Ion-Exchanger Electrodes

Calcium activities as low as 5 x 10 7 M can be measured, with selectivity coefficients ACaMg and ACaK of 0.02 and 0.001, respectively. Such potential response is independent of the pH over the pH range from 5.5 to 11.0. Above pH 11, Ca(OH)+ is formed, while below pH 5.5, protons interfere. Because of its attractive response characteristics, the calcium ISE has proved to be a valuable tool for the determination of calcium ion activity in various biological fluids. 

FIGURE 5-9 Schematic diagram of a calcium ion-selective electrode. 

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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. 

Homogeneous liquid membrane electrodes. This type, which is in limited use, is sometimes considered as a solid ion-exchange electrode as the electroactive species, e.g., calcium dioctylphosphate, after being dissolved in an ethanol-diethyl ether solution of collodion, is left to "dry and can function as an ion-selective pellet in an electrode tip. Orion37 use these electrodes with PVC-gelled membranes for Ca2+, K+, BF4 and N03. ... 

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. 

Ion Exchanger Electrodes One of the most successful liquid membrane electrodes is selective toward calcium. Such an electrode relies on the ability of phosphate ions to form stable complexes with the calcium ion. It uses a liquid cation exchanger, consisting of an aliphatic diester of phosphoric acid... 

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). 

Liquid solid Ion exchange Electrode reactions (including plating) Enzyme reactions peptide synthesis Liquid phase olefin polymerization Cheese making High-fructose corn sweeteners... 

A divalent cation ion exchange electrode that responds to several cations is available. Its response is nearly equal for calcium and magnesium, and it.is useful for measuring water hardness. A copper and a lead electrode are also available. Anion-selective electrodes of this, type are available for nitrate, perchlorate, and chloride. They are the same in principle, except that a liquid anion exchanger is used instead of a cation exchanger. 

Liquid-Membrane lon-Exchai e Electrodes. One design of a liquid-membrane ion-exchange electrode consists of two concentric cylindrical tubes constructed of inert plastic (Fig. 2.7). The inner tube holds the internal reference electrode and an aqueous... 

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. 

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