The Ion-Exclusion Process

Typically, HPICE separator columns contain a totally sulfonated high-capacity cation exchange resin. The separation mechanism occuring at this stationary phase is based on three phenomena  

Di- and tricarboxylic acids such as oxalic and citric acid elute between the excluded and the total permeated volume. Apart from Donnan exclusion, the predominating separation mechanism is, in this case, mainly steric exclusion. The retention is determined by the size of the sample molecule. Since the pore volume of the resin is established by its degree of crosslinking, the resolution can only be improved by applying another or by coupling with another separator column, respectively. 

Characteristic for the stationary phases that are used today in ion-exclusion chromatography are the comparably small particle diameters between 5 pm and 15 pm which enable fast diffusion processes. The structural-technical properties of these phases are listed in Table 4-1. 

Polystyrene/divinylbenzene-based ion-exclusion columns are also offered by Hamilton Co. (Reno, NV, USA) under the trade name PRP-X300. This is a 10-pm material with an exchange capacity of 0.2 mequiv/g [4], It is obtained by sulfonation of PRP-1, a macroporous PS/DVB polymer with reversed-phase properties. Fig. 4-2 shows the separation of various organic acids on this stationary phase. Dilute sulfuric acid was used as the eluent. The much higher retention of succinic acid compared to acetic acid reveals that the retention of organic acids is chararcterized, apart from reversed-phase effects, by the formation of hydrogen bonds. 

Interaction Chemicals also offers two ion-exclusion columns for the separation of organic acids under the trade names ORH-801 and ION-300. Both stationary phases 

Handbook of Ion Chromatography, Fourth Edition. Joachim Weiss. 

In general, organic acid separations can be optimized by changing the pH, because the eluent pH influences the degree of dissociation and, consequently, the solute retention. 

A thermodynamic treatment of the processes occurring at the stationary phase of ion-exclusion columns has been published (in Italian) by Sarzanini and Cavalli [2] in their book on ion chromatography. 

Handbook of Ion Chromatography, Third, Completely Revised and Enlarged Edition. Joachim Weiss Copyright 2004 WILEY-VCH Verlag GmbH Co. KGaA, Weinheim ISBN 3-527-28701-9 

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The ion-exclusion process for sucrose purification has been practiced commercially by Firm Sugar (104). This process operates in a cycHc-batch mode and provides a sucrose product that does not contain the highly molassogenic salt impurities and thus can be recycled to the crystallizers for additional sucrose recovery. 

The ion-exclusion process is particularly suited to sugar processing, e g., sucrose recovery from molasses. 

The ion exclusion process is based on Donnan exclusion. In order for Donnan exclusion to operate in this application, the resin must be in the acid form. It is usually necessary the wash the ion exclusion column with acid between injections in order to remove alkali, alkaline earth and transition metal... 

At the end of the 1970s, ion chromatographic techniques were used to analyze organic ions for the first time. The requirement for a quantitative analysis of organic acids brought about an ion chromatographic method based on the ion-exclusion process, which was first described by Wheaton and Bauman [16] in 1953. 

Instead of ion exclusion, the size exclusion process has been used to perform the separation of NH4S04 and a protein.42 In this case, the adsorption isotherms were simply linear. 

The mechanism for separation of sulfate, chloride and nitrate is not entirely clear. Anions that are completely ionized normally cannot be separated by an ion-exclusion process. A weak hydrophobic effect might account for the slight differences in retention of these anions. 

As notified, the pecrrlarities are only briefly mentioned of ionic interactions among segments of macromolectrles that induce exterrsive coil expartsiotts, as well as between macromolectrles and packing that may lead to ion-exchange, ion-inclusion, and ion-exclusion processes. 

The ion exclusion effect has also been observed on silica gel [ref. 45-48], which appears to be due to silanol groups [ref. 51]. Rinaudo and his coworkers [ref. 46-48] have investigated the exclusion chromatographic behaviour of simple electrolytes and polyelectrolytes on Spherosil (a silica gel), and discussed the dependence of the elution volumes of the sample electrolytes as a function of the sample concentration and the ionic strength of the eluent. The elution volume of a sample electrolyte, V, is dependent not only on the concentration of the electrolyte injected, c, but the sample volume, V. In addition, the sample concentration in the column effluent is no longer the same as that injected, because of the sample band broadening in the elution process. To correlate the different series of experiments, they have thus proposed to plot the dependence of average salt concentration, c = VC /V (v is the volume of solution in... 

The current routes to acrylamide are based on the hydration of inexpensive and readily available acrylonitrile [107-13-1] (C3H3N, 2-propenenittile, vinyl cyanide, VCN, or cyanoethene) (see Acrylonitrile). For many years the principal process for making acrylamide was a reaction of acrylonitrile with H2SO4 H2O followed by separation of the product from its sulfate salt using a base neutralization or an ion exclusion column (68). 

The ratio of reactants had to be controlled very closely to suppress these impurities. Recovery of the acrylamide product from the acid process was the most expensive and difficult part of the process. Large scale production depended on two different methods. If soHd crystalline monomer was desired, the acrylamide sulfate was neutralized with ammonia to yield ammonium sulfate. The acrylamide crystallized on cooling, leaving ammonium sulfate, which had to be disposed of in some way. The second method of purification involved ion exclusion (68), which utilized a sulfonic acid ion-exchange resin and produced a dilute solution of acrylamide in water. A dilute sulfuric acid waste stream was again produced, and, in either case, the waste stream represented a... 

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