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The interaction of Cu(II) ion with humic acid
The Interaction of Cu(ll) Ion with Humic Acid JACOB A. MARINSKY Department of Chemistry, State University of New York at Buffalo, Buffalo, New York 14214
S. GUPTA Swiss Federal Research Station for Agricultural Chemistry and Hygiene of Environment, 3097 Liebefeld, Bern, Switzerland AND
P. SCHINDLER Department of Inorganic, Physical and Analytical Chemistry, University of Bern, Ch 3000, Bern 9, Switzerland Received September 15, 1981; accepted February 12, 1982 The interaction of Cu(II) ion with a humic acid gel was carefully examined as a function of ionic strength and degree of dissociation to test the utility and validity of the fundamental approach developed by the authors for the unambiguous analysis of the protolytic and metal-ion complexation properties of such organic acid substances. Several of the equations developed earlier have been useful for this purpose. Two complexed species, CuS2, where S is believed to be an oxygen atom accessible from ether, aldehyde, ketone, or phenolic groups contained in the three-dimensional array of aromatic rings and their side chains which determine the structure of humic acid, and (RCOOCu+). are believed to be the only Cu(lI)-bound entities formed. The stability constant of the more strongly complexed CuS2 is approximately 1 × 104; the formation constant of (RCOOCu+). is ~18, a value not too different from the formation constant that has been reported for this particular species when formed by the interaction of Cu(II) with mononuclear carboxylic acid molecules. The sizable variation in p K ~ ) v of three orders of magnitude, attributed initially to the heterogeneity of humic acid samples, was found to be more likely a consequence of differences in the accessibility of the functional units (RCOOH),, (RCOO-). and (RCOOCu+)~ in these condensed systems.
soil by 0.1 M EDTA at pH 7. The extraction procedure used was based on the method of Schnitzer and Khan and has been described by Gupta et al. (2). The purified sample contained 55% carbon and 1.9% nitrogen. (2) E M F measurement and electrode calibration. The Ag, AgCI electrode represented as Ag, A g C 1 / / X - 0 . 0 1 M NaNO3, 0.01 M N a C 1 / / X M NaNO3, where X corresponds to the selected ionic strength was prepared as described by Brown (3). It was used in the potentiometric study together with hydrogen- and Cu-ion specific electrodes manufactured by Metrohm and Orion, respec-
INTRODUCTION
In the preceding paper (1) a physical chemical basis for the unambiguous analysis of the protolytic and metal-complexation behavior of humic acid gels during potentiometric investigation is detailed. It was the objective of this investigation to demonstrate its validity and applicability. EXPERIMENTAL
(a) Materials and Methods (1) Humic acid sample preparations. Humic acid sample was extracted from a peat 401
Journalof Colloidand InterfaceScience,Vol,89, No. 2, October1982
0021-9797/82/100401 - 11$02.00/0 Copyright© 1982 by AcademicPress,Inc. All rightsof reproductionin any formreserved.
402
MARINSKY, GUPTA, AND SCHINDLER
tively. A "Wilhelm" type salt bridge (4) connected the test solution to the reference electrode. The H +- and Cu(II)- ion selective electrodes were calibrated in the presence of NO~ at three different concentration levels (0.020, 0.20, and 2.0 M ) by measuring the potential of each of the two cells after the addition of accurately measured increments of standard HNO3 or Cu(NO3)2 solution, containing sufficient additional NO3 to preserve the overall nitrate ion content of the initial cell solution. For example, in the 0.2 M NO~ system, a 0.00500 M Cu(NO3)2 solution, 0.010 M in HNO3 and 0.18 M NaNO3, was added to the cell solution 0.01 M in HNO3 and 0.19 M in NaNO3 for the calibration of the Cu-ion selective electrode. The use of a well-defined quantity of H ÷ ion permitted a simultaneous check of the H +ion electrode response, as well, before and after a particular experiment. In these cells there is a diffusion (liquid junction) potential, Ej, which contributes to the measured potential. For example, the Nernst equation for hydrogen ion measurements is given by E = E ° + ~ln
[H +] + Ej
and Ej can be approximated by Ej = k[H +1 (5).
[ll
The factor k depends on the ionic strength /, and to a lesser degree on the particular salt bridge. It is experimentally available from a set of measurements with the general composition [NO~] = /, [H +] = H, [Na +] = I H. Even at the lowest ionic strength, Ej did not exceed a value of 2.5 mv. All measurements were made at a temperature of 25 _ 0.1°C. Because the Cu(II)ion selective electrode was sensitive to light the cell was always shielded from the light when the Cu(II) measurements were being made. -
(3) The potentiometric titration of humic acid. A 100-rag humic acid sample was Journal of Colloid and Interface Science, Vol. 89, No. 2, October 1982
placed into the cell and 36 ml of XMNaNO3 (X = 2 or 0.2 or 0.02) were added. The mixture was stirred for a period of 30 minutes before 3.5 ml of 0.10 M NaOH (in X M NaNO3) were added. After another 30-minute interval 3.5 ml of 0.1 M HNO3 (in X M NaNO3) were added to the mixture which was stirred once again for 30 minutes. This procedure was repeated and the suspension was finally kept overnight under a saturated N2 atmosphere. After recording the potential measured by a previously calibrated glass electrode small increments of 0.1 M NaOH (in X M NaNO3) were added. After each addition a constant potential was usually reached in 90-120 minutes. The NaOH, HNO3 cycling procedure described above was always used to prepare a humic acid sample for study. Its use was influenced by the observation that after such a recycling procedure irreproducibility of forward and reverse titrations (hysteresis) of the humic acid was essentially eliminated. With this method of sample preparation, however, the concentration of nitrate actually ranged from 0.03 to 0.034, 0.213 to 0.214 and 2.013 to 2.014 at the three experimental concentration levels selected for study. The capacity of the humic acid (0.35 meq per 100 mg) was determined in preliminary experiments by finding the maximum of plots of the second derivative of the change in pH with volume increment of standard base added during its titration.
(4) The examination of Cu(II)-ion binding by humic acid. Samples of humic acid (100 mg) were transferred to polyethylene bottles and pretreated as described above. To fix the degree of neutralization of the humic acid samples for experimental study at ~0.2, 0.4, 0.6, and 0.8, 0.0, 1.4, 2.1, and 2.8 ml of 0.1 M NaOH (X M in NaNO3) were added to the pretreated samples to prepare them for the study of Cu(II)-ion binding by humic acid at these a values and at the three different nitrate ion concentration levels selected for examination. To vary the quantity of Cu(II) ion initially present in these samples different
403
Cu(lI) ION A N D H U M I C A C I D I N T E R A C T I O N
7
6 pm, Opp
r'(HA)v
0.1
0.2
0.5
0.4
0.5
0.6
0.7
0.8
0.9
FIG. 1. Potentiometric study o f h u m i c acid.
amounts of 0.005 M Cu(NO3)2 (in 0.01, 0.19, and 2.0 M NaNO3) were added to each sequence of humic acid samples prepared at a particular fixed a and suspended in either 0.0340 to 0.0333, 0.214 to 0.213, or 2.014 to 2.013 M NaNO3 solution, the eventual supernatant solution concentration after the pretreatment cycle and the partial neutralization step. The samples, blanketed with N2, were sealed by wrapping the plastic caps with cellophane and were shaken. A period of 30 days was required to reach equilibrium identified by small positive and negative fluctuations in the pH and pCu of the supernatant solution in each mixture subsequent to this period of time. RESULTS
(a) Potentiometric Study of Humic Acid The results obtained during the potentiometric study of humic acid in the presence of NaNO3 at three different concentration levels (0.03, 0.2, and 2.0 M ) are presented in Fig. 1. In this figure pK~P?A)~is plotted versus a, the degree of dissociation, each pK~tTA),value being based on the equilibrium
pH of the solution phase during each step of the humic acid neutralization with standard base. No measurements were made of the humic acid water content to permit assessment of the gel pH for estimate of the pK at the site of the reaction. It is interesting to note that this plot of the potentiometric data yields three separate curves that parallel each other. The vertical separation of the two uppermost curves is approximately 0.9 pK units while the middle and lowest curves are separated by only 0.1 pK unit. The near superposition of the pK~A)~ versus c~ curves in 2.0 and 0.2 M NaNO3 can only occur when pNa~-pCN~ approaches zero in both cases. This condition is approximately met when the water content of the humic acid gel is such that the concentration of A- (Na~) approaches 0.2 M. With this concentration level about 0.12 mole/liter of NaNO3 is expected to invade the gel from a 0.2 M NaNO3 solution; 1.9 mole/liter of NaNO3 is predicted to be imbibed from a 2.0 M NaNO3 solution. On this basis the value of pNAs-pCN~ for the two systems is ~0.3 and ~0.3. With the most dilute NaNO3 solution pNa~-pCN~ ~ 0.9. The predicted separation of the curves is, on Journal of Colloid and Interface Science, Vol. 89, No. 2, October 1982
404
MARINSKY, GUPTA, A N D SCHINDLER
-2
log Cub 2 . 0 M NON03
-3
0.2M NON03
O.021d NON03
0 - a = 0.8
A - a = 0.8
~7 - a = O.B
- a=0.6
A - a=0.6
V-a
eg- a=0.4 @- a=0.2
A - a =0.4
~ - a =0.4
& - a =0.2
V-a
=0.6 =0.2
I
I
]
I
-7
-6
-5
-4
-3
tog (Cuf)s FIG. 2. Binding of Cu(II) by a postulated site (unmodified).
this basis, ~0.6 and n 0 in reasonable agreement with observation. Because the concentration of sodium ion is approximately the same in both phases for the 2.0 M NAN03 system its molar activity in both phases must be nearly equal as well. With this estimate of the situation Eq. [5] of the preceding paper (1) should be applicable for evaluation ofpK~A). The assignment of - g int a value of 2.25-2.3 to r, tHA),by extrapolation of the lowest curve in Fig. 1 until it intercepts the ordinate axis is based on the above premise.
(b) Cu(II)-Ion Binding in Humic Acid The Cu(II)-ion binding data, when analyzed with Eqs. [ 12] and [ 13] of the preceding paper (1) were discovered to be correlated best by Eq. [13] over the range of experimental conditions employed. Experiments with relatively small quantities of Cu(II) added, however, yielded D values that were as much as an order of magnitude larger than the eventual plateau reached in the value for D after significant binding of Cu(II) had ocJournal of Colloid and Interface Science, Vol. 89, No. 2, October 1982
curred. This anomalous result was not related to the pH of the system; it appeared uniquely attributable to strongly selective binding of the Cu(II) ion by a small percentage of the humic acid sample rather than to disturbances by heterogeneity factors which, as we have noted earlier (1), are more likely to be characterized by a regular trend in the value of D1. B y presuming that strong specific site binding was responsible for the observed property Of Dl in Eq. [13] (1) correction was made for such an event in the binding data by forcing the resolution of a constant value for D1, This procedure resulted in site saturation by Cu(II) ion which corresponded to 4.63% of the available hydrogen in the humic acid samples. A plot of the logarithm of the quantity of special site-bound Cu versus the logarithm of the free Cu(II) ion yielded straight lines whose slopes were uniquely defined by (1) the initial degree of neutralization of the humic acid prior to addition of the Cu(II) ion and (2) the ionic strength of the aqueous phase. In each case the same degree of site
405
Cu(ll) ION AND HUMIC ACID INTERACTION
z~/x
-2
tog Cub M NON03
0.02 0.2 2.0
-3
-5
o ; a=0.2
• • •
@ A V
/', ~7
; ~ =0.4
•
i
[]
; (z = 0 . 8
; (z = 0 . 6
l
I
{
I
-4
-3
-2
-i
log (Cuf)g FIG. 3. Binding of Cu(II) by a postulated site (modified).
saturation was eventually reached. This result is graphically presented in Fig. 2. The family of curves in Fig. 2 merge into one when the free Cu(II)-ion concentration at the site of the gel is presumed to mimic the free hydrogen-ion behavior in the 0.2 or 2.0 M NaNO3 system (Fig. 3). We have shown that at this ionic strength the concentration of free mobile ions in both phases must be nearly equal so that the average concentration of mobile counterions in the gel phase is defined. The effective concentration of H + ion at the site of reaction (H~+)(f) (see preceding paper (1)) in each of the systems must then be given by the reciprocal of the antilog of the product of the measured pH and -log f, where -log f is defined as the difference between the intrinsic pK of (HA)~ (2.25-2.3) and its pK~A), value based on the solution pH at each experimental a (Fig. 1). In Fig. 3 the measured concentration of Cu(II) ion has been corrected in this way (Cu(II)g = Cu(II)s(f) 2) to estimate the effective concentration of (Cu(II)g) at the surface of the lattice sites in the gel. The Freundlich isotherm generated by this approach can be interpreted by presuming the following hypothetical reaction of Cu
with an unidentified site: Cu + 2S ~ CuS2.
[2]
By presuming (1) that Cu(II) binds to two independent sites (S) and (2) that the saturation capacity of these sites is twice the binding saturation capacity deduced for Cu(II) the above equilibrium is not inconsistent with the isotherm drawn in Fig. 3. Interpolating points from the smooth curve drawn in Fig. 3 to test the feasibility of the hypothetical equilibrium proposed leads to the following estimates of the stability constant for the proposed complexation reaction that are presented in Table I. At high saturation of sites by Cu(II) the TABLE I Selective Binding of Cu(lI) by a Postulated Site ~uuf 3.16 5.62 1.0 1.78 3.16 5.62 1.0
X X × X × X X
Cub 10 4 10 -4 10 -3 10 -3 10 -3 10 -3 10-2
2.69 3.51 4.57 6.02 7.76 1.00 1.2
× X X X × × X
S 10 -3 10-3 10-3 10 -3 10 -3 10 -2 10 -2
2.662 2.499 2.286 1.996 1.648 1.2 0.8
× X X X X X ×
~c~ 10-2 10-2 10-2 10-2 10-2 10-2 10 -2
1.2 1.0 0.87 0.85 0.70 1.2 1.9
× × × X X X X
104 104 104 104 104 104 104
Journal of Colloid and Interface Science, Vol. 89, No. 2, October 1982
MARINSKY, GUPTA, AND SCHINDLER
406
TABLE II Cu(II) Binding by Humic Acid ×Cub (mmole)
Cut, (mmole)
(H+)
(Cu~÷)
(mole/liter)
(mole/liter)
' H A (mmole)
A - (mmole)
1,50 2,40 2.36 2.34 2.25 2.20
1.00 1,10 1.14 1.14 1.21 1.21
Dt
2.0 M NaNO3, a ~ 0.2 2.41 4,56 8.22 1.42 2.30 3.16
× × × × × X
10-3 10-3 10-~ 10-2 10-2 10-2
2.36 4.41 7.67 1,27 1.63 1.63
× × × × × X
10-3 10-3 10-a 10~2 10-2 10-2
1.98 2.16 2,19 2,14 2.16 2.09
× × × × × ×
10-3 10-3 10-3 10-3 10-3 10-3
1.84 8.67 3.43 1.08 2.93 4.58
X × × × × ×
10-6 10-~ 10-5 10-4 10-4 10-4
× × × × × ×
10-t 10-~ 10-t 10-I 10-t 10-t
× × × × X X
10-1 10-j 10-I 10-1 10-1 10-t
1.70 1.54 1.57 1.32 2.55 3.53
× X × × × ×
10.-4 10-4 10-4 10-4 10-4 10-4
2.04 × 10-4 (ave) a = 0.4 9.34 1.66 2.52 3.23 4.15 5.13
× X × × X ×
10-3 10-2 10-2 10-2 10-2 10-2
7.80 1.00 1.23 1.32 1.38 1.43
× × × × × ×
10~3 10-2 10-2 10~2 10-2 10-2
4.92 5.28 6.88 6.72 7.10 7.27
× × × × X ×
10-4 10 -4
10-4 10-4 10-4 10-4
1.22 6.14 2.50 4.37 5.71 6.82
× × × × X ×
10-5 10-s 10.4 10-4 10-4 10-4
1.84 1.81 1.69 1.67 1.62 1.58
× X × X X X
10-1 10-t 10-1 10 -1
10-i 10-t
1.65 1.63 1.68 1.63 1.60 1.57
× × × × × ×
10-I 10-I 10- l 10-I 10-I 10-1
1.49 1.49 1.44 1.15 1.49 1.80
× X × × X X
10--4 10-4 10-4 10-4 10-4 10-4
1.45 X 10-4 (ave) a = 0.6 1.95 3,54 4.71 5.62 6.53 7.48
× X × × X ×
10-2 10-2 10-2 10-2 10-2 10-2
1.35 1.35 1.41 1.62 1.62 1,62
× × × × X ×
10-2 10-2 10-2 10-2 10-2 10-2
1,41 2.17 2.79 3.26 3.31 3.36
× X × X × X
10 -4
10-4 10-4 10-4 10--4 10-4
8.67 7.70 2.01 3.49 4.81 6.51
× X × × X ×
10~s 10-5 10-4 10-4 10-4 10-4
a
=
× × × × × ×
10-~ 10-4 10-4 10`4 10-4 10-3
1.32 1.27 1.22 1.18 1.16 1.14
× X × × × X
10-1 10-1 10-1 10-1 10-l 10-j
2.12 2.01 1.95 1.92 1.85 1.77
X × × × X ×
10-1 10-1 10-1 10-1 10-I 10- l
1,67 1.67 1.67 1.68 1.54 1.38
X X X × X X
10-4 10-4 10-4 10-4 10.-4 10-4
1.60 × 10-4 (ave)
3.80 5.23 6.07 6.75 7.20 7.68
× × X × × ×
10-2 10-2 10-2 10-2 10-2 10-2
1,62 1,62 1.62 1.62 1.62 1.62
× × × X × ×
10-2 10-2 10-2 10-2 10-2 10-2
6.41 7.66 1.09 1.29 1.52 1,98
× × × × × X
10-5 10-n 10-4 10-4 10.4 10-4
3.25 1.19 2.80 4,47 6.82 1.33
0.8 6.61 6.50 6.25 6.06 5.81 5.16
X × X × × ×
10-2 10-2 10-2 10-2 10-2 10-2
2.62 2.49 2,43 2.38 2.36 2.31
X X X × × ×
10-I 10- l 10-I 10- l 10- t 10-1
1.65 1.05 1.17 1.24 1.32 1.55
× × × × × X
10-4-4 10-4 10-4 10--4 10-4 10.-4
1.33 × 10-~ (ave) 0.2 M NaNO3; a ~ 0.2 9.12 X l 0 -3 2.55 × 10-2 3.14 × 10-2
8.33 × 10-3 1.51 X 10-2 1.62 × 10-2
1.94 × 10-3 2.04 X 10-3 2.11 × 10-3
1.70 X 10-5 2.49 × 10-4 4.62 × 10-4
2.49 × 10 -1 2.31 × 10-I 2.19 × 10-i
9.99 X 10-2 1.08 × 10-t 1.15 × 10-4
2.82 X 10-4 3.51 X 10-4 3.51 X 10-4 3.28 × 10-4 (ave)
9.93 1.96 3.62 4.84
× × × ×
10-3 10-2 10-2 10-2
7.93 1.00 1.29 1.51
× × × X
10-3 10-2 10-2 10-2
2,41 2.72 4.31 5,98
× × × ×
10-4 10-4 10-4 10-4
1,27 8.02 6.37 1.82
0.4
a
=
X × × X
10-6 104 10-5 10-4
1.97 1.95 1.84 1.72
X × × ×
10- t 10-I 10-l 10-1
1.51 1.45 1.42 1.45
× × × ×
10-t 10-t 10-t 10-1
3.41 3.38 2.85 3.21
× × × ×
10--4 10-4 10-4 10--4
3,21 × 10-4 (ave) a = 0.6 1.99 3.90 5.40 6.57 7.26 7.70
× X × × × ×
10-2 10-2 10-2 10-2 10-2 10-2
1.29 1.40 1,44 1.50 1.62 1.62
× × × × × ×
10-2 10-2 10-2 10-2 10-2 10-2
7.53 1.39 2.29 2.87 3.49 4.3l
× × × × × ×
10-5 10-4 10-4 10-4 10-3 10-4
1.22 1.59 9.29 2,10 3.80 6.23
X × × × × ×
104 10-s 10-s 10-4 10-4 10--4
1.36 1.32 1.25 1.20 1.15 1.07
X × × X × ×
10-1 10-t 10-t 10-t 10-t 10-t
2.07 1.93 1,85 1,79 1.79 1.82
X × × × × ×
10 -1
10-1 10 -1
10- t 10-t 10-j
3.64 3.36 2.65 2.47 2.44 2.88
X X X × x ×
10-4 10-4 10-4 10--4 10.-4 10-4
2.91 × 10-4 (ave)
Journal of Colloid and Interface Science, VoL 89, No. 2, October 1982
Cu(II)
ION
AND
HUMIC
TABLE
ACID
II--Continued
( H +)
(Cu] ÷)
(mole/liter)
(mole/liter)
1.62 × 10-2 1.62 × 10 -2
2,23 X 10 -5 3,83 × 10 -s
1.89 X 10 --n 1.I1 X 10 -5
6.86 X 10 -2 6.75 X 10 -2
7.71 × 10-2
1.62 X 10 -2
7,93 X 10-s
4.19 X 10-s
8.98 X 10-2
1.62 X 10-2
1,10 X 10 4
1.40 × 10 -4
10.29 X 10 -2
1.62 X 10-2
1.39 X 10-~
14.03 × 10 -2
1.62 × 10-2
1.95 × 10 -4
ZCub( m m o l e )
Cubs (mmole)
407
INTERACTION
H A (mmole)
A - (mmole)
Di
2.57 X 10-1 2.39 × 10 - t
3.09 X 10 ~ 2.99 X 10-4
6.45 X 10 -2
2.25 X 10- I
4.94 X lO~
6.20 × 10 -2
2.15 X 10 -1
3.56 × 10 -4
2.84 X 10 -4
5.91 X 10-2
2.04 X 10 -1
3.44 × 10-4
7.21 × 10 -~
5.38 X 10 -2
1.72 X 10 - I
3,89 X 10-4
a = 0.8 3.99 X 10 -2 5.93 × 10 -2
3 . 7 0 × 10 -4 (ave) 0.02 M N a N O s ; a ~ 0.2 2.49 X 10-s
2.41 × 10 -s
1.04 X 10-s
1.68 × 10-7
2.98 × 10 - l
5.24 × 10 -2
3.04 × 10-4
4.96 X 10 -s
4.63 X 10-s
1.05 X 10 -s
7.71 k 10 -7
2.96 × 10- I
5.35 × 10-2
2.88 X 10 -4
1.89 X 10 -2 3.24 × 10-2
1.30 X 10-2 1.30 X 10 -2
1.23 X 10 -s 1.33 X 10 -s
2.10 X 10 -s 1.30 X 10 -¢
2.83 X 10 - I 2.73 × 10 -1
6.07 × 10-2 5.79 × 10-2
3.22 X 10 -4 2.05 X 10~
4.66 × 10-2
1.45 × 10 -2
1.50 X 10 -s
2 A 6 X 10-4
2.57 X 10 - t
6.07 x 10 -2
3.07 × 10 -4 2.85 X 10-4 (ave)
a = 0.4 9.99 X 10-2
8.79 X 10-s
8.02 X 10-5
8.54 X 10-8
2.06 X 10 -1
1.43 X 10 - l
3.05 X 104
1.99 X 10-2
1.24 X 10 -2
1.47 X 10 -~
1.13 X 10 -6
2.02 X 10-1
1.41 X 10- I
4.96 × 10 -4
3.94 × 10 -2
1.60 × 10 -2
1.96 X 10-4
9.72 × 10 4
1.94 X 10 - t
1.33 X 10 -1
3.27 X 10 -4
5.60 X 10-2
1.62 X 10 -2
3.69 × 10 -4
6.37 × 10-s
1.87 X 10 - I
1.23 X 10 -1
2.99 X 10-4
6.87 X 10-2
1.62 X 10 -2
4.48 X 10 -4
1.68 X 10 -4
1.80 X 10 -1
1.17 × 10 -1
2.26 × 10 4
7.73 X 10 -2
1.62 X 10 -2
5.36 X 10 -4
3.18 × 10 -4
1.72 X 10 - j
1.17 × 10 -1
2.18 × 10-4 3.12 X 10 -4 (ave)
aO.6 2.00 × 10 -2 3.99 × 10 -2 5.94 × 10-2
1.34 × 10-2 1.60 × 10 -2 1.62 × 10 -2
2.04 × 10 -s 4.76 × 10 -s 1.00 X 10-s
8.28 X 10-8 1.58 × 10 4 9.21 × 10 ~s
1.39 × 10- I 1.37 X 10- I 1.34 X 10 -1
2.05 × 10 - I 1.89 × 10 -1 1.73 X 10 -1
3.31 × 104 3.45 X 10 -4 4.61 × 10-4
7.77 X 10-2 9.12 X 10-2
1.62 × 10-2 1.62 X 10-2
1.16 × 104 2.01 X 10 -4
3.39 X 10-5 1.23 X 10-4
1.32 X 10 -1 1.25 X 10-1
1.56 X 10 - I 1.49 X 10 - I
2.18 × 10-4 2.35 × 10 -4
9.95 X 10 -2
1.62 × 10 -2
3.08 X 10 -4
3.30 X 10 4
1.16 X 10-~
1.50 X 10- j
2.67 X 10-4 3.09 X 10 -4 (ave)
a = 0.8 4.00 X 10 -2
1.62 × 10 -2
5.30 X 10 -2
1.14 × 10 -7
6.97 X 10 -2
2.56 X 10-1
3.09 X 10-4
7.95 X 10 -2 9.86 × 10-2
1.62 X 10 -2 1.62 × 10-2
3.20 X 10 -~ 3.92 X 10 -~
6.47 × 10~ 1.86 × 10 -5
6.78 × 10 -2 6.71 × 10 -2
2.22 × 10 - t 2.00 X 10 - I
4.84 X 10 -4 3.02 × 10 -4
11.74 X 10-2
1.62 × 10-2
9.82 X 10 -*
9.75 X 10-~
6.24 × 10 -2
1.86 × 10- I
4.78 X 10-4
14.38 X 10-2
1.62 X 10 -2
2.04 X 10 4
6.05 × 10 4
5.11 × 10-2
1.71 × 10 - j
5.75 X 10 -4 4.24 × 10-4 (ave)
value of S which is based on a small difference between two relatively large numbers (So - 2Cub) in these computations can be strongly affected by a small uncertainty in these numbers. As a consequence the value of 13cus2 is very sensitive to the uncertainty in this term which can account for the rise in the value of 13 observed in Table I as the saturation binding of Cu is more nearly approached.
The forced value of DI that is resolved with Eq. [13] (see Ref. (1)) to facilitate estimate in Figs. 2 and 3 of the postulated selective binding of Cu(II) in the various experimental situations employed are listed in Table II. The gross experimental binding data (ZCUb), the estimated quantity of Cu(II) presumed bound to the postulated S sites (Cubs), the experimentally measured concentrations of H + ion and Cu(II) ion, and the quantity of Journal of Colloid and Interface Science, Vol. 89, No. 2, October 1982
408
MARINSKY, GUPTA, AND SCHINDLER
undissociated (HA) and dissociated (A-) acid (monomer basis), corrected for Cu bound to the A- functional units (ZCub - CUbs) are also presented in Table II. There are several instances in which the value of D1 deviates quite sizably from the relatively constant value obtained for this parameter at each ionic strength (DI "~ 1.6 X 10-4 in 2.0 MNaNO3, ~3.3 × 10 4 in 0.2 M NaNO3, and -~3.4 × 10-4 in 0.03 M NaNO3); we believe this result is not significant. However, the listed difference between each average value of DL is, and most probably arises from, the difference in activity coefficients of H ÷ and Cu 2÷ at the three different ionic strengths studied. In the 0.03 M NaNO3 system DI is least affected by deviations arising from ionic interactions and this source of deviation should be most easily and accurately correctable by multiplying D1 by the mean molal activity coefficient ratio of HNO3 and Cu(NO3)2
[3]
At this low ionic strength the activity coefficients for the pure electrolyte components should be representative of their activity coefficients in the electrolyte mixture. Recalling now that D1 = MCuAt~int +//~int/~,iOHA)~2and that 13HA i n t " lS available from the intercept value of pK~I~), in 0.2 MNaNO3 (Fig. 1) the value o f ~int ~,c~ + ~ (5.0 × 10-4)(3.6 × 10 4) "~ 1.8 × 101. The magnitude of ~CuA~ ' i+, nt though somewhat smaller than we expect for the carboxylate ligand, is not remarkably smaller than the values published for the CuA ÷ species formed with, for example: acetoxyacetic acid,/3 = 17; benzoic acid,/3 = 32; formic acid,/3 = 24; chloroacetic acid,/3 = 44; salicylic acid,/3 -- 24; isobutyric acid,/3 = 93; ethyl malonic acid, /3 = 55; and dimethyl malonic acid, 13 = 5 (6).
Journal of Colloid and Interface Science,
The possibility that a third Cu(II)-complexed species is formed has been examined as well. In one estimate of this possibility it was assumed that with the formation of CuA + (CuRCOO) + a much weaker acid, i.e., an aromatic carboxylic acid or a phenol, if conveniently located could be bound as well. The following sequence of reactions was presumed to occur Cu(II) + (RCOO-) ~ Cu(RCOO) +,
[41
Cu(RCOO) + + R'OH Cu(RCOO)(R'O) + H +, H + + RCOO- ~ RCOOH.
[51 [6]
The net reaction is then Cu(II) + 2(RCOO-) + R'OH ~Cu(RCOO)(R'O) + RCOOH
[7]
and the mass action expression to describe the equilibrium is
D1 ('~HNO3)4 -- 3.6 × 10-4 ( 0 " 8 5 ) 4 3'Cu(NO3)3 (0.72) 3 = 5.03 × 10-4.
(c) Extended Analysis of Cu(II)-Ion Binding Data
Vol. 89, No. 2, October 1982
[Cu(RCOO)(RO ')] [RCOOH] K-- [Cu(II)][(RCOO_)]2[(R,OH) ] .
[8]
Analysis of the possibility of this complexation path has been performed by computing the concentration of Cu(II) in the gel at the surface of the reaction site as before. The concentration of WOH, unaffected by the pH of the solution in the range studied and barely affected by the small fraction of Cu(II) that could be bound to it, should remain essentially constant and can be incorporated into the Kterm. The Cu(II)-ion binding data, obtained in the 2.0 M NaNO3 system, were employed by taking the difference, ~CUD -- CUsD, as before, to define the total Cu(II) bound to RCOO- alone and to RCOO and RO'. Since we know from our earlier analysis that Cu(RCOO-) + is formed in significant quantity a plot of K', the exchange quotient,
Cu(II) ION
K' =
AND
(2~Cub- Cusb)(RCOOH) (Cu(Ii))0c)2(RCOO_)2 ,
HUMIC
ACID
409
INTERACTION
.08
[9] .O7
versus (RCOOH)/(RCOO-) is expected to give a straight line of slope equal to fl[(CuRCOO)l~+ and an intercept equal to fl(Cu(RCOOXR,O)),(Rr~OH). The graphical representation of the data so expressed are presented in Fig. 4. The intercept is zero to demonstrate the absence of any measurable quantity of the postulated species. The slope is 5. Dividing this number by the mean molal activity coefficient, 0.43, of Cu(NO3)2 at the experimental ionic strength yields a fl(CuRCOO+)~ value of 12 in good agreement with the earlier estimates of the magnitude of this parameter. The second possibility examined was the formation of a bidentate complex as discussed in the preceding paper (Eqs. [17][24]). The sequence of equilibria considered a possibility was Cu(II) + H2AA' ~ CuAA' + 2H +, (H ÷) + HAA'- ~ H2AA'
[10] [11]
B
.OE
o°
.0~ Kch .0¢ .03 ,02 .0t I .002
I .004
I .006
I .008
r .OI0
I .0t2
I .0t4
(H +) (f)
FIG. 5. Extended analysis of Cu(II)-ion binding in humic acid-2. to give the net reaction Cu(II) + HAA'- ~ CuAA' + H +.
[12]
Analysis of the binding data in 2.0 M NaNO3 examined the following expression for the quotient, Koh
14
Kch =
(~Cub - CU~b)(H+)s (Cu(II))sOC)(HAA' -)
[13]
by plotting Kch versus H~+(f). This graphical representation of the data is presented in Fig. 5. The intercept which represents the equilibrium expressed by Eq. [12] is again zero in value to demonstrate the absence of the chelate species. The slope which corresponds to/~(RCOOCu),+again yields a value of 5 or ~ 12 after correction for the nonideality of Cu(NO3) 2 in the 2.0 M NaNO3 solution.
lO K' 8
6
4
2 DISCUSSION I
I
1.0
2.0
RCOOH / RCOO-
FIG. 4. Extended analysis of Cu(II)-ion binding in humic acid-1.
The above analysis of the data compiled in this research has led to the conclusion that interaction of Cu(II) ion with the humic acid samples employed results in the formation
Journal of Colloid and Interface Science, Vol. 89, No. 2, October 1982
410
MARINSKY, GUPTA, AND SCHINDLER
of two separate species. The first, more selective reaction, appears to be (1) independent of system pH and (2) to require the interaction of two sites simultaneously with each Cu(II) ion. The number of such accessible sites is approximately 9% of the accessible hydrogen capacity. It seems quite likely that the sites are polar oxygen atoms accessible in the ether, aldehyde, ketone, and phenolic groups contained in the three-dimensional array of aromatic rings and their side chains that determine the structure of humic acid. The less selective reaction is attributed to the formation of RCOOCu ÷. As has been pointed out the formation constant that is resolved for this species is of the same order of magnitude that has been observed with simple carboxylic acids; although the pK of the carboxylic acid may vary considerably in magnitude the pK of the RCOOCu ÷ species formed with it is not strongly affected (6) to indicate that the reaction assessment is not unreasonable. It is surprising that the analysis of the data in a system as heterogeneous as humic acid is expected to be can lead to the rather uncomplicated resolution of the data that has been accomplished. According to our earlier estimate of the effect of heterogeneity it would have been more reasonable to expect the D1 quotient to become smaller as the degree of dissociation increased. We have observed that the formation constant of the (RCOOCu) ÷ species formed with various carboxylic acids does not vary appreciably in value with the pK of the acid (6). Because of this general observation the observed increase in the pK of humic acid (pK~A), changes three orders of magnitude over the complete neutralization range) was not expected to be duplicated by the variation in pK~d~A+), of the (RCOOCu) ÷ species as it apparently was. There is no other explanation, however, for the successful resolution of the sorption isotherm in Fig. 3 that was affected by applying correction factors deJournal of Colloid and Interface Science, VoL 89, No. 2, October 1982
rived from the potentiometric analysis of the humic acid data for assessment of the effective Cu(II)-ion concentration at the gel-lattice site surfaces. This result leads us to reassess the heterogeneity factor in humic acid as follows: Because of the much limited accessibility of the rigid matrix units in the humic acid their effective concentration as we have pointed out earlier is lowered appreciably relative to their effective concentration in equilibria of similar functional units in small molecule systems. Their freedom of motion is certainly limited to fluctuations restricted by their attachment to the gel matrix. Their relative positions in a sample, because of matrix rigidity, can determine the accessibilities of dissociated and undissociated species, i.e., their activity coefficient ratio. With an increase in the degree of dissociation increase in electrostatic repulsion of the ionized units due to their closer approach will lead to an increase in their accessibility relative to the undissociated units. This could result in an increase in the effective concentration of (RCOO-), relative to (RCOOH)~ to yield the observed increase in the apparent pK with neutralization of the humic acid. In fact, the combined effect of geometry and charge could explain the initially anomolously low p 1~ ~ int (hA), value increasing 3 logarithmic units over the complete neutralization range. The observed squared dependence of the formation constant of the Cu(II)-bound species, [(RCOOCu)~] + on the net deviation term deduced from the acid equilibrium study is then explained by the fact that the electrostatic attraction of the oppositely charged species reduces its accessibility as much as the repulsion of [RCO0]; units increases their accessibility. In closing we would like to point out that the fundamental approach developed by the authors in the preceding paper (1) for the unambiguous analysis of the protolytic and metal-ion complexation properties of humic acid gels has indeed facilitated the achieve-
Cu(lI) ION AND HUMIC ACID INTERACTION m e n t o f this objective in this study. Certainly the u n a m b i g u o u s description o b t a i n e d with this a p p r o a c h to the equilibria e n c o u n t e r e d in the C u ( I I ) - h u m i c acid system studied appear to substantiate the validity a n d utility o f the f u n d a m e n t a l l y based equations develo p e d for this purpose. REFERENCES 1. Marinsky, J. A., Gupta, S., and Schindler, P., J. Colloid Interface Sci. 89, 412 (1982).
41 1
2. Gupta, S., H~ini, H., and Schindler, P., "Mobilisierung und immobillisierung Von Metallen durch die Organische Bodensubstanz." Swiss Federal Research Station for Agricultural Chemistry and Hygiene of Environment, 3097 Liebefeld, Bern, Switzerland, March 1980. 3. Brown, A. S., J. Amer. Chem. Soc. 56, 646 (1934). 4. Forsling, W. S., Hietanen, S., and Sill6n, L. G., Acta Chim. Scand. 6, 901 (1952). 5. Biedermann, G., and Sill6n, L. G., Arkiv. Kemi 5, 425 (1953). 6. SiU6n,L. G., and Martell, A. E., "Stability Constants of Metal-Ion Complexes." The Chemical Society, Burlington House, Wl, London, 1964.
Journal of Colloid and Interface Science, Vol. 89, No. 2, October 1982
S. GUPTA Swiss Federal Research Station for Agricultural Chemistry and Hygiene of Environment, 3097 Liebefeld, Bern, Switzerland AND
P. SCHINDLER Department of Inorganic, Physical and Analytical Chemistry, University of Bern, Ch 3000, Bern 9, Switzerland Received September 15, 1981; accepted February 12, 1982 The interaction of Cu(II) ion with a humic acid gel was carefully examined as a function of ionic strength and degree of dissociation to test the utility and validity of the fundamental approach developed by the authors for the unambiguous analysis of the protolytic and metal-ion complexation properties of such organic acid substances. Several of the equations developed earlier have been useful for this purpose. Two complexed species, CuS2, where S is believed to be an oxygen atom accessible from ether, aldehyde, ketone, or phenolic groups contained in the three-dimensional array of aromatic rings and their side chains which determine the structure of humic acid, and (RCOOCu+). are believed to be the only Cu(lI)-bound entities formed. The stability constant of the more strongly complexed CuS2 is approximately 1 × 104; the formation constant of (RCOOCu+). is ~18, a value not too different from the formation constant that has been reported for this particular species when formed by the interaction of Cu(II) with mononuclear carboxylic acid molecules. The sizable variation in p K ~ ) v of three orders of magnitude, attributed initially to the heterogeneity of humic acid samples, was found to be more likely a consequence of differences in the accessibility of the functional units (RCOOH),, (RCOO-). and (RCOOCu+)~ in these condensed systems.
soil by 0.1 M EDTA at pH 7. The extraction procedure used was based on the method of Schnitzer and Khan and has been described by Gupta et al. (2). The purified sample contained 55% carbon and 1.9% nitrogen. (2) E M F measurement and electrode calibration. The Ag, AgCI electrode represented as Ag, A g C 1 / / X - 0 . 0 1 M NaNO3, 0.01 M N a C 1 / / X M NaNO3, where X corresponds to the selected ionic strength was prepared as described by Brown (3). It was used in the potentiometric study together with hydrogen- and Cu-ion specific electrodes manufactured by Metrohm and Orion, respec-
INTRODUCTION
In the preceding paper (1) a physical chemical basis for the unambiguous analysis of the protolytic and metal-complexation behavior of humic acid gels during potentiometric investigation is detailed. It was the objective of this investigation to demonstrate its validity and applicability. EXPERIMENTAL
(a) Materials and Methods (1) Humic acid sample preparations. Humic acid sample was extracted from a peat 401
Journalof Colloidand InterfaceScience,Vol,89, No. 2, October1982
0021-9797/82/100401 - 11$02.00/0 Copyright© 1982 by AcademicPress,Inc. All rightsof reproductionin any formreserved.
402
MARINSKY, GUPTA, AND SCHINDLER
tively. A "Wilhelm" type salt bridge (4) connected the test solution to the reference electrode. The H +- and Cu(II)- ion selective electrodes were calibrated in the presence of NO~ at three different concentration levels (0.020, 0.20, and 2.0 M ) by measuring the potential of each of the two cells after the addition of accurately measured increments of standard HNO3 or Cu(NO3)2 solution, containing sufficient additional NO3 to preserve the overall nitrate ion content of the initial cell solution. For example, in the 0.2 M NO~ system, a 0.00500 M Cu(NO3)2 solution, 0.010 M in HNO3 and 0.18 M NaNO3, was added to the cell solution 0.01 M in HNO3 and 0.19 M in NaNO3 for the calibration of the Cu-ion selective electrode. The use of a well-defined quantity of H ÷ ion permitted a simultaneous check of the H +ion electrode response, as well, before and after a particular experiment. In these cells there is a diffusion (liquid junction) potential, Ej, which contributes to the measured potential. For example, the Nernst equation for hydrogen ion measurements is given by E = E ° + ~ln
[H +] + Ej
and Ej can be approximated by Ej = k[H +1 (5).
[ll
The factor k depends on the ionic strength /, and to a lesser degree on the particular salt bridge. It is experimentally available from a set of measurements with the general composition [NO~] = /, [H +] = H, [Na +] = I H. Even at the lowest ionic strength, Ej did not exceed a value of 2.5 mv. All measurements were made at a temperature of 25 _ 0.1°C. Because the Cu(II)ion selective electrode was sensitive to light the cell was always shielded from the light when the Cu(II) measurements were being made. -
(3) The potentiometric titration of humic acid. A 100-rag humic acid sample was Journal of Colloid and Interface Science, Vol. 89, No. 2, October 1982
placed into the cell and 36 ml of XMNaNO3 (X = 2 or 0.2 or 0.02) were added. The mixture was stirred for a period of 30 minutes before 3.5 ml of 0.10 M NaOH (in X M NaNO3) were added. After another 30-minute interval 3.5 ml of 0.1 M HNO3 (in X M NaNO3) were added to the mixture which was stirred once again for 30 minutes. This procedure was repeated and the suspension was finally kept overnight under a saturated N2 atmosphere. After recording the potential measured by a previously calibrated glass electrode small increments of 0.1 M NaOH (in X M NaNO3) were added. After each addition a constant potential was usually reached in 90-120 minutes. The NaOH, HNO3 cycling procedure described above was always used to prepare a humic acid sample for study. Its use was influenced by the observation that after such a recycling procedure irreproducibility of forward and reverse titrations (hysteresis) of the humic acid was essentially eliminated. With this method of sample preparation, however, the concentration of nitrate actually ranged from 0.03 to 0.034, 0.213 to 0.214 and 2.013 to 2.014 at the three experimental concentration levels selected for study. The capacity of the humic acid (0.35 meq per 100 mg) was determined in preliminary experiments by finding the maximum of plots of the second derivative of the change in pH with volume increment of standard base added during its titration.
(4) The examination of Cu(II)-ion binding by humic acid. Samples of humic acid (100 mg) were transferred to polyethylene bottles and pretreated as described above. To fix the degree of neutralization of the humic acid samples for experimental study at ~0.2, 0.4, 0.6, and 0.8, 0.0, 1.4, 2.1, and 2.8 ml of 0.1 M NaOH (X M in NaNO3) were added to the pretreated samples to prepare them for the study of Cu(II)-ion binding by humic acid at these a values and at the three different nitrate ion concentration levels selected for examination. To vary the quantity of Cu(II) ion initially present in these samples different
403
Cu(lI) ION A N D H U M I C A C I D I N T E R A C T I O N
7
6 pm, Opp
r'(HA)v
0.1
0.2
0.5
0.4
0.5
0.6
0.7
0.8
0.9
FIG. 1. Potentiometric study o f h u m i c acid.
amounts of 0.005 M Cu(NO3)2 (in 0.01, 0.19, and 2.0 M NaNO3) were added to each sequence of humic acid samples prepared at a particular fixed a and suspended in either 0.0340 to 0.0333, 0.214 to 0.213, or 2.014 to 2.013 M NaNO3 solution, the eventual supernatant solution concentration after the pretreatment cycle and the partial neutralization step. The samples, blanketed with N2, were sealed by wrapping the plastic caps with cellophane and were shaken. A period of 30 days was required to reach equilibrium identified by small positive and negative fluctuations in the pH and pCu of the supernatant solution in each mixture subsequent to this period of time. RESULTS
(a) Potentiometric Study of Humic Acid The results obtained during the potentiometric study of humic acid in the presence of NaNO3 at three different concentration levels (0.03, 0.2, and 2.0 M ) are presented in Fig. 1. In this figure pK~P?A)~is plotted versus a, the degree of dissociation, each pK~tTA),value being based on the equilibrium
pH of the solution phase during each step of the humic acid neutralization with standard base. No measurements were made of the humic acid water content to permit assessment of the gel pH for estimate of the pK at the site of the reaction. It is interesting to note that this plot of the potentiometric data yields three separate curves that parallel each other. The vertical separation of the two uppermost curves is approximately 0.9 pK units while the middle and lowest curves are separated by only 0.1 pK unit. The near superposition of the pK~A)~ versus c~ curves in 2.0 and 0.2 M NaNO3 can only occur when pNa~-pCN~ approaches zero in both cases. This condition is approximately met when the water content of the humic acid gel is such that the concentration of A- (Na~) approaches 0.2 M. With this concentration level about 0.12 mole/liter of NaNO3 is expected to invade the gel from a 0.2 M NaNO3 solution; 1.9 mole/liter of NaNO3 is predicted to be imbibed from a 2.0 M NaNO3 solution. On this basis the value of pNAs-pCN~ for the two systems is ~0.3 and ~0.3. With the most dilute NaNO3 solution pNa~-pCN~ ~ 0.9. The predicted separation of the curves is, on Journal of Colloid and Interface Science, Vol. 89, No. 2, October 1982
404
MARINSKY, GUPTA, A N D SCHINDLER
-2
log Cub 2 . 0 M NON03
-3
0.2M NON03
O.021d NON03
0 - a = 0.8
A - a = 0.8
~7 - a = O.B
- a=0.6
A - a=0.6
V-a
eg- a=0.4 @- a=0.2
A - a =0.4
~ - a =0.4
& - a =0.2
V-a
=0.6 =0.2
I
I
]
I
-7
-6
-5
-4
-3
tog (Cuf)s FIG. 2. Binding of Cu(II) by a postulated site (unmodified).
this basis, ~0.6 and n 0 in reasonable agreement with observation. Because the concentration of sodium ion is approximately the same in both phases for the 2.0 M NAN03 system its molar activity in both phases must be nearly equal as well. With this estimate of the situation Eq. [5] of the preceding paper (1) should be applicable for evaluation ofpK~A). The assignment of - g int a value of 2.25-2.3 to r, tHA),by extrapolation of the lowest curve in Fig. 1 until it intercepts the ordinate axis is based on the above premise.
(b) Cu(II)-Ion Binding in Humic Acid The Cu(II)-ion binding data, when analyzed with Eqs. [ 12] and [ 13] of the preceding paper (1) were discovered to be correlated best by Eq. [13] over the range of experimental conditions employed. Experiments with relatively small quantities of Cu(II) added, however, yielded D values that were as much as an order of magnitude larger than the eventual plateau reached in the value for D after significant binding of Cu(II) had ocJournal of Colloid and Interface Science, Vol. 89, No. 2, October 1982
curred. This anomalous result was not related to the pH of the system; it appeared uniquely attributable to strongly selective binding of the Cu(II) ion by a small percentage of the humic acid sample rather than to disturbances by heterogeneity factors which, as we have noted earlier (1), are more likely to be characterized by a regular trend in the value of D1. B y presuming that strong specific site binding was responsible for the observed property Of Dl in Eq. [13] (1) correction was made for such an event in the binding data by forcing the resolution of a constant value for D1, This procedure resulted in site saturation by Cu(II) ion which corresponded to 4.63% of the available hydrogen in the humic acid samples. A plot of the logarithm of the quantity of special site-bound Cu versus the logarithm of the free Cu(II) ion yielded straight lines whose slopes were uniquely defined by (1) the initial degree of neutralization of the humic acid prior to addition of the Cu(II) ion and (2) the ionic strength of the aqueous phase. In each case the same degree of site
405
Cu(ll) ION AND HUMIC ACID INTERACTION
z~/x
-2
tog Cub M NON03
0.02 0.2 2.0
-3
-5
o ; a=0.2
• • •
@ A V
/', ~7
; ~ =0.4
•
i
[]
; (z = 0 . 8
; (z = 0 . 6
l
I
{
I
-4
-3
-2
-i
log (Cuf)g FIG. 3. Binding of Cu(II) by a postulated site (modified).
saturation was eventually reached. This result is graphically presented in Fig. 2. The family of curves in Fig. 2 merge into one when the free Cu(II)-ion concentration at the site of the gel is presumed to mimic the free hydrogen-ion behavior in the 0.2 or 2.0 M NaNO3 system (Fig. 3). We have shown that at this ionic strength the concentration of free mobile ions in both phases must be nearly equal so that the average concentration of mobile counterions in the gel phase is defined. The effective concentration of H + ion at the site of reaction (H~+)(f) (see preceding paper (1)) in each of the systems must then be given by the reciprocal of the antilog of the product of the measured pH and -log f, where -log f is defined as the difference between the intrinsic pK of (HA)~ (2.25-2.3) and its pK~A), value based on the solution pH at each experimental a (Fig. 1). In Fig. 3 the measured concentration of Cu(II) ion has been corrected in this way (Cu(II)g = Cu(II)s(f) 2) to estimate the effective concentration of (Cu(II)g) at the surface of the lattice sites in the gel. The Freundlich isotherm generated by this approach can be interpreted by presuming the following hypothetical reaction of Cu
with an unidentified site: Cu + 2S ~ CuS2.
[2]
By presuming (1) that Cu(II) binds to two independent sites (S) and (2) that the saturation capacity of these sites is twice the binding saturation capacity deduced for Cu(II) the above equilibrium is not inconsistent with the isotherm drawn in Fig. 3. Interpolating points from the smooth curve drawn in Fig. 3 to test the feasibility of the hypothetical equilibrium proposed leads to the following estimates of the stability constant for the proposed complexation reaction that are presented in Table I. At high saturation of sites by Cu(II) the TABLE I Selective Binding of Cu(lI) by a Postulated Site ~uuf 3.16 5.62 1.0 1.78 3.16 5.62 1.0
X X × X × X X
Cub 10 4 10 -4 10 -3 10 -3 10 -3 10 -3 10-2
2.69 3.51 4.57 6.02 7.76 1.00 1.2
× X X X × × X
S 10 -3 10-3 10-3 10 -3 10 -3 10 -2 10 -2
2.662 2.499 2.286 1.996 1.648 1.2 0.8
× X X X X X ×
~c~ 10-2 10-2 10-2 10-2 10-2 10-2 10 -2
1.2 1.0 0.87 0.85 0.70 1.2 1.9
× × × X X X X
104 104 104 104 104 104 104
Journal of Colloid and Interface Science, Vol. 89, No. 2, October 1982
MARINSKY, GUPTA, AND SCHINDLER
406
TABLE II Cu(II) Binding by Humic Acid ×Cub (mmole)
Cut, (mmole)
(H+)
(Cu~÷)
(mole/liter)
(mole/liter)
' H A (mmole)
A - (mmole)
1,50 2,40 2.36 2.34 2.25 2.20
1.00 1,10 1.14 1.14 1.21 1.21
Dt
2.0 M NaNO3, a ~ 0.2 2.41 4,56 8.22 1.42 2.30 3.16
× × × × × X
10-3 10-3 10-~ 10-2 10-2 10-2
2.36 4.41 7.67 1,27 1.63 1.63
× × × × × X
10-3 10-3 10-a 10~2 10-2 10-2
1.98 2.16 2,19 2,14 2.16 2.09
× × × × × ×
10-3 10-3 10-3 10-3 10-3 10-3
1.84 8.67 3.43 1.08 2.93 4.58
X × × × × ×
10-6 10-~ 10-5 10-4 10-4 10-4
× × × × × ×
10-t 10-~ 10-t 10-I 10-t 10-t
× × × × X X
10-1 10-j 10-I 10-1 10-1 10-t
1.70 1.54 1.57 1.32 2.55 3.53
× X × × × ×
10.-4 10-4 10-4 10-4 10-4 10-4
2.04 × 10-4 (ave) a = 0.4 9.34 1.66 2.52 3.23 4.15 5.13
× X × × X ×
10-3 10-2 10-2 10-2 10-2 10-2
7.80 1.00 1.23 1.32 1.38 1.43
× × × × × ×
10~3 10-2 10-2 10~2 10-2 10-2
4.92 5.28 6.88 6.72 7.10 7.27
× × × × X ×
10-4 10 -4
10-4 10-4 10-4 10-4
1.22 6.14 2.50 4.37 5.71 6.82
× × × × X ×
10-5 10-s 10.4 10-4 10-4 10-4
1.84 1.81 1.69 1.67 1.62 1.58
× X × X X X
10-1 10-t 10-1 10 -1
10-i 10-t
1.65 1.63 1.68 1.63 1.60 1.57
× × × × × ×
10-I 10-I 10- l 10-I 10-I 10-1
1.49 1.49 1.44 1.15 1.49 1.80
× X × × X X
10--4 10-4 10-4 10-4 10-4 10-4
1.45 X 10-4 (ave) a = 0.6 1.95 3,54 4.71 5.62 6.53 7.48
× X × × X ×
10-2 10-2 10-2 10-2 10-2 10-2
1.35 1.35 1.41 1.62 1.62 1,62
× × × × X ×
10-2 10-2 10-2 10-2 10-2 10-2
1,41 2.17 2.79 3.26 3.31 3.36
× X × X × X
10 -4
10-4 10-4 10-4 10--4 10-4
8.67 7.70 2.01 3.49 4.81 6.51
× X × × X ×
10~s 10-5 10-4 10-4 10-4 10-4
a
=
× × × × × ×
10-~ 10-4 10-4 10`4 10-4 10-3
1.32 1.27 1.22 1.18 1.16 1.14
× X × × × X
10-1 10-1 10-1 10-1 10-l 10-j
2.12 2.01 1.95 1.92 1.85 1.77
X × × × X ×
10-1 10-1 10-1 10-1 10-I 10- l
1,67 1.67 1.67 1.68 1.54 1.38
X X X × X X
10-4 10-4 10-4 10-4 10.-4 10-4
1.60 × 10-4 (ave)
3.80 5.23 6.07 6.75 7.20 7.68
× × X × × ×
10-2 10-2 10-2 10-2 10-2 10-2
1,62 1,62 1.62 1.62 1.62 1.62
× × × X × ×
10-2 10-2 10-2 10-2 10-2 10-2
6.41 7.66 1.09 1.29 1.52 1,98
× × × × × X
10-5 10-n 10-4 10-4 10.4 10-4
3.25 1.19 2.80 4,47 6.82 1.33
0.8 6.61 6.50 6.25 6.06 5.81 5.16
X × X × × ×
10-2 10-2 10-2 10-2 10-2 10-2
2.62 2.49 2,43 2.38 2.36 2.31
X X X × × ×
10-I 10- l 10-I 10- l 10- t 10-1
1.65 1.05 1.17 1.24 1.32 1.55
× × × × × X
10-4-4 10-4 10-4 10--4 10-4 10.-4
1.33 × 10-~ (ave) 0.2 M NaNO3; a ~ 0.2 9.12 X l 0 -3 2.55 × 10-2 3.14 × 10-2
8.33 × 10-3 1.51 X 10-2 1.62 × 10-2
1.94 × 10-3 2.04 X 10-3 2.11 × 10-3
1.70 X 10-5 2.49 × 10-4 4.62 × 10-4
2.49 × 10 -1 2.31 × 10-I 2.19 × 10-i
9.99 X 10-2 1.08 × 10-t 1.15 × 10-4
2.82 X 10-4 3.51 X 10-4 3.51 X 10-4 3.28 × 10-4 (ave)
9.93 1.96 3.62 4.84
× × × ×
10-3 10-2 10-2 10-2
7.93 1.00 1.29 1.51
× × × X
10-3 10-2 10-2 10-2
2,41 2.72 4.31 5,98
× × × ×
10-4 10-4 10-4 10-4
1,27 8.02 6.37 1.82
0.4
a
=
X × × X
10-6 104 10-5 10-4
1.97 1.95 1.84 1.72
X × × ×
10- t 10-I 10-l 10-1
1.51 1.45 1.42 1.45
× × × ×
10-t 10-t 10-t 10-1
3.41 3.38 2.85 3.21
× × × ×
10--4 10-4 10-4 10--4
3,21 × 10-4 (ave) a = 0.6 1.99 3.90 5.40 6.57 7.26 7.70
× X × × × ×
10-2 10-2 10-2 10-2 10-2 10-2
1.29 1.40 1,44 1.50 1.62 1.62
× × × × × ×
10-2 10-2 10-2 10-2 10-2 10-2
7.53 1.39 2.29 2.87 3.49 4.3l
× × × × × ×
10-5 10-4 10-4 10-4 10-3 10-4
1.22 1.59 9.29 2,10 3.80 6.23
X × × × × ×
104 10-s 10-s 10-4 10-4 10--4
1.36 1.32 1.25 1.20 1.15 1.07
X × × X × ×
10-1 10-t 10-t 10-t 10-t 10-t
2.07 1.93 1,85 1,79 1.79 1.82
X × × × × ×
10 -1
10-1 10 -1
10- t 10-t 10-j
3.64 3.36 2.65 2.47 2.44 2.88
X X X × x ×
10-4 10-4 10-4 10--4 10.-4 10-4
2.91 × 10-4 (ave)
Journal of Colloid and Interface Science, VoL 89, No. 2, October 1982
Cu(II)
ION
AND
HUMIC
TABLE
ACID
II--Continued
( H +)
(Cu] ÷)
(mole/liter)
(mole/liter)
1.62 × 10-2 1.62 × 10 -2
2,23 X 10 -5 3,83 × 10 -s
1.89 X 10 --n 1.I1 X 10 -5
6.86 X 10 -2 6.75 X 10 -2
7.71 × 10-2
1.62 X 10 -2
7,93 X 10-s
4.19 X 10-s
8.98 X 10-2
1.62 X 10-2
1,10 X 10 4
1.40 × 10 -4
10.29 X 10 -2
1.62 X 10-2
1.39 X 10-~
14.03 × 10 -2
1.62 × 10-2
1.95 × 10 -4
ZCub( m m o l e )
Cubs (mmole)
407
INTERACTION
H A (mmole)
A - (mmole)
Di
2.57 X 10-1 2.39 × 10 - t
3.09 X 10 ~ 2.99 X 10-4
6.45 X 10 -2
2.25 X 10- I
4.94 X lO~
6.20 × 10 -2
2.15 X 10 -1
3.56 × 10 -4
2.84 X 10 -4
5.91 X 10-2
2.04 X 10 -1
3.44 × 10-4
7.21 × 10 -~
5.38 X 10 -2
1.72 X 10 - I
3,89 X 10-4
a = 0.8 3.99 X 10 -2 5.93 × 10 -2
3 . 7 0 × 10 -4 (ave) 0.02 M N a N O s ; a ~ 0.2 2.49 X 10-s
2.41 × 10 -s
1.04 X 10-s
1.68 × 10-7
2.98 × 10 - l
5.24 × 10 -2
3.04 × 10-4
4.96 X 10 -s
4.63 X 10-s
1.05 X 10 -s
7.71 k 10 -7
2.96 × 10- I
5.35 × 10-2
2.88 X 10 -4
1.89 X 10 -2 3.24 × 10-2
1.30 X 10-2 1.30 X 10 -2
1.23 X 10 -s 1.33 X 10 -s
2.10 X 10 -s 1.30 X 10 -¢
2.83 X 10 - I 2.73 × 10 -1
6.07 × 10-2 5.79 × 10-2
3.22 X 10 -4 2.05 X 10~
4.66 × 10-2
1.45 × 10 -2
1.50 X 10 -s
2 A 6 X 10-4
2.57 X 10 - t
6.07 x 10 -2
3.07 × 10 -4 2.85 X 10-4 (ave)
a = 0.4 9.99 X 10-2
8.79 X 10-s
8.02 X 10-5
8.54 X 10-8
2.06 X 10 -1
1.43 X 10 - l
3.05 X 104
1.99 X 10-2
1.24 X 10 -2
1.47 X 10 -~
1.13 X 10 -6
2.02 X 10-1
1.41 X 10- I
4.96 × 10 -4
3.94 × 10 -2
1.60 × 10 -2
1.96 X 10-4
9.72 × 10 4
1.94 X 10 - t
1.33 X 10 -1
3.27 X 10 -4
5.60 X 10-2
1.62 X 10 -2
3.69 × 10 -4
6.37 × 10-s
1.87 X 10 - I
1.23 X 10 -1
2.99 X 10-4
6.87 X 10-2
1.62 X 10 -2
4.48 X 10 -4
1.68 X 10 -4
1.80 X 10 -1
1.17 × 10 -1
2.26 × 10 4
7.73 X 10 -2
1.62 X 10 -2
5.36 X 10 -4
3.18 × 10 -4
1.72 X 10 - j
1.17 × 10 -1
2.18 × 10-4 3.12 X 10 -4 (ave)
aO.6 2.00 × 10 -2 3.99 × 10 -2 5.94 × 10-2
1.34 × 10-2 1.60 × 10 -2 1.62 × 10 -2
2.04 × 10 -s 4.76 × 10 -s 1.00 X 10-s
8.28 X 10-8 1.58 × 10 4 9.21 × 10 ~s
1.39 × 10- I 1.37 X 10- I 1.34 X 10 -1
2.05 × 10 - I 1.89 × 10 -1 1.73 X 10 -1
3.31 × 104 3.45 X 10 -4 4.61 × 10-4
7.77 X 10-2 9.12 X 10-2
1.62 × 10-2 1.62 X 10-2
1.16 × 104 2.01 X 10 -4
3.39 X 10-5 1.23 X 10-4
1.32 X 10 -1 1.25 X 10-1
1.56 X 10 - I 1.49 X 10 - I
2.18 × 10-4 2.35 × 10 -4
9.95 X 10 -2
1.62 × 10 -2
3.08 X 10 -4
3.30 X 10 4
1.16 X 10-~
1.50 X 10- j
2.67 X 10-4 3.09 X 10 -4 (ave)
a = 0.8 4.00 X 10 -2
1.62 × 10 -2
5.30 X 10 -2
1.14 × 10 -7
6.97 X 10 -2
2.56 X 10-1
3.09 X 10-4
7.95 X 10 -2 9.86 × 10-2
1.62 X 10 -2 1.62 × 10-2
3.20 X 10 -~ 3.92 X 10 -~
6.47 × 10~ 1.86 × 10 -5
6.78 × 10 -2 6.71 × 10 -2
2.22 × 10 - t 2.00 X 10 - I
4.84 X 10 -4 3.02 × 10 -4
11.74 X 10-2
1.62 × 10-2
9.82 X 10 -*
9.75 X 10-~
6.24 × 10 -2
1.86 × 10- I
4.78 X 10-4
14.38 X 10-2
1.62 X 10 -2
2.04 X 10 4
6.05 × 10 4
5.11 × 10-2
1.71 × 10 - j
5.75 X 10 -4 4.24 × 10-4 (ave)
value of S which is based on a small difference between two relatively large numbers (So - 2Cub) in these computations can be strongly affected by a small uncertainty in these numbers. As a consequence the value of 13cus2 is very sensitive to the uncertainty in this term which can account for the rise in the value of 13 observed in Table I as the saturation binding of Cu is more nearly approached.
The forced value of DI that is resolved with Eq. [13] (see Ref. (1)) to facilitate estimate in Figs. 2 and 3 of the postulated selective binding of Cu(II) in the various experimental situations employed are listed in Table II. The gross experimental binding data (ZCUb), the estimated quantity of Cu(II) presumed bound to the postulated S sites (Cubs), the experimentally measured concentrations of H + ion and Cu(II) ion, and the quantity of Journal of Colloid and Interface Science, Vol. 89, No. 2, October 1982
408
MARINSKY, GUPTA, AND SCHINDLER
undissociated (HA) and dissociated (A-) acid (monomer basis), corrected for Cu bound to the A- functional units (ZCub - CUbs) are also presented in Table II. There are several instances in which the value of D1 deviates quite sizably from the relatively constant value obtained for this parameter at each ionic strength (DI "~ 1.6 X 10-4 in 2.0 MNaNO3, ~3.3 × 10 4 in 0.2 M NaNO3, and -~3.4 × 10-4 in 0.03 M NaNO3); we believe this result is not significant. However, the listed difference between each average value of DL is, and most probably arises from, the difference in activity coefficients of H ÷ and Cu 2÷ at the three different ionic strengths studied. In the 0.03 M NaNO3 system DI is least affected by deviations arising from ionic interactions and this source of deviation should be most easily and accurately correctable by multiplying D1 by the mean molal activity coefficient ratio of HNO3 and Cu(NO3)2
[3]
At this low ionic strength the activity coefficients for the pure electrolyte components should be representative of their activity coefficients in the electrolyte mixture. Recalling now that D1 = MCuAt~int +//~int/~,iOHA)~2and that 13HA i n t " lS available from the intercept value of pK~I~), in 0.2 MNaNO3 (Fig. 1) the value o f ~int ~,c~ + ~ (5.0 × 10-4)(3.6 × 10 4) "~ 1.8 × 101. The magnitude of ~CuA~ ' i+, nt though somewhat smaller than we expect for the carboxylate ligand, is not remarkably smaller than the values published for the CuA ÷ species formed with, for example: acetoxyacetic acid,/3 = 17; benzoic acid,/3 = 32; formic acid,/3 = 24; chloroacetic acid,/3 = 44; salicylic acid,/3 -- 24; isobutyric acid,/3 = 93; ethyl malonic acid, /3 = 55; and dimethyl malonic acid, 13 = 5 (6).
Journal of Colloid and Interface Science,
The possibility that a third Cu(II)-complexed species is formed has been examined as well. In one estimate of this possibility it was assumed that with the formation of CuA + (CuRCOO) + a much weaker acid, i.e., an aromatic carboxylic acid or a phenol, if conveniently located could be bound as well. The following sequence of reactions was presumed to occur Cu(II) + (RCOO-) ~ Cu(RCOO) +,
[41
Cu(RCOO) + + R'OH Cu(RCOO)(R'O) + H +, H + + RCOO- ~ RCOOH.
[51 [6]
The net reaction is then Cu(II) + 2(RCOO-) + R'OH ~Cu(RCOO)(R'O) + RCOOH
[7]
and the mass action expression to describe the equilibrium is
D1 ('~HNO3)4 -- 3.6 × 10-4 ( 0 " 8 5 ) 4 3'Cu(NO3)3 (0.72) 3 = 5.03 × 10-4.
(c) Extended Analysis of Cu(II)-Ion Binding Data
Vol. 89, No. 2, October 1982
[Cu(RCOO)(RO ')] [RCOOH] K-- [Cu(II)][(RCOO_)]2[(R,OH) ] .
[8]
Analysis of the possibility of this complexation path has been performed by computing the concentration of Cu(II) in the gel at the surface of the reaction site as before. The concentration of WOH, unaffected by the pH of the solution in the range studied and barely affected by the small fraction of Cu(II) that could be bound to it, should remain essentially constant and can be incorporated into the Kterm. The Cu(II)-ion binding data, obtained in the 2.0 M NaNO3 system, were employed by taking the difference, ~CUD -- CUsD, as before, to define the total Cu(II) bound to RCOO- alone and to RCOO and RO'. Since we know from our earlier analysis that Cu(RCOO-) + is formed in significant quantity a plot of K', the exchange quotient,
Cu(II) ION
K' =
AND
(2~Cub- Cusb)(RCOOH) (Cu(Ii))0c)2(RCOO_)2 ,
HUMIC
ACID
409
INTERACTION
.08
[9] .O7
versus (RCOOH)/(RCOO-) is expected to give a straight line of slope equal to fl[(CuRCOO)l~+ and an intercept equal to fl(Cu(RCOOXR,O)),(Rr~OH). The graphical representation of the data so expressed are presented in Fig. 4. The intercept is zero to demonstrate the absence of any measurable quantity of the postulated species. The slope is 5. Dividing this number by the mean molal activity coefficient, 0.43, of Cu(NO3)2 at the experimental ionic strength yields a fl(CuRCOO+)~ value of 12 in good agreement with the earlier estimates of the magnitude of this parameter. The second possibility examined was the formation of a bidentate complex as discussed in the preceding paper (Eqs. [17][24]). The sequence of equilibria considered a possibility was Cu(II) + H2AA' ~ CuAA' + 2H +, (H ÷) + HAA'- ~ H2AA'
[10] [11]
B
.OE
o°
.0~ Kch .0¢ .03 ,02 .0t I .002
I .004
I .006
I .008
r .OI0
I .0t2
I .0t4
(H +) (f)
FIG. 5. Extended analysis of Cu(II)-ion binding in humic acid-2. to give the net reaction Cu(II) + HAA'- ~ CuAA' + H +.
[12]
Analysis of the binding data in 2.0 M NaNO3 examined the following expression for the quotient, Koh
14
Kch =
(~Cub - CU~b)(H+)s (Cu(II))sOC)(HAA' -)
[13]
by plotting Kch versus H~+(f). This graphical representation of the data is presented in Fig. 5. The intercept which represents the equilibrium expressed by Eq. [12] is again zero in value to demonstrate the absence of the chelate species. The slope which corresponds to/~(RCOOCu),+again yields a value of 5 or ~ 12 after correction for the nonideality of Cu(NO3) 2 in the 2.0 M NaNO3 solution.
lO K' 8
6
4
2 DISCUSSION I
I
1.0
2.0
RCOOH / RCOO-
FIG. 4. Extended analysis of Cu(II)-ion binding in humic acid-1.
The above analysis of the data compiled in this research has led to the conclusion that interaction of Cu(II) ion with the humic acid samples employed results in the formation
Journal of Colloid and Interface Science, Vol. 89, No. 2, October 1982
410
MARINSKY, GUPTA, AND SCHINDLER
of two separate species. The first, more selective reaction, appears to be (1) independent of system pH and (2) to require the interaction of two sites simultaneously with each Cu(II) ion. The number of such accessible sites is approximately 9% of the accessible hydrogen capacity. It seems quite likely that the sites are polar oxygen atoms accessible in the ether, aldehyde, ketone, and phenolic groups contained in the three-dimensional array of aromatic rings and their side chains that determine the structure of humic acid. The less selective reaction is attributed to the formation of RCOOCu ÷. As has been pointed out the formation constant that is resolved for this species is of the same order of magnitude that has been observed with simple carboxylic acids; although the pK of the carboxylic acid may vary considerably in magnitude the pK of the RCOOCu ÷ species formed with it is not strongly affected (6) to indicate that the reaction assessment is not unreasonable. It is surprising that the analysis of the data in a system as heterogeneous as humic acid is expected to be can lead to the rather uncomplicated resolution of the data that has been accomplished. According to our earlier estimate of the effect of heterogeneity it would have been more reasonable to expect the D1 quotient to become smaller as the degree of dissociation increased. We have observed that the formation constant of the (RCOOCu) ÷ species formed with various carboxylic acids does not vary appreciably in value with the pK of the acid (6). Because of this general observation the observed increase in the pK of humic acid (pK~A), changes three orders of magnitude over the complete neutralization range) was not expected to be duplicated by the variation in pK~d~A+), of the (RCOOCu) ÷ species as it apparently was. There is no other explanation, however, for the successful resolution of the sorption isotherm in Fig. 3 that was affected by applying correction factors deJournal of Colloid and Interface Science, VoL 89, No. 2, October 1982
rived from the potentiometric analysis of the humic acid data for assessment of the effective Cu(II)-ion concentration at the gel-lattice site surfaces. This result leads us to reassess the heterogeneity factor in humic acid as follows: Because of the much limited accessibility of the rigid matrix units in the humic acid their effective concentration as we have pointed out earlier is lowered appreciably relative to their effective concentration in equilibria of similar functional units in small molecule systems. Their freedom of motion is certainly limited to fluctuations restricted by their attachment to the gel matrix. Their relative positions in a sample, because of matrix rigidity, can determine the accessibilities of dissociated and undissociated species, i.e., their activity coefficient ratio. With an increase in the degree of dissociation increase in electrostatic repulsion of the ionized units due to their closer approach will lead to an increase in their accessibility relative to the undissociated units. This could result in an increase in the effective concentration of (RCOO-), relative to (RCOOH)~ to yield the observed increase in the apparent pK with neutralization of the humic acid. In fact, the combined effect of geometry and charge could explain the initially anomolously low p 1~ ~ int (hA), value increasing 3 logarithmic units over the complete neutralization range. The observed squared dependence of the formation constant of the Cu(II)-bound species, [(RCOOCu)~] + on the net deviation term deduced from the acid equilibrium study is then explained by the fact that the electrostatic attraction of the oppositely charged species reduces its accessibility as much as the repulsion of [RCO0]; units increases their accessibility. In closing we would like to point out that the fundamental approach developed by the authors in the preceding paper (1) for the unambiguous analysis of the protolytic and metal-ion complexation properties of humic acid gels has indeed facilitated the achieve-
Cu(lI) ION AND HUMIC ACID INTERACTION m e n t o f this objective in this study. Certainly the u n a m b i g u o u s description o b t a i n e d with this a p p r o a c h to the equilibria e n c o u n t e r e d in the C u ( I I ) - h u m i c acid system studied appear to substantiate the validity a n d utility o f the f u n d a m e n t a l l y based equations develo p e d for this purpose. REFERENCES 1. Marinsky, J. A., Gupta, S., and Schindler, P., J. Colloid Interface Sci. 89, 412 (1982).
41 1
2. Gupta, S., H~ini, H., and Schindler, P., "Mobilisierung und immobillisierung Von Metallen durch die Organische Bodensubstanz." Swiss Federal Research Station for Agricultural Chemistry and Hygiene of Environment, 3097 Liebefeld, Bern, Switzerland, March 1980. 3. Brown, A. S., J. Amer. Chem. Soc. 56, 646 (1934). 4. Forsling, W. S., Hietanen, S., and Sill6n, L. G., Acta Chim. Scand. 6, 901 (1952). 5. Biedermann, G., and Sill6n, L. G., Arkiv. Kemi 5, 425 (1953). 6. SiU6n,L. G., and Martell, A. E., "Stability Constants of Metal-Ion Complexes." The Chemical Society, Burlington House, Wl, London, 1964.
Journal of Colloid and Interface Science, Vol. 89, No. 2, October 1982