POTENTIOMETRIC TITRATION OF WEAK POLYACIDS
POTENTIOMETRIC TITRATION OF WEAK POLYACIDS
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DOI：
10.1016/0014-3057(70)90005-4
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发表时间：
1970-01-01
影响因子：
6
通讯作者：
MANDEL, M
中科院分区：
文献类型：
--
作者：
MANDEL, M
建议通过α的级数展开来表示弱多元酸的pK（K=表观解离常数）解离度（α）的增加，在滴定过程中不表现出构象转变； pK= pK0+Φ1α+Φ2α2+ ….聚丙烯酸的实验结果表明，对于含有过量钠盐的水溶液，α 的二次多项式表示 pK 对解离度的依赖性，且在实验精度范围内。在没有这种过量的情况下，这个二次多项式仍然是一个相当好的近似值。三个系数 pK0、Φ1 和 Φ2 显示显着取决于盐浓度 (C3)。所提出的经验关系是根据Marcus（13）从热力学和泊松-玻尔兹曼方程推导出的多元酸滴定平衡表达式来讨论的，没有考虑沿大分子链的特定相互作用（带电和不带电基团的非随机分布）以及聚电解质带电位点和小离子之间离子对的形成。 pK0 对盐浓度的依赖性部分归因于介质效应。正系数Φ1与不带电聚电解质的大分子基团VS（分配给稀溶液中每个大分子的电中性子体积）的平均分布以及接近α= 0的聚离子周围的平均电势随α的增加有关。将该系数的值与使用聚离子电荷的固定平均分布的模型计算得出的相应值进行比较。系数 Φ2 在低盐浓度下为负值，在较高盐浓度下为正值，显示还取决于由于链扩展而导致解离的大分子基团平均分布的扰动（接近 α = 0）。最后要强调的是，所提出的 α 的 pK 依赖性表示与适用于接近 α= 0·5 的有限 α 值区域的多元酸滴定曲线的经验 Henderson-Hasselbalch 方程并不不一致。
It is proposed to represent the increase with degree of dissociation (α) of pK(K= apparent dissociation constant) for weak polyacids, not exhibiting a conformational transition during titration, by a series expansion in α; pK= pK0+Φ1α+Φ2α2+ …. Experimental results for poly-(acrylic acid) show that a second degree polynomial in α represents the dependence of pKon the degree of dissociation within experimental accuracy for aqueous solutions containing an excess of sodium salt. In the absence of such an excess, this second degree polynomial remains a fairly good approximation. The three coefficients pK0,Φ1andΦ2, are shown to depend significantly on the salt concentration (C3). The proposed empirical relationship is dicussed in terms of the expression derived for the titration equilibrium of polyacids by Marcus(13)from thermodynamics and the Poisson-Boltzmann equation, not taking into account specific interactions along the macromolecular chain (non-random distribution of charged and uncharged groups) and formation of ion-pairs between charged sites on the polyelectrolyte and small ions. The dependence of pK0on salt concentration is partially attributed to medium effects. The positive coefficientΦ1is shown to be related to the mean distribution of macromolecular groupsVS(an electrically neutral sub-volume assigned to each macromolecule in a dilute solution) for theunchargedpolyelectrolyte and the increase with α of the mean electric potential around the poly-ion nearα= 0. Values of this coefficient are compared to corresponding values derived from model calculations using a fixed mean distribution of polyion charges. The coefficientΦ2, negative at low salt concentrations and positive at higher ones, is shown to depend also on the perturbation (nearα= 0) of the mean distribution of the macromolecular groups inVSon dissociation, due to chain-expansion. Finally it is emphasized that the proposed representation for the pK-dependence on α is not inconsistent with the empirical Henderson-Hasselbalch equation for the titration curve of polyacids valid for a limited region of α-values nearα= 0·5.