Chromatography Equilibrium chemistry

in chromatography substances separated partition between stationary phase , mobile phase. analyte dissolved in mobile phase, , passes on stationary phase. separation occurs because of differing affinities of analytes stationary phase. distribution constant, kd can defined as

k


d



=



a


s




a


m







{\displaystyle k_{\mathrm {d} }={\frac {a_{\mathrm {s} }}{a_{\mathrm {m} }}}}

where , equilibrium activities in stationary , mobile phases respectively. can shown rate of migration, ν, related distribution constant by

ν
¯



∝


1

1
+
f

k


d






.


{\displaystyle {\bar {\nu }}\propto {\frac {1}{1+fk_{\mathrm {d} }}}.}

f factor depends on volumes of 2 phases. thus, higher affinity of solute stationary phase, slower migration rate.

there wide variety of chromatographic techniques, depending on nature of stationary , mobile phases. when stationary phase solid, analyte may form complex it. water softener functions selective complexation sulfonate ion exchange resin. sodium ions form relatively weak complexes resin. when hard water passed through resin, divalent ions of magnesium , calcium displace sodium ions , retained on resin, r.

rna + m



⇌


{\displaystyle \rightleftharpoons }

rm + na

the water coming out of column relatively rich in sodium ions , poor in calcium , magnesium retained on column. column regenerated passing strong solution of sodium chloride through it, resin–sodium complex again formed on column. ion-exchange chromatography utilizes resin such chelex 100 in iminodiacetate residues, attached polymer backbone, form chelate complexes of differing strengths different metal ions, allowing ions such cu , ni separated chromatographically.

another example of complex formation in chiral chromatography in used separate enantiomers each other. stationary phase chiral , forms complexes selectively enantiomers. in other types of chromatography solid stationary phase, such thin layer chromatography analyte selectively adsorbed onto solid.

in gas–liquid chromatography (glc) stationary phase liquid such polydimethylsiloxane, coated on glass tube. separation achieved because various components in gas have different solubility in stationary phase. glc can used separate literally hundreds of components in gas mixture such cigarette smoke or essential oils, such lavender oil.

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