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]. 

Often sensors only display Hofmeister selectivity or are affected by anions of a similar nature to that of interest. The development of sensors that will detect small concentrations of the anion of interest in the presence of strong interferents, possibly in higher concentrations, will be of great use. 

The majority of ionophores for nitrate that are commercially available are based on quaternary ammonium salts, such as tridodecylmethylammonium and tetraoctadecylammonium salts. Electrodes based on these salts, however, lack selectivity and, most often, the selectivity pattern of the electrodes follows the classical Hofmeister selectivity series. The response of these electrodes, then, is primarily toward C104 with a log of 3.0 relative to nitrate, determined by the separate solution method (3, 24). In fact, in many cases, it would be necessary to pretreat sample solutions to remove some of the more highly interfering anions (EPA method 9210). 

From a mechanistic point of view, ion exchange reactions follow the hard and soft acids and bases (HSAB) principle. Materials containing the relatively soft imidazolium ion are therefore particularly efficient ion exchange materials for soft anionic species. For this reason, the measured distribution coefficients between the solid and liquid phases are often particularly high with soft anions and follow Hofmeister selectivity [142]. 

Clearly, the second relationship reflects the Hofmeister selectivity sequence (irais of higher lipophilicity are preferred over more hydrophilic ones), while the former predicts a selectivity that is additionally dictated by the binding constants between the ions and the ionophore. 

While ionophore-free membranes based on classical ion exchangers are still in use for the determination of lipophilic ions, such sensors often suffer from insufficient selectivity, as it is governed solely by the lipophilicity pattern of ions, also known for anions as the Hofmeister sequence. This pattern for cations is Cs+ > Ag+ >K+ > NH > Na+ > Li+ > Ca2+ > Mg2+ and for anions CIOT > SCN- > I > Sal- > N03- > Br > N02- > Cl- > OAc- HC03- > SO - > HPO4. While the ion exchanger fixes the concentration of hydrophilic analyte ions in the membrane on the basis of the electroneutrality condition within the membrane, the second key membrane component is the ionophore that selectively binds to the analyte ions. The selectivity of... 

With the help of selectivity coefficients, such as in Eqs. (4.51) and (4.52) a general order of affinity can be given. For most clays the Hofmeister series (the same series as was given in Eq. (2.17) for the affinity of ions to oxide surfaces)... 

A number of studies have reported the abilities of different eluting salts to produce both different resolution and different selectivities.17 23 As expected, polyvalent ions are stronger eluters than monovalent ions. Some studies have suggested a correlation between eluting ability and ranking in the Hofmeister series. Others fail to observe this correlation, but nevertheless note differences in elution behavior. Overall, it is impossible to predict which eluting ions, if any, may produce a... 

Pedersen and Frensdorff studied the binding and extraction of the cation by crown ethers, which require coextraction of an anion. Without the presence of an anion host, solvation directs selectivity, giving rise to Hofmeister bias selectivity favoring low anion charge density. Anions initially in aqueous solution must be dehydrated (at least partially) and are then resolvated in the solvent phase. Empirically, the HBD ability of the solvent medium is the single most important determinant of the solvation of small, inorganic anions.100... 

In considering the differential or selective binding of one anion or another, the intrinsic properties of anions mean that we are not on a level playing field it is considerably easier to bind some anions than others. In general, in the absence of specific chemical recognition between anion and host, anion binding selectivity, particularly in solvent extraction experiments or in the detection of anions by membrane-based ion selective electrodes, follows the order of anion hydrophobicity. This order is termed the Hofmeister series, or lyotropic series and was first outlined in 1888 from experiments based on the... 

The majority of the devices mentioned thus far rely on the Hofmeister series for anion selectivity. However, for anions that deviate from this series, organometallic receptors can be utilised. The type of ligand or metal centre will influence the sensor selectivity due to the characteristics of the electron acceptance of the complex. An interesting development that is being explored here is the use of calixarenes. These have previously found use as cation-selective species, but with suitable substitution are now being incorporated within anion-selective devices. Compounds suitable as receptors for halides [61],benzoate [61] and acetate [62] have been developed. Reinhoudt and his co-workers have reported the production of a POj-selective CHEMFET based on a uranyl cation immobilised within a salophene ligand (Fig. 5), which shows selectivity over more lipophilic anions such as Br" and NOj [63]. 

Sol-gel methods have been used to form membranes on sensor surfaces by the mixing of the ion-sensitive molecule with alkoxysilanes which then crosslink to form a hard, transparent coating on the sensor. For example, tridodecyl-methylammonium chloride-doped films were used as a chloride sensor and showed Hofmeister-type selectivity [91]. Long cure times (several days), however, represent a disadvantage in this process [91]. 

As earlier reported for electrochemical sensing, often the active chromo-phore will be dispersed in a polymeric matrix. For example, Mohr and Wolfbeis reported a nitrate sensor [121] where the active chromophore is a rhodamine B dye which had been modified with an octadecyl side chain to render it hydrophobic and prevent leaching. The dye was dispersed in a plasticised PVC membrane containing a hydrophobic anion carrier (tridodecylmethylammo-nium chloride). On exposure to nitrate, the fluorescence of the dye increased. This membrane, however, only displayed Hofmeister-type selectivity and was also affected by pH. Replacing the quaternary ammonium anion carrier with a palladium phospine chloride carrier led to selectivity for nitrite [ 122], probably due to a preferential interaction between Pd and nitrite ion. 

Several of the polymer membrane anion-selective electrodes described in the literature use quaternary ammonium salts as ion carriers (ionophores) (7). These electrodes respond according to the Hofmeister series (CIO4 > SCN > I > NO3 > Br - N3 > NC>2 > Cl > HCO3 acetate) (2, 5), which is the order of relative lipophilicity of the anions. Therefore, in strict terms, electrodes that respond according to this series could be considered "nonselective". 

A hydrophobic cobyrinate (Figure 2, structure 2) was used to prepare solvent polymeric membranes (10). The typical membrane composition was 1% (w/w) ionophore, 66% (w/w) plasticizer and 33% (w/w) polymer. Electrodes prepared with this ionophore, dioctyl sebacate (DOS) and poly(vinyl chloride) (PVC) presented, at pH 6.6, the selectivity pattern shown in Figure 3. The response of the electrodes was near-Nernstian for salicylate, thiocyanate, and nitrite. Their selectivity behavior clearly deviates from that of the Hofmeister series, with nitrite being the anion that presents the larger deviation. 

ISE Based on a Hydrophobic Cobalamin. Cobalamin 4 (Figure 2) is a hydrophobic vitamin B12 derivative that retains a proximal base (an imidazole ring), which provides the molecule with a resemblance to the original vitamin (Figure 1). The selectivity pattern of electrodes prepared with ionophore 4 at pH 5.5 is shown in Figure 4. These electrodes exhibit an anion selectivity pattern that differs from that of the Hofmeister series and from that of the previously reported cobyric acid derivatives (see section above). Indeed, electrodes based on 4 are selective for... 

The potentiometric behavior of electrodes based on these films was studied (Figure 8). These ISEs presented sub-Nemstian slopes for thiocyanate (from -40 to -53 mV/decade, depending on the buffer used), and had detection limits of 5xl0 7 M. The response time of the electrodes was typically less than 25 s. The selectivity pattern observed was thiocyanate > perchlorate > iodide > nitrite - salicylate bromide > chloride > bicarbonate > phosphate. This anion-selectivity behavior does not follow the Hofmeister series, with thiocyanate and nitrite being the ions that deviate the most from it. This indicates that there is a selective interaction of the immobilized porphyrin with the two anions. 

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