By Constantin Sychov on Friday, June 18, 2004 - 06:15 am:

Dear all,

I want to know whether am I correct or not.

Statement is following:

1. When ion exchanger is fully ionized, the retention depends upon the degree of ionization of adsorbates, i.e. for acidic components it increases with the increase in pH, and for basic it deacreases with increase of pH.

2. When adsorbates are fully ionized, the retention depends upon the degree of ionization of weak exchanger, i.e. the dependance upon pH is REVERSED. In fact, here the retention is closely connected with capacity of a phase.

Am I right? (I`m not ion chromatographist)

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By tom jupille on Friday, June 18, 2004 - 06:04 pm:

You are correct -- with one additional assumption: that your mobile phase salt (driving ion) is fully ionized. If you are working near the pKa of your driving ion, then its ionization will also be pH dependent, and the retention vs pH curve can become complex.

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By Chris Pohl on Friday, June 18, 2004 - 06:23 pm:

Constantin,

One added comment relative to your question, you haven't included a description of the most common situation in the separation of anions in ion chromatography: both the analyte and stationary phase are fully ionized. In this case, the pH has no effect on retention per se except inasmuch as hydroxide acts as an eluent ion. Under these circumstances, retention is a function of ionic strength and the relative affinity of analyte and eluent species for the stationary phase.

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By Anonymous on Sunday, June 20, 2004 - 07:06 pm:

Your statement is not clear. Your statement is only correct, if the method is designed to do ion-exchange in both cases. However, if the method is designed to do ion-exclusion, the results are exactly the opposite of the second part of your statement. For example if you use a positively charged ion-exchanger, the retention of bases will increase with an increase in pH, just as the retention of acids increases with an increase in pH. If you use a negatively charged ion exchanger, the retention of acids will decrease with an increase in pH due to ion-exclusion, and the retention of bases will also decrease with pH due to a loss of ionization of the bases.

Your second statement is also related exclusively to the transition from ion-exchange to no ion-exchange.

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By Constantin Sychov on Monday, June 21, 2004 - 08:36 am:

Thanks, Tom, Chris. My question was only about pH dependance.
To ananimous: the question was about ion exchange, but not ion exclusion.
Thanks all!