Potassium ISE

An electrode with a plastic membrane containing valinomycin as the active carrier is now predominantly used in clinical analyzers. Nearly four decades of experience with this sensor have proven that it fulfils all demands concerning sensitivity, selectivity and lifetime. An anionic interference that can be observed during measurements in undiluted urine may be eliminated by the use of silicone rubber instead of polyvinyl chloride in the membrane or by pre-dilution of urine. Despite some experimental trials, no other ionophore has replaced valinomycin as the active compound in potassium ISEs. This is basically due to the better stability and lipophilicity of this compound in comparison to the others proposed. 

The two most well known liquid membrane electrodes are cation-selective the potassium and calcium electrodes. For the potassium ISE, the soft plastic membrane has the neutral hydrophobic ionophore valinomycin immobilised in it. This electrode has a 10000-fold selectivity for potassium ions over sodium ions. Eor the calcium ISE, the liquid ion exchanger membrane is a water-immiscible calcium chelator. Calcium ions are... 

Recommendation on the Expression of Results of Sodium and Potassium ISE Measurements ... 

Table 5 shows the variation of sodium and potassium ISE vs SCE at different concentrations of TRIS/TRIS-HCl. 

Mousavi et al. have used PEDOT-CNT composite as ion-to-electron transducer in the fabrication of potassium ISEs [19]. In this work, PEDOT was electrochemically S5mthesized using negatively charged multi-walled CNTs (MWCNTs) as counterions. Results from cyclic voltammetric (CV) and electrochemical impedance spectroscopic (EIS) measurements shown in Fig. 11.1, reveal that the PEDOT-MWCNT film exhibits higher redox capacitance than a film based on PEDOT doped with chloride [Cr) ions, i.e., PEDOT-Cl. This sufficiently high redox capacitance is one of the conditions necessary for stable potential in all-solid-state ISEs having an ECP as the solid contact [23]. 

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. 

Neutral carriers are organic complexing agents which are capable of sequestering and transporting ionic species in a hydrophobic organic phase. The antibiotics, valino-mycin and nonactin were the first neutral carriers to be incorporated in an ISE These macrocyclic neutral carriers contain a polar internal cavity and an outer hydro-phobic shell. The excellent selectivity exhibited by valinomycin for potassium ions is... 

Although rum ammonia levels are not routinely measured, it is a useful indicator of Reye s syndrome and should be monitored in newborns at risk of developing hyperammonemia Ammonia is produced in many analytically useful enzyme reactions and the ammonium ISE has been used as the base sensor in several enzyme electrodes (see next section). In addition to valinomycin, other antibiotics such as the nonactin homalogs and gramicidins also behave as ionophores. The nonactin homolo were originally studied for their ability to selectively bind potassiiun ions It was then discovered that ammonium ions were preferred over potassium ions, and the selectivity coefficient Knh+ = 0.12 was reported. Since ammonia is present at fairly low levels in serum, this selectivity is not sufficient to to accurately measure NH4 in the presence of K. An extra measure of selectivity can be gained by using a gas permeable membrane to separate the ammonia gas from the sample matrix... 

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). 

Fig. 3.4. Hydrophobic anion effect on ise- The membrane contains valinomycin with equivalent concentration of potassium tetraphenyl-borate dissolved in 2-nitro-p-cymene. - ise dependence on the activity of KCl o - f jgg dependence on the activity of KCIO4. The curves were calculated using equation (3.3.13). (After W. E. Morf, G. Kahr and W. Simon.)...

Analysers for clinical purposes have been designed [129-131] and even a bedside analyser for monitoring Na, K, Cs and jJ-D-glucose in patients blood [127] or a blood potassium analyser for use during open-heart surgery [109]. A computer-controlled interference correction has been proposed [44], in which the standards are mixed to match the electrode potential obtained in the test solution. A simple caUbration in flow systems [61] involves dilution of the standard solution and monitoring of the ISE potential as a function of the diluent volume and dilution time. 

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. 

The most important application of the valinomycin macroelectrode is for the determination of potassium in serum [9, 126,141,174] and in whole blood [45, 71, 224]. This electrode with a polymeric membrane is a component of most automatic instruments for analysis of electrolytes in the serum. It has also been used for monitoring the K level during heart surgery [168]. The valinomycin ISE is also useful for determination of Rb [33]. 

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).

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... 

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