Sodium ISE

Explain clearly (using equations) why the sodium ISE is more sensitive than the calcium one. 

Sodium ISEs (especially for biological applications) contain monensin XXVII (see [98, 99, 107,203]). Synthetic ligands for sodium ISEs were gradually developed the latest type, XXVIII [64] has properties sufficient for the ISE to be comparable to the sodium glass electrode (see [91 ] for determination of Na in urine). 

From the above, it is clear that the tetraesters and close derivatives can form the basis of excellent sodium ISEs. Studies on device lifetime showed that the sensors could be expected to be used for months at a time [19], and could analyse several thousand blood samples before the signal would become unacceptably affected by membrane coating or leaching of membrane components [20]. 

In some systems, known as continuous-flow analy2ers, the reaction develops as the sample —reagent mixture flows through a conduit held at constant temperature. In such systems, the reaction cuvettes are replaced by optical reading stations called flow cells. In most analy2ers, whether of discrete- or continuous-flow type, deterrnination of electrolyte tests, eg, sodium and potassium levels, is done by a separate unit using the technique of ion-selective electrodes (ISE) rather than optical detection. 

Ion Selective Electrodes Technique. Ion selective (ISE) methods, based on a direct potentiometric technique (7) (see Electroanalytical techniques), are routinely used in clinical chemistry to measure pH, sodium, potassium, carbon dioxide, calcium, lithium, and chloride levels in biological fluids. 

The Zincke reaction has also been adapted for the solid phase. Dupas et al. prepared NADH-model precursors 58, immobilized on silica, by reaction of bound amino functions 57 with Zincke salt 8 (Scheme 8.4.19) for subsequent reduction to the 1,4-dihydropyridines with sodium dithionite. Earlier, Ise and co-workers utilized the Zincke reaction to prepare catalytic polyelectrolytes, starting from poly(4-vinylpyridine). Formation of Zincke salts at pyridine positions within the polymer was achieved by reaction with 2,4-dinitrochlorobenzene, and these sites were then functionalized with various amines. The resulting polymers showed catalytic activity in ester hydrolysis. ... 

I he diaryImethanes (105). The protons on the methylene group of lliese compounds are sufficiently acidic to be removed by strong b.ises such as sodium amide or butyl lithium. Alkylation of the resulting carbanion with w-C2-chloroethyl)dimethylamine affords,... 

Clinical chemistry, particularly the determination of the biologically relevant electrolytes in physiological fluids, remains the key area of ISEs application [15], as billions of routine measurements with ISEs are performed each year all over the world [16], The concentration ranges for the most important physiological ions detectable in blood fluids with polymeric ISEs are shown in Table 4.1. Sensors for pH and for ionized calcium, potassium and sodium are approved by the International Federation of Clinical Chemistry (IFCC) and implemented into commercially available clinical analyzers [17], Moreover, magnesium, lithium, and chloride ions are also widely detected by corresponding ISEs in blood liquids, urine, hemodialysis solutions, and elsewhere. Sensors for the determination of physiologically relevant polyions (heparin and protamine), dissolved carbon dioxide, phosphates, and other blood analytes, intensively studied over the years, are on their way to replace less reliable and/or awkward analytical procedures for blood analysis (see below). 

Examples of the use of FIA with ISE detection involve the determination of nitrate and total nitrogen in environmental samples [48, 49, 125, 166], potassium, sodium [125], calcium [51] and urea [124] in serum or major nutrients in fertilizers [73]. An interesting combination of an ISFET sensor with the FIA principle [52] is shown in fig. 5.17. This is a simultaneous determination of potassium, calcium and pH in serum during dialysis on an artificial kidney. 

The results obtained with ISEs have been compared several times with those of other methods. When the determination of calcium using the Orion SS-20 analyser was tested, it was found that the results in heparinized whole blood and serum were sufficiently precise and subject to negligible interference from K and Mg ([82]), but that it is necessary to correct for the sodium error, as the ionic strength is adjusted with a sodium salt [82], and that a systematic error appears in the presence of colloids and cells due to complexa-tion and variations in the liquid-junction potential [76]. Determination of sodium and potassium with ISEs is comparable with flame photometric estimation [39, 113, 116] or is even more precise [165], but the values obtained with ISEs in serum are somewhat higher than those from flame photometry and most others methods [3, 25, 27, 113, 116]. This phenomenon is called pseudohyponatremia. It is caused by the fact that the samples are not diluted in ISE measurement, whereas in other methods dilution occurs before and during the measurement. On dilution, part of the water in serum is replaced by lipids and partially soluble serum proteins in samples with abnormally increased level of lipids and/or proteins. 

Of the K ISEs, the best properties are those of the electrode based on the ionophore valinomycin XXII [51,196], marked by high selectivity for potassium with respect to sodium. Esters of phthalic acid (9,10) are used as membrane solvents and it is preferable for the reasons given in section 3.3 if the membrane contains the potassium salt of hydrophobic anion XV or XVI [119, 166]. The ISE containing cyclic polyether XXV is useful for only some applications [183] because its selectivity for potassium with respect to sodium is much smaller than with the valinomycin ISE. 

Figure 4.13 — (A) Serially arranged ISEs for the simultaneous determination of sodium, potassium, calcium and chloride ions. (B) Serially and parallelly arranged ISEs for the simultaneous determination of T, Br", Cl and F . WE working electrode PC personal computer Ej-E r, Cr, Br" and F ISEs, respectively C C3 amalgamated-lead columns (1.5 and 2.5 cm long, respectively) AgCl column RE reference electrode PHM pH/mV-meter S sample injection CS carrier stream P pump R chart recorder W waste. (Reproduced from [126] and [127] with permission of Elsevier Science Publishers and Pergamon Press, respectively).

IK. 13 Zinc is a much more reactive metal than cadmium, as expected from the discussion on pages 877-879. Yet both are ised to protect iron from rusting. How is this possible 18.14 Sodium hypo phosphite. NaH.PQ . has been suggested as a repbeemem of sodium nitrite. NaNCK. as a meat preservative to prevent botulism Draw the structure of each anion. 

For comparison, we briefly consider results for metallic fluids. Early experiments for sodium (Tc = 2573 K) [63] indicate a parabolic coexistence curve, but the more accurate results for rubidium (Tc = 2017 K), cesium (Tc = 1924K) [27,28] and mercury (Tc = 1751K) [29] ensure an Ising-like... 

From (6.44), we see that in order to obtain a selective membrane, the value of the ion-exchange constant must be small and the sodium ion mobility in the hydrated layer relative to that of the hydrogen ion must also be small. The expansion of the selectivity coefficient to include selectivity to other ions involves inclusion of more complex ion-exchange equilibria, and the use of a more complex form of the Nernst-Planck equation. This rapidly leads to intractable algebra that requires numerical solution (Franceschetti et al 1991 Kucza et al 2006). Nevertheless, the concept of the physical origin of the selectivity coefficient remains the same. Electrochemical impedance spectroscopy has been successfully used in analysis of the ISE function (Gabrielli et al 2004). 

Undoubtedly, while the direct method is more relevant, because the analyte activity in water plasma is actually measured, the reporting on blood sodium, potassium and chloride in terms of concentration in plasma is preferred by medical professionals, whatever method of measurement is used. This is justified by the fact that before ISEs had been invented, sodium, potassium and chloride were all determined by indirect methods, with flame emission spectroscopy (FES) for Na+ and K+, and coulometry for Cl. ... 

For the above reasons, the IFCC recommendations on activity coefficients [19] and the measurement of and conventions for reporting sodium and potassium [21] and chlorides [25] by ISEs were developed. At the core of these recommendations is the concept of the adjusted active substance concentration (mmol/L), as well as a traceable way to remove the discrepancy between direct and indirect determinations of these electrolytes in normal sera. Extensive studies of sodium and potassium binding to inorganic ligands and proteins, water binding to proteins, liquid-junction effects and the influence of ionic strength have demonstrated that the bias between sodium and potassium reports obtained from an average ISE-based commercial... 

Using 150 pL of sample, a complete report containing the concentration of ionized magnesium, ionized calcium, sodium, potassium, chloride, pH and recalculated to pH — 7.40, the so-called pH-cor-rected ionized magnesium and calcium concentration can be obtained after a total measurement time of less than 2 min. The lifetime of the Mg-ISE is 2000 blood samples [4]. 

Measurements with sodium ISFETs. After the prepared ISFETs were conditioned in 10 3 M NaCl solution for a few hours to stabilize the potential response, their potential response was measured vs. Ag/AgCl reference electrode with a source-follower circuit (ISFET mV/pH meter Shindengen Electric Mfg. Co. Ltd.) at 25 °C in the dark. The frozen horse serum (Working Certified Reference Serum for ISEs ... 

---