Cation-sensitive glass electrodes

Table 15.2 Composition of glasses for cation-sensitive glass electrodes...

G. Eisenman, The electrochemistry of cation-sensitive glass electrode, in Advarwes... 

Redox potential pH Ionic activities Inert redox electrodes (Pt, Au, glassy carbon, etc.) pH-glass electrode pH-ISFET iridium oxide pH-sensor Electrodes of the first land and M" /M(Hg) electrodes) univalent cation-sensitive glass electrode (alkali metal ions, NHJ) solid membrane ion-selective electrodes (F, halide ions, heavy metal ions) polymer membrane electrodes (F, CN", alkali metal ions, alkaline earth metal ions)... 

Procedure 1 Cell (VI) is constructed using a univalent cation-sensitive glass electrode (Ag electrode for Ag+) and the emf is measured by titrating metal ion M+ with ligand L ... 

Ion solvation has been studied extensively by potentiometry [28, 31]. Among the potentiometric indicator electrodes used as sensors for ion solvation are metal and metal amalgam electrodes for the relevant metal ions, pH glass electrodes and pH-ISFETs for H+ (see Fig. 6.8), univalent cation-sensitive glass electrodes for alkali metal ions, a CuS solid-membrane electrode for Cu2+, an LaF3-based fluoride electrode for l , and some other ISEs. So far, method (2) has been employed most often. The advantage of potentiometry is that the number and the variety of target ions increase by the use of ISEs. 

The sensor for ion solvation was a silver wire for Ag+ and an univalent cation-sensitive glass electrode for Li+ and Na+. In detennining AG (M+, R -> S) from Eq. (6.13), they assumed that the LJPs at PC/(PC + DMSO) and PC/DMSO were negligible (Ej = 0). At the same time, they detennined in PC the step-wise formation constants (/ ) for the complexation of Ag+, Li+ and Na+ with DMSO (see Section 6.3.3), and used them to calculate the Gibbs energies of transfer of those ions from PC to (PC + DMSO) and to neat DMSO. Equations (6.14) and (6.15) were used in the calculation ... 

Rechnitz, G. A Progress in cation sensitive glass electrodes. Anal. Chem. 37, 29A (1965). 

TABLE I. Some Compositions of Glasses for Cation Sensitive Glass Electrodes... 

Rechnitz GA, Zamochnick SB (1964) Application of cation-sensitive glass electrodes to the study of alkali metal complexes. II. Use of a potential comparison method. Talanta 11 1061-1065... 

Here again, it is difficult to give detailed procedures, because each of the large number of possible reactions has its own set of optimal parameters. One problem which often arises is the proper pH value. The optimal pH value for the biologically active material does not always coincide with that of the ion-selective electrode. This may be because H" ions are directly sensed by a cation-sensitive glass electrode, or because the species to be indicated is associated (for example, NHj ion formation when NH3 is to be detected, or HCN formation when measuring CN"). In the case of urease, a compromise can be reached between pH 7 [183] and 8.5 [184]. When this is not possible, the reaction and indication must be broken down into two separate steps. This was done in the first attempts to measure urea in whole blood [190]. In such cases it is best to use a flow cell apparatus in which the biological reaction is separated from the indicator electrode, and the optimal indicator electrode pH is established immediately before... 

The term log in Equation 13.44 is equal to — plon, and so ion-selective electrodes (ISE) can be used to monitor changes in pM during a titration. For example, a cation-selective glass electrode that is sensitive to silver ion can be used to follow changes in pAg in titrations with silver nitrate. A calcium-sensitive electrode can be used for the titration of calcium with EDTA. The electrode should not respond to sodium ion since the disodium salt of EDTA is usually used. If the electrode responds to a second ion in the solution whose activity remains approximately... 

The resulting ammonium ions can be detected with a cation-sensitive glass membrane. Alternatively, one could use a gas-sensing electrode for ammonia in place of the glass electrode, so that interferences from H , Na", and K are reduced. 

T. M. El-Shamy, The electrode properties of some cation sensitive glasses, PhD Thesis, University of Sheffield, 1966. 

Specific ion electrodes, similar in design to the glass electrode, have been developed to analyze for a variety of cations and anions. One of the first to be used extensively was a fluoride ion electrode that is sensitive to F- at concentrations as low as 0.1 part per million and hence is ideal for monitoring fluoridated water supplies. An electrode that is specific for Cl- ions is used to diagnose cystic fibrosis. Attached directly to the skin, it detects the abnormally high concentrations of sodium chloride in sweat that are a characteristic symptom of this disorder. Diagnoses that used to require an hour or more can now be carried out in a few minutes as a result, large numbers of children can be screened rapidly and routinely. 

In all cases some sensitivity to hydrogen ions remains in any potentiometric determination with these modified glass electrodes the hydrogen ion concentration of the solution must be reduced so as to be not more than 1 per cent of the concentration of the ion being determined, and in a solution containing more than one kind of alkali metal cation, some interference will be encountered. 

Glass electrodes are responsive to univalent cations. The selectivity for these cations is achieved by varying the composition of a thin ion-sensitive glass membrane. 

The membrane of the glass electrode used for pH measurements is selectively permeable to hydrogen ions and from this basic concept a whole range of ion-selective electrodes have been developed. Varying the composition of the glass membrane can change the permeability of the glass and several cation-sensitive... 

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