PHZone Refining

The technique of pH-zone-refining CCC primarily developed by Weisz and Scher in collaboration with Ito, deserves special mention because it is unique to CCC. It is applicable to organic acids and bases and is clearly the technique of choice for obtaining highly purified dyes and samples of the impurities contained therein.

For a very brief explanation it is convenient to view the technique as a kind of two-phase acid-base titration carried out in a CCC column. To illustrate this, imagine a series of aromatic carboxylic acids that differ slightly in pKa as well as lipophilicity. Typically a simple solvent system such as methyl t-butyl ether (MTBE) and water is mutually saturated and the phases put in separate containers. The organic phase is acidified with trifluoroacetic acid (TFA) and the column is filled with it as the stationary phase. In practice, either phase can serve as the stationary phase but the organic phase is chosen in this example. The sample of mixed acids is dissolved, or suspended if incompletely soluble, in a portion of the acidified stationary phase and introduced at the column inlet. The aqueous phase is made basic with ammonia and pumped into the rotating column. The acidified sample will dissolve in a relatively small zone of acidified MTBE at the column inlet and as the basic mobile aqueous phase is introduced it will neutralize portions of the sample, which will then be extracted into the aqueous phase. However, on moving down the column, the zone will encounter more TFA, undergo re-acidification and again be transferred to the stationary organic phase. This neutralization and transfer will proceed, as in a titration, first with the acid of lowest pKa, modified also by the lipophilicity of the particular acid, those with lowest pKa and lowest lipophilicity migrating fastest.

Monitoring the eluent by UV absorption is usually not informative, but monitoring the pH shows a series of broad steps of increasing pH containing very pure samples of the individual acids with impurities concentrated at the interfaces between the zones. Bases can be separated in a similar fashion. This technique provides a sample capacity of several grams.

An example showing the separation of two isomeric nitrochloromethoxybenzoic acids by pH-zone refining is presented in Figure 6. Monitoring the UV absorbance of the effluent yields only a flat trace, which does not indicate the separation. However, the effluent pH shows sharp steps at the start and end of the component zones. Using a 310mL-column and a sample size of 20 g, the separation yielded 9 g of the pure 2-nitro compound, 5.3 g of the 6-nitro isomer and 2 g of a mixture of the two.

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