Environ. Sci. Technol. 1986, 20, 349-354
A Unified Physicochemical Description of the Protonation and Metal Ion Complexation Equilibria of Natural Organic Acids (Humic and Fulvic Acids). 1, Analysis of the Influence of Polyelectrolyte Properties on Protonation Equilibria in Ionic Media: Fundamental Concepts Jacob A. Marlnsky" and James Ephralm Chemistry Department, State University of New York at Buffalo, Buffalo, New York 14214
The high sensitivity of the potentiometric properties of weakly acidic polyelectrolytes (gel or linear) to medium ionic strength has been quantitatively explained. Advantage can be taken of this highly useful assessment to facilitate a valid physical chemical description of protonation phenomena encountered with natural organic polyacids such as humic and fulvic acids.
I. Introduction Elucidation of the protonation properties of humic substances is complicated by two factors, their polymeric nature and their heterogeneity. The first of these is responsible for the strong dependence of their potentiometric properties on the neutral salt concentration level of the aqueous medium. The second is due only to the presence of a mixture of functional groups of different acid strength in a typical humic substances molecule. In order to be able to quantify the acidic properties of a particular humic substance, the contribution of each factor to the observed potentiometric properties must be separated. The only possible way to accomplish this is through a capability for assessing accurately the contribution of polymeric properties to the potentiometric properties measured experimentally. Once this is accomplished the heterogeneity factor, which is unaffected by the neutral salt concentration level, is accessible for refinement through specific heterogeneity estimates. In the various attempts that have been made to model the proton binding by humic substances, this need for separate consideration of the polyelectrolyte effect prior to consideration of the effect on hydrogen binding exerted by the presence of a mixture of acidic groups of different acid strength has not been appreciated. Indeed, in most of the models accommodation of the observed properties is sought in only one of the two factors. For example, in the site-binding model, one of the several most often employed, intrinsic pK values and abundances are assigned to the smallest number of monoprotic acids that reproduce the potentiometric data (1-4). However, because of neglect of electrolyte concentration effects, the parameters so obtained are not applicable to data obtained at salt concentration levels different from those used in their studies. In other attempts to interpret the potentiometric behavior of humic substances continuum models have been applied. In this approach polyelectrolyte perturbations become an intrinsic component of the distribution function (5-9). The existence of a continuum of binding sites that is implied in these distribution models is believed by us to overestimate the complexity of the problem as well. In nature there is the tendency to resist the statistical, undirected distribution of structural components in the course of the development of a product. There are vectors (e.g., environmental) that influence the path to the eventual natural product. This natural course must limit somewhat the site distribution in humic substances to 0013-936X/86/0920-0349$01.50/0
narrower boundaries than provided by the continuum approach. The fact that there are differences between humic substances from different sources (aquatic, marine, soil) is attributable to the environmental vectors alluded to above. Finally in the third kind of model (9-18) generally employed all deviations are attributed to the polyelectrolyte nature of the humic substances. However, Posner (17), as early as 1964, recognized that this kind of model could not account for the effect of ionic strength on the potentiometric properties. He correctly concluded that the observed potentiometric properties were additionally complicated by the presence of different acidic functional groups in the humic substances molecule. It is the purpose of this paper to present the carefully documented basis, already developed (19), for a well-defined experimentalapproach to the quantitative separation of the contribution of simple neutral salt concentration levels to the potentiometric properties of the weakly acidic polymer substances. This earlier research led to the development of procedures for determining the flexibility and salt permeability characteristics of such substances. Methods have also been developed to use such characterization of weakly acidic polymeric substances for quantitative estimate of their effect, as a function of excess salt, on the computed values of the apparent pK at any degree of neutralization. These methods, described in the course of their documentation in the text that follows, have been shown to be applicable to fulvic acids (20).
II. Fundamental Concepts In the development of this section of the manuscript the two different properties, salt permeability and salt impermeability, assignable to weakly cross-linked gels are discussed and analyzed. The sensitivity of potentiometric properties to electrolyte concentration levels as a consequence of electrolyte penetration or repulsion by the matrix and the method for identifying experimentally which of the two matrix properties is applicable to the particular polymeric substance under investigation are then presented. Finally all the concepts and procedures shown to be applicable to the cross-linked weakly acidic polyelectrolyte gels are demonstrated to be equally applicable to their linear analogues as well. This result documents the applicability of all concepts developed to any charged polymeric molecule no matter whether its existence in the system as a separated phase is physically discernable or not. A. Sensitivity of the Potentiometric Properties of Weakly Acidic Gels to the Presence of Neutral Salt. 1. Salt-Permeable Matrix. The Donnan model and its use in the development of the concepts under discussion in this section of the manuscript have been presented in great detail in earlier publications (19, 21-24) by one of US (JAM). As a consequence only its essential features are outlined in the presentation that follows: Consider the disposition of diffusible components, MX and HX, be-
0 1986 American Chemical Society
Environ. Sci. Technol., Vol. 20, No. 4, 1986
349
I
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u 3 r n NaPSS 0.OOi m NaPSS
10-'rn NaPSS Y
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3 00
04
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08
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Figure 1. Potentiometric propertles of the flexible Sephadex CM50-120 gel measured at three different sodium polystyrenesulfonate concentration levels (Donnan potential term neglected).
tween solution and gel phases during each step of the potentiometric titration of a weakly acidic (or weakly basic) gel, (HA),, in the presence of simple electrolyte, MX. With equilibrium established during each step of the neutralization the chemical potential, p, of each component is identical in both phases over the course of the titration with standard base. By choosing the standard state of each component to be the same in both phases, as well, the equilibrium distribution of HX and MX between the two phases is expressed by eq 1given: In this equation n is [(aHXaMX)/(aHXaMX)]=
n R T ( V H X - VMX)
(1)
the osmotic pressure of the water component in the gel phase; V and a refer, respectively, to the partial molar volume and activity associated with HX and MX; and the bar, placed directly above the a, is used to identify the gel phase. Since the term a/RT(VMx - VHX)is very small even in highly constrained gels (when a = 200 atm, VMx - VHx = 0.0012 L, with the employment of a Na+ salt and - VHX)is
