1 1 1 1 k Id Im Ia

10"pH

10"pH +Ka

0.999

The distribution ratios KDM and KAM are correlated to the concentrations $,D x ci;1;D and x ci;1;A of the volatile forms in the donor and in the acceptor solutions, respectively:

pi,D

pi,A

For large concentration gradients (cD — cA)/dM and very intensive stirring in the set-up shown in Figure 1 or in flow-through dialysis cells with high flow-rates the membrane diffusional transport becomes rate-determining particularly for relatively thick membranes with small pores. Eqn [21] then follows:

The right-hand terms of eqns [26] and [27] are approximations for low partial pressures pi;D and pi;A. ci,g,D and ci,g,A are the concentrations of the analyte in the gas phase at the interface between the donor or the acceptor solution, respectively, and the membrane gas phase. For very small partial pressures pi,D and pi,a, Kam«Kdm (pi,D»Pi,A) and without hydrodynamic transport limitations, eqn [18] can be simplified to eqn [28]:

Example 2: Dialysis of Volatile Substances Through Hydrophobic and Microporous membranes. To separate a nonvolatile base B", its corresponding volatile acid BH is produced according to the equilibrium:

The donor pH value is chosen according to eqn [22] so that there is a 99.9% degree of conversion into the permeable form of the analyte:

For fast-flowing donor solutions with 1/kD p 0 the mass transport is determined by the gas diffusion through the membrane, the partial pressure of the analyte in the donor solution and the phase-transfer resistances. The partial pressure of the volatile analyte can be increased by decreasing the partial pressure of the water using high ionic strengths in the donor solution. For highly volatile analytes, thin and highly porous membranes, and fast-flowing solutions, the overall mass transport is controlled by the phase transfer resistance (k = 1/a).

Ka is the acid-dissociation constant of BH.

The acceptor pH value should be adjusted according to eqn [23] to trap the analyte in its nonvolatile form:

Reverse osmosis The driving force of reverse osmosis is the difference between the outer pressure ph and the osmotic pressure difference n. The mass transfer can be described according to eqn [29]:

0.999

with:

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