Liquid ion-exchange electrodes

This experiment describes the preparation of liquid ion-exchange electrodes for Gk and Ga +. The liquid ion-exchange solutions are incorporated into PVG membranes and fixed to the end of glass tubing. The internal solutions are either NaGl or GaGk, and a Ag/AgGl reference electrode is situated in the internal solution. 

The so-called "trapped sites" of classical mobile-site, liquid ion exchanger electrodes belong to a category of compounds known as ion association extractants. Examples are long-chain diesters of phosphoric acid and tricaprylylmethylammonium (Aliquat) ions. The latter cation was studied extensively by Freiser and co-workers (1-3 ) in the design of anion sensors. 

For a copper-sensitive liquid ion-exchanger electrode, hydrogen (il ++H+ = 10) and iron(II) (il ++Fe++ = 140) interfere seriously, whereas nickel(II) (Kca i = 0.01) interferes only moderately. 

Miniature liquid ion-exchanger electrodes have been made of such small size that ion activities in intracellular fluid can be measured. 

W26. Wise, W. M., Kurey, M. J., and Baum, G., Direct potentiometric measurement of K in blood serum with liquid ion exchange electrode. Clin, Chem. 16, 103-106 (1970). 

Figure 2.7. Schematic representation of the construction of a liquid-liquid ion-exchange electrode.

The lower limit of detection for liquid ion-exchange electrodes is determined primarily by the solubility of the ion exchanger in aqueous media. As with crystalline solid-state electrodes, Nernstian response is obtained until the activity of the solution is within a factor of about 100 of the solubility of the membrane salt. Then the response deviates and levels off at a constant potential reflecting this solubility. 

Typical characteristics of some commercially available liquid ion-exchange electrodes are presented in Table 2.6. 

Table 2.6. Typical Properties ofSelected Commercial Liquid-Liquid Ion-Exchange Electrodes... 

What is the maximum concentration of interfering ions that can be tolerated for a 1% interference level when measuring 10 M Ca + with a calcium-sensitive liquid ion-exchange electrode For a 10% interference level The interfering ions and... 

There have been complaints over expensive and cumbersome membrane replacement with liquid ion-exchanger electrodes, but such technical inconveniences can be minimised by confining the liquid ion-exchanger or carrier sensors in PVC matrix membranes [6,24,54], Such practical considerations with the above attention to principles and proposals [9] for concentration calibration of ion-selective electrodes for use in biological fluids can all facilitate the biological scientist s confident use of ion-selective electrodes. 

A microcapillary version of the potassium liquid ion-exchanger electrode has been used in situ for following potassium ion gradients along the proximal convoluted tubule of a rat kidney [281]. The mean tubular fluid to plasma potassium ion concentration ratio falls significantly from 0.89 for the first convolution to 0.81 for the last convolution of the proximal tubule. Such a disproportionate reabsorption of potassium does not support a common cationic pump mechanism whereby ions and water are reabsorbed in the same proportion as they are first delivered as in the case of sodium [281]. 

Water hardness is the total calcium and magnesium ion concentration in a water sample and is expressed as the concentration of calcium carbonate. Temporary hardness is that part of the total hardness that disappears on boiling. Whilst not being accepted as a standard method, the use of ion-selective electrodes allows a rapid measurement of water hardness and can be used to determine changes in hardness. The direct potentiometric method is not recommended for the ion-selective electrode but an indirect potentiometric method involving ethylenediaminetetraacetic acid titration is recommended. The ion-selective electrode that is used is a liquid ion-exchange electrode that responds to the divalent ions magnesium and calcium. 

Dynamic properties of i.s.e.s. differ greatly for various electrode types and constructions. When the capacitance of analyte/active surface interface is the only cause of response delay, then relaxation time (or time constant of first-order step-response characteristic) is in the order of milliseconds. When the transport of ions across the dynamic Prandtl layer to the surface of the i.s.e. is the main factor (i.e., this transport is the slowest process of equilibrium reinstallation), for a mixing velocity of about lOcm/s a relaxation time of several seconds occurs. This is typical of solid-membrane electrodes with the exception of glass ones. On the other hand, the limited rate of the exchange process in the liquid membrane, the small diffusion flux of the tested ions into the membrane, the slow dynamics for the creation of diffusion potential and the solubility of the active component of the membrane in the testing solution are the main reasons for the slow response of liquid ion-exchanger electrodes (time constants 10-30 s or even more). 

Fig. 3. Ion-selective electrodes. (A) solid-membrane electrode, (B) glass electrode, (C) tip of glass microelectrode, (D) through-flow tubular electrode, (E) liquid ion-exchanger electrode, (F) electrode with restorable heterogeneous membrane layer...

Figure 2.4. Cross section of a liquid ion exchange electrode.

Fig. 1, Effect of reference electrode on potassium determination. Single measurements in NaCl or KCl with reference electrode (glass micropipet with 5 y m tip) against a 3 M KCl agar bridge 2 mm in diameter, upper lines, and with liquid ion-exchanger electrode against the reference electrode, lower lines. The reference electrode was filled with a solution of 3 M lithium acetate with or without addition of agar 2 g/100 ml.

Liquid ion-exchange electrodes where the membrane consists of a solvent in which is dissolved an ion-selective carrier, e.g. valinomycin which binds or dioctyl phosphate which binds calcium. This latter membrane is used for the measurement of ionized calcium in serum. 

Maj-Zurawska M, Sokalski T, Hulanicki A (1988) Interpretation of the selectivity and detection limit of liquid ion-exchanger electrodes. Talanta 35 281-286... 

In the liquid ion-exchanger electrodes of Corning, a ceramic frit imbedded in the electrode body serves as an interface-stabilizer. As shown in Fig. 28, the inner shunt electrode, with its own electrolyte, dips directly into the exchanger phase. 

Gavach et al. were probably the first to apply liquid membrane-based ion-selective electrodes (ISEs) for the titration of long chain alkyl methyl ammonium salts with sodium tetraphenyl borate. Birch et al." were also among the first researchers to use liquid ion-exchange electrodes responsive to ionic surfactants in 1972. 

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