A Weak Acid Transport Across a Highly-charged Collodion Membrane:Stagnant Layer and Membrane Phase as Rate-determing Steps
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1) Experimental studies have been made on permeability coefficients to a weak electrolyte of benzoic acid across a highly-charged polystyrene sulfonic acid-collodion membrane whose effective negative charge density is 0.12 <I>M</I>. The differential permeability coefficient <I>P<SUB>m</SUB></I> is defined as <I>P<SUB>m</SUB></I>=Δ<I>lJ<SUB>m</SUB></I>/ (<I>C</I><SUB>2</SUB>-<I>C</I><SUB>1</SUB>), where <I>J<SUB>m</SUB></I> is the flux of benzoic acid in moles per sec across unit area of the membrane that separates two aqueous solutions of the weak electrolyte at different concentrations <I>C</I><SUB>2</SUB> and <I>C</I><SUB>1</SUB> (<I>C</I><SUB>2</SUB><<I>C</I><SUB>1</SUB>). Al is the membrane thickness.<BR>2 It was found that at γ=<I>C</I><SUB>2</SUB>/<I>C</I><SUB>1</SUB>=constant, <I>P<SUB>m</SUB></I> is nearly constant of 2.5-4×10<SUP>-9</SUP> when <I>C</I><SUB>2</SUB> is above 10<SUP>-3</SUP>. But below 10<SUP>-3</SUP>, Pm dependence on <I>C</I><SUB>2</SUB> is more complicated and dependent on γ. <I>P<SUB>m</SUB></I> decreases as <I>C</I><SUB>2</SUB> decreases at γ=50, but <I>P<SUB>m</SUB></I> increases as <I>C</I><SUB>2</SUB> decreases at γ=2. These observations are not in agreement with the results on transport of strong electrolytes such as KC1 across a charged membrane in which <I>P<SUB>m</SUB></I> has been shown to decrease to zero as <I>C</I><SUB>2</SUB> decreases at both low and high γ.<BR>3 Considering concentration distributions of dissociated (φ-) and undissociated (Hφ) species of benzoic acid in aqueous solutions and the membrane phase, it is found that the transport of φ- across the stagnant layer on the side of <I>C</I><SUB>2</SUB> solution is a rate-determing step at low γ, and the concentration dependence of <I>J<SUB>m</SUB></I> is given by an equation of <I>J<SUB>m</SUB></I>=constant X (Cφ-)″, where (Cφ-)″ is the φ- concentration in the<I> C<SUB>2</SUB></I> solution. Furthermore, the transport of Hφacross the membrane phase is a rate-determing step at high γ (for example 5, 10, 50) and the concentration dependence of <I>J<SUB>m</SUB></I> is given as <I>J<SUB>m</SUB></I>=constant × (C<SUB>Hφ</SUB>)″-(C<SUB>Hφ</SUB>)″ where (C<SUB>Hφ</SUB>)″ and (C<SUB>Hφ</SUB>)″ ' is the HO concentrations in the <I>C</I><SUB>2</SUB> and <I>C</I><SUB>1</SUB> solutions, respectively.
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- A Weak Acid Transport Across a Highly-charged Collodion Membrane:Stagnant Layer and Membrane Phase as Rate-determing Steps