A cation selective electrode based on copper(II) and nickel(II) hexacyanoferrates: dual response mechanisms, selective uptake or adsorption of analyte cations
Introduction
Ion-selective electrodes (ISEs) are one of the most important chemical analysis methods for monitoring analyte ion concentrations (activities) in aqueous systems1, 2, 3. Liquid membrane-type ISEs for various analyte ions have been improved and sophisticated with the developments of various synthetic receptor molecules for various analyte ions4, 5, 6, 7, 8. Compared to liquid membrane-type ISEs, not many new types of ISEs have been developed based on inorganic materials although there have been some extensive studies concerning the response mechanism of solid-membrane ISEs9, 10, 11such as silver halides12, 13, 14, metal sulfides15, 16, 17, 18, 19and single-crystal lanthanum trifluoride20, 21, 22.
For the precipitate-based ISEs such as silver halides and metal sulfides, the adsorption–desorption process of primary ions on their surfaces has been found to be one of the most important characteristics for the functions of solid membrane ISEs12, 14, 16. Relevant to this, the following crystal-face-specific response of single-crystal CdS-based ISEs was reported: the (0001)Cd face was found to respond to S2− (SH−) ions rather than to Cd2+ ions and the (0001̄)S face to Cd2+ ions rather than to S2− (SH−) ions[18]. This result was explained by a model of reversible adsorption–desorption at the solid/solution interface; the Cd atom acts as an adsorption site for S2− (SH−) and the S atom for Cd2+ ions[18]. The potentiometric selectivities of such precipitate-based ISEs were determined by the solubility products; the smaller the solubility of the precipitate, the higher the observed potentiometric selectivity for the component ion was observed[9]. Therefore, there are invariable selectivities of the potentiometric responses of these precipitated-based ISEs, I−>Br−>Cl− for silver halides and Hg2+>Ag+>Cu2+>Pb2+, Cd2+ for metal sulfides.
On the other hand, the potentiometric selectivities of LaF3 and some inorganic materials having 3D framework structures with interstitial sites are obviously based on the size of the analyte ions; for example, LaF3 is known to respond only to F− and OH− ions[23]and various types of manganese oxides, LiMnO224, 25, Na0.44MnO2[25]and KMn8O16[25]were found to respond selectively to Li+, Na+ and K+ ions, respectively. For these ISEs, the formation of vacancy sites on the solid surface, which acts as a selective site for analyte ion uptake from the adjacent solution sample, seems to be a requisite for ion-selective charge separation, thereby discriminating analyte ions based on the size and shape: non-stoichiometric dissolution of F− ions was found during the membrane conditioning in primary ion-free electrolyte solutions such as NaNO3 and NaCl prior to the potentiometric measurements and positively charged vacancies were formed on the LaF3 surface[22]. The preferential dissolution of Na+ and K+ ions from Na0.44MnO2 and KMn8O16 membranes, respectively, was also found during the membrane conditioning and reported to be essential for the potentiometric responses of these membrane to the respective primary ions[25].
Clear distinction between the above two response processes, (i) selective adsorption based on solubility products of the primary ions, such as silver halides and metal sulfides and (ii) selective uptake into imprinted ionic vacancy on the ISE membrane surfaces, i.e. LaF3 type, seems to be important per se and also valuable for further designing new solid membrane ISEs. For this purpose, copper and nickel hexacyanoferrate (Prussian Blue analogs) membrane electrodes were studied for their potentiometric properties because they are not only slightly water-soluble salts of heavy metal and hexacyanoferrate ions in water system but also have a 3D network structure, in which interstitial ions such as K+ ions are incorporated to maintain the electroneutrality26, 27. For Prussian Blue analogs, while their extensive voltammetric studies were published28, 29, 30, 31, 32, 33including voltammetric ion-sensors for alkali metal ions34, 35and electrocatalysis for reduction of oxygen[36], hydrogen peroxide[37], Fe(III)[38]and nitrite[39], only a few studies were reported on their potentiometric properties40, 41, 42.
In the present paper, the potentiometric selectivities of copper and nickel hexacyanoferrate membrane electrodes will be discussed in relation to the sizes and dehydration energies of analyte cations as well as the solubilities. It will be shown that dual response mechanisms (selective uptake or adsorption) work, depending upon the analyte cations, for ion selective charge separation at the solid/aqueous interface.
Section snippets
Chemicals
K4[Fe(CN)6]·3H2O and K3[Fe(CN)6]·4H2O were obtained from Wako Pure Chemicals Industries (Osaka, Japan) and used as received. All metal nitrate salts obtained from Wako Pure Chemicals Industries (Osaka, Japan) and RbCl and CsCl obtained from Nacalai Tesque (Kyoto, Japan) were of analytical-reagent grade and used as received. n-Butylamine, tert-butylamine, N-methyl-iso-butylamine and N-methyl-n-butylamine were used as free forms obtained from Tokyo Kasei Kogyo Co. (Tokyo, Japan). Other ammonium
Potentiometric responses to K+ ion of the copper hexacyanoferrate membrane electrode as a function of applied potentials for the membrane deposition
Fig. 1 shows the potentiometric responses to K+ ions of the copper hexacyanoferrate membrane electrode deposited at various applied potentials: with the applied potential below 0.40 V vs SCE, the electrode gave a near-Nernstian (55 mV/decade) slope at K+ concentrations above 1×10−4 M (Fig. 1). Weaker responses to K+ ion were however observed with increasing the applied potentials: The slopes exhibited by the copper hexacyanoferrate membrane electrode were 30, 10 and −2 mV/decade at the applied
Conclusion
The mechanism for the selective potentiometric responses of metal hexacyanoferrate membrane ISEs was shown to be dual depending upon different series of analyte cations examined. One is the selective uptake of analyte ions such as alkali metal and ammonium ions into vacancies formed at the surface of metal hexacyanoferrate membranes by the interstitial K+ ion dissolution during the membrane conditioning prior to the measurement. The other is the selective adsorption of the primary ions such as
Acknowledgements
This work was financially supported by grants including the one for the Priority Area of ``Electrochemistry of Ordered Interfaces'' (No. 10131216) from the Ministry of Education, Science and Culture, Japan.
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†Present address: Division of Environmental Health Sciences, Graduate School of Nutritional and Environmental Sciences, University of Shizuoka, 52-1 Yada, Shizuoka 422, Japan.