Chromatographic behaviour of 29 common cations has been studied in alcohol-formic acid ratios of 9:1, 7:3, 5:5, 3:7, and 1:9. The plot of Rf value of a metal ion against %formic acid provides an S-shaped curve, the Rf value increasing with increase in the acid concentration. Sn2+ and Sb3+ show exceptional behaviour. In butanol-formic acid (1:9), these have Rf values (Sn= 0.50; Sh= 0.62) oven though most metal inns either 'tail' from the 'point of application or have zero Rf valves. This

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... r and other structural effects have been explained in terms of the polarity of the chromato­graphic system.