Anti-Hofmeister selectivity

Although not strictly relevant to amperometric sensor technology, various metalloporphyrins [Co(III), Mn(III), Fe(III) Fig. 45] have been shown to sense anions potentiometrically with selectivity sequences dependent on the centrally bound metal (Amman et al., 1986 De et al., 1994). For example the anti-Hofmeister selectivity sequence SCN" > I" > CIO4 > N02 > Br > Cl- > NOJ was exhibited by PVC membrane electrodes containing [87]. 

Receptors 2.64 and 2.65 (derived from ds-l,3,5-tris(aminomethyl)cyclohexane) have both been incorporated into membrane ion-selective electrodes. The two hosts differ dramatically in their degree of rigidity and preorganisation, and the size of the cavity that they create and consequently display very different response characteristics. The more rigid 2.65 shows Hofmeister-like selectivity while the more flexible tris(2-aminoethylamine) derivative (2.64) proved to be distinctly anti-Hofmeister. The Hofmeister series was developed in 1888, based on the ability of ions to salt-out proteins and correlates the hydrophobic tendencies of various ions. 

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