N and a glass electrode MG-511 produced by Denki Kagaku Keiki Co., Ltd. was used. The pH values were measured without any supporting electrolyte. The interval of titration was set at 30 sec but, if equilibrium was not attained during that period, the interval was extended automatically. Calibration was done with use of standard pH buffers, i.e. K-phthalate (pH 7) and K-oxalate (pH 4), according to Japanese Industrial Standard (JIS K8802-1958).
4.60 4.86 4.64 4.77 4.86 5.35 4.89 4.72 2.89 2.91 4.76 4.88 log ~ , s 5.0946 5.1863 4.8461 4.8831 4.5272 4.5317
Shitara
acid
4.44 5.25 2.89 3.24 2.42 3.20 4.00 4.00 1.18 1.29 0.90 1.02 log 13.6 2.5673 2.5849 3.9444 3.9363 ---
n .2
0.60 0.58 --1.65 --1.64 --1.91 --1.91
0.33 0.34 0.27 0.27 0.27 0.25 0.37 0.36 0.57 0.53 0.23 0.25
log b = p ( [ H A ] / [ H ] ) ; i = o . 5 .3
1.67 1.77 3.97 3.92 3.69 3.70
2.91 2.92 2.92 2.92 2.88 2.90 2.85 2.85 2.80 2.84 3.15 3.14
---
--
----
0.3 0.3 0.2 0.2 0.1 0.1 0.2 0.2 0.4 0.5 0.1 0.1
pHi- =0.5 a ~ = 0 . 5 * '
× 1 0 - 3 : 7 . 6 0 8 × 1 0 - ' = 13.1
× 1 0 - 3 : 7 . 6 0 8 × 10-4 = 1 3 . 0
1 . 9 7 6 × 1 0 - 2 : 1 . 5 2 1 × 10 -3 = 1 3 . 0
9.95
9.87
0 . 5 7 5 x 1 0 - a : 3 . 0 4 3 x 10 -4 = 1.9
1 . 0 5 5 × 1 0 - 3 : 4 . 5 6 5 × 10 .4 = 2.3
1 . 0 9 3 × 1 0 - 3 : 4 . 5 6 5 x 10 -4 = 2.4
1 . 0 9 4 × 10 - 3 : 4 . 5 6 5 × 10-4 = 2.4
1 . 1 0 5 × 1 0 - 3 : 4 . 5 6 5 x 10-4 = 2.4
1 . 0 9 8 × 1 0 - s : 4 . 5 6 5 × 10-4 = 2.4
T A ( e q u i v . / l ) : A l ( m o l e / l ) .5
N o t e : t o t a l v o l u m e 30 ml a n d t i t r a t e d b y 0 . 1 N N a O H ( S h i t a r a - - p o l y a c r y l i c a c i d ) o r 1N N a O H ( m a l o n i c - - a c e t i c acid). ,1 a n d ,2 C o n s t a n t s in m o d i f i e d H e n d e r s o n - - H a s s e l b a c h e q u a t i o n , p H = p K a - - n l o g ( 1 - - ~ ) / , ~ . ,3 H A ; u n d i s s o c i a t e d acid g r o u p . ~; a v e r a g e n u m b e r o f acid g r o u p b o a n d to m e t a l ion. "4 ~ ; d e g r e e o f d i s s o c i a t i o n . , s T A ; t o t a l c o n c e n t r a t i o n o f acid. ,6 ~1 a n d 132; ( i o n i z a t i o n c o n s t a n t ) -l.
Acetic
S u c c i n i c acid
M a l o n i c acid
P o l y ~ c r y l i c acid
Alginic acid
Tsubame
Sanage
Kuragari
p K a *~
Sample
C o m p l e x a t i o n c o n s t a n t s o f AI w i t h h u m i c a n d t h e o t h e r a c i d s in 1M N a C I O ,
TABLE I
50
4.515 4 . 5 1 5 av. 4 . 5 1 5
4 . 9 7 7 av. 4 . 9 7 6 4.973 4.982 4 . 9 9 0 av. 4 . 9 8 6
Glutaric acid Adipic acid
1,2,3-Pro- 5.744 p a n e t r i c a r - 5 . 7 4 5 av. 5 . 7 4 4
4.606
4.603 av. 4 . 6 0 5
2.676 av. 2 . 6 7 2 2.668
3.743
3.750
av. 3 . 7 4 6
3.440 3 . 4 3 3 av. 3 . 4 3 7
---_
---
-------_
l o g 53*~
Note. Total volume 30 ml and titrated by 1N NaOH. ,1 ~1--~3 ( i o n i z a t i o n c o n s t a n t ) -i. ,2 L = dissociated acid group. ,3 T L = t o t a l c o n c e n t r a t i o n o f acid.
1,3,5-Pen- 5.186 5.184 t a n e t r i c a r - 5 . 1 8 2 av" boxylic acid
boxylic acid
4 . 1 3 0 av. 4 . 1 3 1 4.133 4.238 4 . 2 3 3 av. 4 . 2 3 5
4.911 av. 4 . 9 0 8 4.906
----2.976 av. 2 . 9 8 0 2.983 3.986 3 . 9 8 4 av. 3 . 9 8 5
0-Phthalic acid
av. 5 . 1 5 8
av. 5 . 0 1 7
av. 3 . 9 7 2
av. 4 . 5 3 7
4.538 4.537 3.972 3.972 5.018 5.016 5.160 5.155
Acetic acid Benzoic acid Malonic acid Succinic acid
l o g ~2*~
l o g ~*~
Acid
,.
4.53
4.52
av. 4 . 5 3
5.09 5 . 1 3 av. 5 . 1 1
3 . 8 5 av. 3 . 8 5 3.85 3.79 3 . 7 9 av. 3 . 7 9
4.07 av. 4 . 0 6 4.05
av. 3 . ~ 5
av. 6.8
av. 2 . 7 0
av. 2 . 6 5
4.00
4.01
3.97 3.96
3.92 3.92 3.98 3.98
3.17 3.21
3.66 3.68 3.22 3.11 1.76 1.80 3.91 3.91
k=PLn=o.~2pH~=o. 5
2.66 2.63 2.77 2.83 6.7 6.8 3.94 3.96
log
C o m p l e x a t i o n c o n s t a n t s o f AI w i t h s e v e r a l l o w m o l e c u l a r c o m p o u n d s in 0 . 1 M NaCIO4
T A B L E II
3 . 2 9 × 1 0 - 3 : 7 . 6 1 × 1 0 -4 = 4 . 3
4.3
6.5
4 . 9 7 × 1 0 - 3 : 7 . 6 1 × 10-* L 3 . 3 0 × 1 0 - 3 : 7 . 6 1 × 1 0 -4 =
6.5
4 . 9 8 × 1 0 -3:7.b3 × 1 0 -4 =
6.5
6.4
4 . 9 5 x 1 0 - 3 : 7 . 6 1 x 1 0 -4 = 5 . 9 8 × 1 0 - 3 : 9 . 1 3 x 1 0 -4 =
6.4
4 . 9 5 × 1 0 - 3 : 7 . 1 6 x 10 -4 =
1 . 2 9 x 1 0 - 2 : 9 . 1 3 x 1 0 -4 = 1 4
1 . 9 9 x 1 0 - 2 : 1 . 5 2 x 1 0 -3 = 1 3
TL(mole/l):Al(mole/l),3
Conditional stability c o n s t a n t s
The potential difference (E, mV) was measured in 1M NaC104 solution (humic acids etc.) or 0.1M NaClO4 (low molecular weight organic acids). The value was then converted to the hydrogen ion concentration using a calibration curve prepared from an HCIO4 standard solution at the same NaC104 level. In these measurements, the KC1 solution of the reference electrode was replaced by NaC1 solution to avoid interference due to the precipitation of KC104. The ionic strength of a polymeric acid system, u = 1, was selected after a preliminary test so that gelation was almost complete without A13÷. Titration was begun after allowing the mixed solution to stand for 2--3 h. The NaOH standard solution was added using a microsyringe. Enough time was allowed for equilibrium to be attained and, in most cases, the interval was 0.5-10 min. Each determination was made in a range of pH (---log [H] ) less than 4 to minimize the effect of the hydrolysis of A1 ions, because the hydrolysis constant of A1, Q,1, is about 10 -s (Baes and Mesmer, 1976). To keep the pH low, a suitable amount of perchloric acid was added if necessary. The ratio of ligand to metal (mole or equivalent to mole) was kept as high as possible. These ratios are shown in Table I and II. Calculation
Conditional stability constants (b) of high molecular-weight polymeric acids, including humic acids, and some low molecular-weight acids were determined with a modification of Bjerrum's method (Gregor et al., 1955) by calculating p ( [ H A ] / [ H ] ) = log ka.le = log b, against n, where [HA] is the concentration of the undissociated acid group, ka the dissociation constant, k the formation constant and n the average number of ligands to the total concentra tion of metal ions. Constants for low molecular-weight acids were obtained by Bjerrum's method. In these cases, the formation of mixed complexes, such as M L p X q , where M is the metal, L the ligand and X hydronium or hydroxyl ion and p and q are constants, was not taken into consideration for the first approximation. m
RESULTS
The p H titration curves
Fig. 1 shows the pH titration curves of Shitara humic acid, polyacrylic acid, alginic acid and acetic acid, with and without A1 ions. In these figures, the dotted lines are hypothetical curves obtained by additions of titration curves of the organic acids to those of A1 with a correction as described below. It is shown that the titration curve for an acid HA and a strong base B follows the equation, t = C B / C H A = ( K w / [ H ] - - [ H ] ) / C H A + (1 + [ H ] K H A ) - I , where t is the degree of neutralization, C B and CHA the cor4centrations of the added
I0
I0
(b)
9
9
7
7
6
6
5
5
" o
.........
; .........
;,
"
'# ~ "
"o'.,
.........
0'2 ........
'o'3
/
8 7
6
6
.," ~
+ f " ll" l
" .~
5
"
.
0 O.INHCI ml
.~'
,o (d9
~7
5
÷
o ......
,o9
8
÷ ..'Ib'"
I O.INNaOH ml
.
.
.
0 O.INHCI ml
.
.
.
.
.
.
.
.
|
I
.
.
.
.
.
.
.
.
.
!
.
.
.
.
.
2
0.1N NaOH ml
Fig. 1. The potentiometric titration curves of Shitara humic acid (a), polyacrylic acid ( b ) , alginic acid (c) and acetic acid (d) with and without Al. Total volume 25 ml; o - a c i d , A = A I , ® = acid with Al and + = quasi-theoretical values assuming the simple mixing of acid and A l .
base and the acid, respectively, Kw the ionic product of water ([H] .[OH] ), and KHA the dissociation constant of the acid (Tanaka and Nakagawa, 1966). When two titration curves are added together, the first term in the above equation will be summed up also so that correction of this term is necessary. The effect of this correction was almost negligible between pH 4 and 10. In a mixed solution of acetic and oxalic acids, the curve thus corrected agreed well with the measured one (not shown in a figure). The titration curves for humic acid and other acids with AI crossed the dotted curves at about pH 5, i.e. excess proton was set free in the acidic range whereas the hydroxyl ion increased more than expected if the hydrolysis of the Al ion took place at a pH of more than about 5 (Fig. 1). This phenomenon seems due to coordination of the ligands of acids with cations instead of water. The same results were obtained regardless of the type of the humic acid or the ratio of humic acid to Al. Similar observations were made by earlier Kawaguchi and Kyuma (1959) and Yoshida and Nakao (1971). The effect of acetic acid on Al titration was similar to that of humic acid although
the effect of the former was slight (Fig. l(d)). Thus, coordination of the carboxyl group with the A1 ion appears c o m m o n to carboxylic acids. Conditional stability constants (b, or K,) Modified Bjerrum plots for humic and other acids are shown in Fig. 2 and the values of p ( [ H A ] / [ H ] ) at n = 0.5 are tabulated in Table I. If successive formation constants are separated far enough, p ( [ H A ] / [ H ] ) at n = 0.5 equals log b, - log [MA] [ H ] / [ M ] [HA] (Gregor et al., 1955). But, as seen in Fig. 2, the separation was not large enough, although circumstances differed from sample to sample. Thus, the p ( [ H A ] / [ H ] ) value should be taken as an index describing the relative tendency of complex formation at a condition of n =0.5. m
w
L~
o
~
o.,
It
o
"~
0.6
!
o
E
I
ii I
-0.2
°'"
'
I
-0.1
0
I
I
I
I
I
..I
0.1
0.2
0.3
0.4
0.5
0.6
Fig. 2. Modified Blerrum plot of AI humates, polyaerylate, alginate, malonate, sueeinate and acetate.
Conditional ,c~ability measurements are reported to be theoretically incomplete for polydisperse systems, because stability can be influenced by experimental conditions such as degree of dissociation, cation concentration etc. (Gamble et al., 1970; Stevenson, 1976; Saar and Weber, 1979). The effect of sample concentration was relatively small in Cu 2÷ fulvate system (Gamble et al., 19"/0) or in the solution of low Cd 2÷ to fulvic acid ratio (Saar and Weber, 19"/9). In our measurements, as indicated in Table I, the ratio of A1 to humie or other polymeric acids (ca. 1 mole: 2.4 equiv.) or the degree of dissociation (~ 0.1--0.3) was low and pH values (< 4) remained within a limited range. Therefore, the values of conditional stability could be used for relative comparison. For humic acids, the n values in the modified Henderson-Hasselbalch equation were higher than the ones usually observed, viz. 1--2 (Table I). This may
be due to the fact that different types of acid groups occur together even though the equation seems applicable within a restricted range of ~ (0.1--0.4). The values of p ( [ H A ] / [ H ] } at n = 0.5 were slightly greater in Shitara (A t y p e ) and Tsubame (P) than in Kuragari (B) and Sanage (Rp). These results appear reasonable because the amounts of acid functional groups and strongly dissociating groups in these humic acids are in t h e order of A > B , P> Rp (Arai and Kumada, 1977). The degree of dissociation ~ at n = 0.5 differed between Shitara (0.3) and Tsubame (0.2) ~.nd b e t w e e n Kuragari (0.2) and Sanage (0.1) (Table I). Consequently, if it were possible to compare t h e m at identical conditions, the difference among the p ( [ H A ] / [ H ] ) values o f various types of humic acid would become clearer. The values of p ( [ H A ] / [ H ] ) at n = 0.5 for the humic acids were between those for alginic and polyacrylic acids, though there remained some uncertainty about the value of polyacrylic acid because the pH at n = 0.5 was so low that the constants Ka and n in the modified Henderson-Hasselbalch equation might also include some uncertainty. The values of p C [ H A l / [ H ] ) at n = 0.5 of humic and other polymeric acids were less than t h a t of malonic acid and greater than those of acetic and succinic acids. Bjerrum plots of low molecular weight carboxylic acids are shown in Fig. 3, and the pL values at n = 0.5 are listed in Table II. The c o m p o u n d s used ranged from mono- to tri-basic acids. Among polybasic acids, the number of carbon atoms between two carboxyl groups ranged from 1 to 4. Two aromatic acids, benzoic and phthalic acids, were also included. The value n = 0.5 was attained at pH less than 4 in each case. The pL at n = 0.5 would be log k, if the successive complexations were sufficiently separated and mixed complexes were absent. ~_~
.o
~_ o
~
=o
_
.-
~o o~e
;~oo:~e
\
C:].~
0
2.5
I
I
I
I
i
I
I
:
I
30
3.0
4.0
4.5
5,0
5,0
6.0
6,5
7.0
pL F i g . 3. Bjei~rum plot of AI c o m p l e x with several low m o l e c u l a r w e i g h t c a r b o x y l i c
acids.
The pL value at ~ = 0.5 was greater in polybasic acid than in a monobasic acid, greater in a tribasic acid than in a dibasic acid and decreased with the increase in the number of carbon atoms b e t w e e n two functional groups. The decrease in pL value was large between malonic acid and succinic acid but was slight among succinic, glutaric and adipic acids (Table II). The tendencies ob-
served among acetate, malonate, succinate etc were similar to those observed for Zn, Pb and Cu complexes (Yasuda et al., 1960). DISCUSSION The effect of the hydrolysis of A1 ion will be considered first. The hydrolysis constant Q,1 = [MOH]/[M] is about 10 -s, so the formation constant of hydroxide KMO n -=[MOH]/[M] [OH] - Q I ~ / K w amounts to 109. This value is far greater than the stability of Al acetate, about 103, which was calculated ignoring hydrolysis. Thus it may be possible for a mixed complex with hydroxyl groups to occur even though the concentration of acetate ion, ca. 10 -3 mole/l was far greater than that of hydroxyl, ca. 10 -'° in the system. The hydrolysis, however, did not appear to affect the determination of pH nor the concentration of the dissociated ligand. On the other hand, there is a possibility that some mixed complexes with protons may occur in the acidic systems described in the preceding section. On these premises, the stability of the A1 complex will be discussed by comparing the values of p L or p{[HA]/[H] ) at n = 0.5 with each other. The examination of potentiometric titration curves suggests that the carboxyl group can be coordinated with the A1 ion regardless of its dissociation constant {Fig. 1). Moreover, the stability of an A1 complex with a low molecular-weight acid was shown to increase with decreasing distance between two adjacent carboxylic groups in a molecule or with increasing basicity {Table II). According to these results, complex formation of A1 with a polymeric carboxylic acid of high molecular weight can be ascribed partly to the inherent ability of carboxyl group to bind A1 and partly to intensified binding caused by certain arrangements of ligands or by the increased basicity in the polymer. The bonding between A1 and carboxyl may not be a primary bond but an electrostatic bond because the A1 ion is a typical "hard" acid {Pearson, 1963). Recent studies, however, have indicated various types of bonding in metal complexes with humic substances. Ferric ions {"hard" acids) could form complexes of inner spheric character {Gamble et al., 1976; Senesi et al., 1977). Yariv et al. {1966) suggested a water bridge combining organic molecules with exchangeable cations on montmorillonite. This mechanism was supported by (3amble et al. (1977) for an Mn2÷--fulvate complex, where the water of hydration was kept unexchanged with the ligand (Gamble et al., 1976). Mn2+--humate was also shown to be an outer sphere complex (McBride, 1978). Mn 2÷ ion is of a "borderline" nature between " h a r d " and "soft" acids. The cupric ion (a "borderline" acid) was considered to form a covalent complex (Gamble et al., 1976) or electrostatic and partly covalent complex with humic substances (McBride, 1978). Concerning the additional electrostatic field effect or polymeric effect, Gamble {1973) concluded that Na ÷ and K ÷ ions are held by these forces in the fulvate system. Gregor et al. (1955) analysed the complex formation of Cu 2÷ with various carboxylic acids ranging from acetic to polyacrylic acids
10 and they explained polymeric effects as due to the stabilization of 8-membered chelates by the powerful field effects of the polyelectrolyte chains. Similarly, some contribution of electrostatic field effects to the stabilization of A1 complexes seems probable, because the stability of A1 complexes tended to increase in the compounds having easily disssociating ligands, e.g. alginic acid>humic acids>polyacrylic acids, or malonic acids>other low molecular weight acids (Tables I and II). In the acid mediums used in this experiment, the reacting functional group might be easily dissociated (Type I according to Gamble, 1970, or S cluster of Arai and Kumada, 1977), whereas weakly acidic groups might react only with difficulty. Among the former are the ortho-carboxy-phenol(salicylate) and ortho
11
tures or dissociations could contribute in different ways to the formation of A1 complexes, at least in fulvic and humic acids. ACKNOWLEDGEMENTS
We are indebted to Dr. N. Nakasuka for several helpful discussions and to Mr. Stephen Duah-Yentumi for reading through the English manuscript and wish to thank Mrs. Iwatsuki for technical assistance. The research was supported in part by a "Grant-in-Aid" from the Ministry for E d u c a t i o n Japan, on soil organic matter {subject No. 347096).
REFERENCES
Arai, S. and Kumada, K., 1977. An interpretation of the conductometric titration curve of humic acid. Geoderma, 19: 21--35. Babko, H. and Rychkova, T.N., 1949. Salicylate complex of aluminum. Zhur. Obschei Khim., (J. Gen. Chem.), 18: 1617--1725. C.A., 43: 2536d (1949). Baes, C.F. Jr. and Mesmer, R.E., 1976. The Hydrolysis of Cations. Wiley, New York, N.Y., pp. 112--123. Beckwith, R.S., 1959. Titration curves of soil organic matter. Nature, 184 : 745. Bloom, P.R. and McBride, M.B., 1979. Metal ion binding and exchange with hydrogen ions in acid washed peat. Soil Sci. Soc. Am. J., 43: 687--692. Bresnahan, W.T., Grant, C.L. and Weber, J.H., 1978. Stability constants for the complexation of copper(II) ions with water and soil fulvic acids measured by an ion selective electrode. Anal. Chem., 50: 1675--1679. Coleman, N.T., McClung, A.C. and Moore, D.P., 1956. Formation constants for Cu(II)peat complexes. Science, 123: 330--331. Gamble, D.S., 1970. Titration curves of fulvic acid: the analytical chemistry of a weak acid polyelectrolyte. Can. J. Chem., 48: 2662--2669. Gamble, D.S., 1973. Na ÷ and K+ binding by fulvic acid. Can. J. Chem., 51: 3217--3222. Gamble, D.S., Schnitzer, M. and Hoffman, I., 1970. Cu:÷-fulvic acid chelation equilibrium in 0.1 M KCI at 25.0°C. Can. J. Chem., 48: 3197--3204. Gamble, D.S., Langford, C.H. and Tong, J.P.K., 1976. The structure and equilibria of a manganese(II) complex of fulvic acid studied by ion exchange and nuclear resonance. Can. J. Chem., 54: 1239--1245. Gamble, D.S., Schmitzer, M. and Skinner, D.S., 1977. MnII--fulvic acid complexing equilibrium measurements by electron spin resonance spectroscopy. Can. J. Soil Sci., 57: 47--53. Geering, H.R. and Hodgeson, J.F., 1969. Micronutrient cation complexes in soil solution, III. Characterization of soil solutior ligands and their complexes with Zn ~* and Cu ~*. Soil Sci. Soc. Am. Proc., 33: 54--59. Greenland, D.J., 1971. Interactions between humic and fulvic acids and clays. Soil Sci., 111 : 34--41. Gregor, H.P., Luttinger, L.B. and Loebl, E.M., 1955. Metal--polyelectrolyte complexes, I. The polyacrylic acid---copper complex. J. Phys. Chem., 59: 34--39. Himes, F.L. and Barber, S.A., 1957. Chelating ability of soil organic matter. Soil Sci. Soc. Am. Proc., 21: 363--373. Kawaguchi, K. and Kyuma, K., 1959. On the complex formation between soil humus and polyvalent cations. Soil Plant Food, 5: 54---63.
12 Kumada, K., Sato, O., Ohsumi, Y. and Ohta, S., 1967. Humus composition of mountain soils in central Japan with special reference to the distribution of P type humic acid. Soil Sci. Plant Nutr., 13: 151--158. Langford, C.H. and Khan, T.R., 1975. Kinetics and equilibrium of binding of Fe 3÷ by a fulvic acid: a study by stopped flow methods. Can. J. Chem., 53: 2979--2984. McBride, M.B., 1978. Transition metal bonding in h~,mic acid: an ESR study. Soil Sci., 126: 200--209. Pearson, R.G., 1963. Hard and soft acids and bases. J. Am. Chem. Soc., 85: 3533--3539. Rosell, R.A., Miglierina, A.M. and De Novilla, L.Q., 1977. Stability constants of some complexes of Argentine humic acids and micronutrients. Soil O~ganic Matter Studies, II: 15--21. Saar, R.A. and Weber, J.H., 1979. Complexation of cadmium(II) with water- and soilderived fulvic acids: effect of pH and fulvic acid concentration. Can. J. Chem., 57 : 1263-1268. Schnitzer, M. and Desjardins, J.G., 1962. Molec,:lar and equivalent weights of the organic matter of a podzol. Soil Sci. Soc. Am. Proc., 26: 362--365. Schnitzer, M. and Skinner, S.I.M., 1965. Organo-metallic interactions in soils, 4. Carboxyl and hydroxyl groups in organic matter and metal retention. Soil Sci., 99: 278--284. Schnitzer, M. and Khan, S.U., 1972. Humic substances in the Environment. Dekker, New York, N.Y., pp. 203--251. Senesi, N., Griffith, S.M. and Schnitzer, M., 1977. Binding of Fe 3÷ by humic materials. Geochim. Cosmochim. Acta, 41: 969--976. Stevenson, F.J., 1976. Stability constants of Cu ~÷, Pb ~÷, and Cd ~÷ complexes with humic acids. Soil Sci. Soc. Am. J., 40: 665--672. Takamatsu, T. and Yoshida, T., 1978. Determination of stability constants of metal-humic acid complexes by potentiometric titration and ion-selective electrodes. Soil Sci., 125 : 377--386. Tanaka, M. and Nakagawa, M, 1966. San-enki heiko to chuwa tekitei (acid-base equilibrium and neutralization analysis). In: M. Kotake et al. (Editors), Jikken Kagaku Koza (experimental chemistry). Maruzen, Tokyo, II-7, pp. 1--14 (in Japanese). Van Dijk, H., 1971. Cation binding of humic acids. Geoderma, 5: 53--67. Wagner, G.H. and Stevenson, F.J., 1965. Structural arrangement of functional groups in soil humic acids as revealec by infrared analysis. Soil Sci. Soc. Am. Proc., 29: 43--48. Wood, J.C., Moschopedis, S.E. and Den Hertog, W., 1961. Studies in humic acid chemistry. II. Humic anhydrides. Fuel, 40: 491--502. Yariv, S., Russel, J.D. and Farmer, V.C., 1966. Infrared study of the adsorption of benzoic acid and nitrobenzene in montmorillonite. Israel J. Chem., 4: 201--213. Yasuda, M., Yamasaki, K. and Ohtaki, H., 1960. Stability of complexes of several carboxylic acids formed with bivalent metals. Bull. Chem. Soc. Jpn., 33: 1067--1070. Yoshida, M. and Nakao, Y., 1971. Behavior of aluminum--humic acid complexes as revealed by titration curves. J. Sci. Soil Manure, Jpn., 42 : 333--337 (in Japanese with English abstract).