Donnan Active Dialysis

Donnan dialysis provides an alternative to column ion-exchange, filtration, or precipitation methods that are used to preconcentrate ions and/or remove interfering 

Since NaOH is at the higher concentration, the driving force is to make the ionic concentrations equal on both sides. But Na is prevented from traveling across the membrane. Instead, most of the Cl is replaced by OH . The Cl is never completely recovered on the receiver side, but most will travel across because the OH is at a much higher concentration. Since OH is at a much higher concentration, its dilution by transport to the sample is not greatly affected. 

The exact amount of sample ion transported across is affected by the contact time, the mixing of the solutions and probably other factors. That is why standards used to calibrate the instrument should undergo the same process as the 

Finally, consider the case where the sample is flowing past the membrane and the receiver remains static. In this case, only the volume and concentration of the sample and the concentration of the receiver solution limit the concentration effect of the sample into the receiver solution. Obviously, sample anions can only be transported across the membrane if there are receiving ions available for transport back across. Electric neutrality must be preserved. 

Electrochemical dialysis is a refinement to the dialysis method. The transfer of ions through a membrane is driven by the application of an electric field. 

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Even for this simplest of all situations we had to make a fairly drastic assumption of high concentration of NaCl, in order to get from (6.11) to (6.13). The situation is considerably more complicated when different multivalent ions are present in the solution, although the basic argument is the same. In biological fluids, such as whole blood, the value of the Donnan potential across the dialysis membrane can be tens of millivolts. (In the above derivation of the Donnan potential, concentrations instead of activities have been used for purely historical reasons.)... 

Depending on the type of membrane used, dialysis may be either passive or active (Donnan type). Passive dialysis employs a neutral jwrous membrane with a fine pore structure. This type of dialysis is used primary to remove sample matrix materials such as proteins and/or fiber or other solids [8,9]. Donnan dialysis can do the same, but can also perform preconcentration of the ions in sample. In Donnan dialysis, an ion exchange membrane is used to exchange sample ions for receiving solution ions [10-13]. Each type of dialysis is described below. 

Donnan dialysis is an ion exchange process in which an ion exchange membrane is placed between two solutions, i.e., the feed (I) and receiver (II) solutions. The membrane is permeable to counter-ions and excludes co-ions. The ionic strength of the feed solution is relatively low compared with that of the receiver electrolyte solution. At equilibrium, if concentrations are used instead of activities, there is the following Donnan relationship,... 

Active Donnan dialysis is employed most commonly with ion chromatography and is useful for clean-up of sample solutions at extreme pHs. Electrodialysis can be used with ion chromatography for the off-hne analysis of strongly alkaline samples containing trace amounts of common inorganic anions. ... 

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