Ch_322b_24.02

# The moving charges through an aqueous solution

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The moving charges through an aqueous solution conduct a current. H 3 NCHCO 2 + - R -H + +H + H 2 NCHCO 2 - H 3 NCHCO 2 H -H + +H + + dipolar ion (no net charge) cation anion migrates towards cathode migrates towards anode R R

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The Experiment The isoelectric point experiment measures the current conducted by an aqueous solution of an amino acid (the ability to transport charge) as the pH is varied. The isoelectric point is the pH where there is minimal current flow. This minimal current results from a low concentration of chemical species with a net charge. The dominant species in solution at this pH will have no net charge.
Example: Alanine CH 3 CHCOOH NH 2 (2-aminopropanoic acid) At very low pH , the dominant species is the cation: CH 3 CHCOO H NH 3 + + H 2 O pK a = 2.3 CH 3 CHCO 2 - + H 3 O + NH 3 + The positive charge increases the acidity of the carboxylic acid. Near pH = 7, the dominant species is the zwitterion: CH 3 CHCO 2 - + H 2 O NH 3 + CH 3 CHCO 2 - + H 3 O + NH 2 zwitterion pK a = 9.7 Above pH = 12 , the dominant species is the anion: CH 3 CHCO 2 - NH 2 The isoelectric point ( pI ) of an amino acid with a single amino and a single carboxyl function is the average of the pK a and the pK a . pI = pK a + pK a 2 For alanine, pI = 2.3 + 9.7 2 = 6.0 At pH = 6.0 , there will be minimal current flow because the dominant species in solution is the zwitterion that does not have a net charge.

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The concentrations of the three species of the alanine system vary according to the pH of the solution, as shown below.
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