Potentiometric sensors testing

Besides these potentiometric sensors there are also amperometric sensors using the principle of ion conductive solid electrolytes. In addition to the heating voltage those sensors are also equipped with a second voltage supply, inducing a current, which varies depending on the concentration of the test gas. Fig. 3.19 shows a schematic view of these so-called saturating current probe. 

Composite potentiometric sensors involve systems based on ion-selective electrodes separated from the test solution by another membrane that either selectively separates a certain component of the analyte or modifies this component by a suitable reaction. This group includes gas probes, enzyme electrodes and other biosensors. Gas probes are discussed in this section and chapter 8 is devoted to potentiometric biosensors. 

L. C. Clark first suggested in 1956 that the test solution be separated from an amperometric oxygen sensor by a hydrophobic porous membrane, permeable only for gases (for a review of the Clark electrode see [88]). The first potentiometric sensor of this type was the Severinghaus CO2 electrode [150], with a glass electrode placed in a dilute solution of sodium hydrogenocarbonate as the internal sensor (see fig. 4.10). As an equilibrium pressure of CO2, corresponding to the CO2 concentration in the test solution, is established in the... 

Several classical ion-selective electrodes (some of which are commercially available) have been incorporated into continuous systems via suitable flow-cells. In fact, Lima et al. [112] used a tubular homogeneous crystal-membrane (AgjS or AgCl) sensor for the determination of sulphide and chloride in natural and waste waters. However, the search for new active materials providing higher selectivity and/or lower detection limits continues. Thus, Smyth et al [113] tested the suitability of a potentiometric sensor based on calix[4]arene compounds for use in flow injection systems. They found two neutral carriers, viz. methyl-j3-rerr-butylcalix[4]aryl acetate and... 

At this point, we have the data generated by our potentiometric sensor array inserted in EasyNN. The next step to follow is to select the training, internal validation and test subsets. To do so, click /Edit/ Change Range of Types... and a window appears in which the fraction of samples for each task can be selected. 

Solid-state reference electrodes for potentiometric sensors are currently under research. The main problem to be faced in developing this type of electrode lies in connecting the ionic conducting (usually aqueous) solution with an electronic conductor. Since the reference electrode has to maintain a defined potential, the electrochemical reaction with components of the electrolyte has to be avoided. Oxides, mixed oxides, and polyoxometalate salts of transition elements can be proposed for preparing solid-state reference electrodes. Tested compounds include tungsten and molybdenum oxides (Guth et al., 2009). 

The hrst neutral ionophore molecules, valinomycin and nonactin, were borrowed from nature, but soon thereafter good success was also achieved with synthetic ionophores. Simon s results initiated intensive research aiming to design, prepare, and test different types of organic molecules in ion-selective potentiometric sensor preparations. This work is still in progress today. 

Techniques for using a silicon-based light addressable potentiometric sensor (LAPS) to measure the electrical properties of phospholipid bilayer membranes were developed. Membrane conductance, capacitance, and potential could all be measured when the membrane was painted on an aperture between the silicon surface and a controlling electrode. The sensor was tested by observing changes in membrane properties on the addition of simple ion carriers and channels. 

The key component of a potentiometric sensor is the ion-selective electrode (ISE), an electrode or electrode assembly with a potential that is dependent on the concentration of an ionic species in the test solution and is used for electroanalysis. This interfacial potential at the electrode surface is caused by the selective ion exchange reaction. Ion-selective electrodes are often membrane type electrodes. The well-known glass pH electrode, which is selectively sensitive to hydrogen ions, is a typical ISE and has been used for years for the measurement of acidity or basicity of aqueous solutions in... 

Smiechowski and Lvovich monitored the levels of acidity and basicity in industrial lubricant. The sensor was based on electrochemical impedance methodology. An iridium oxide potentiometric sensor was developed in both a conventional and MEMS configuration. Tests of the sensors in diesel lubricant showed good correlation between TAN, TEN, and the voltage output of each sensor [7]. Widera et al. used a potentiometric iridium oxide electrode as an indicating electrode with a silver/silver chloride reference electrode for the off-line monitoring of fuel acidity. The data showed that the iridium oxide sensor responds to compounds present in fuel that have acid-base character and it is possible to determine the acidity of different fuels and discriminate between unstressed and thermally stressed fuels. Experimental results indicated the ability to correlate the response of the iridium oxide sensor with the total acid numbers of different fuels [20]. 

Typical construction and its output characteristics of ZrO -based sensors are shown in Fig. 2.7. The cell in oxygen sensors is usually shaped like a test tube where the inner and outer surfaces are each coated with ultrathin layers of porous platinum which act as the cathode and anode electrodes. The output of this potentiometric sensor is due to the combined effect of chemical and electrical processes. At high temperatures >650 °C, zirconium dioxide exhibits two mechanisms (Park et al. 2009) (1) ZrO partly dissociates to produce oxygen ions, which can be transported through the material when a voltage is applied and (2) ZrO behaves like a solid eleetrolyte for oxygen. 

Artificial noise systems, some of them made of an array of unselective or selective potentiometric sensor spots [87, 88], have already been tested in food analysis, for example, for detecting food quality decline. 

M. Arvand and H.A. Sarnie, A biomimetic potentiometric sensor based on molecularly imprinted polymer for the determination of memantine in tablets. Drug Test. Anal, 5 (6) 461-467, 2013. 

M. Najafi and R. Mehdipour, Molecularly imprinted polymer-based potentiometric sensor for 2-aminopyridine as a potential impurity in piroxicam, Drug Test. Anal., 3 (2) 132-137,2011. 

The principle of pH electrode sensing mechanisms which are based on glass or polymer membranes is well investigated and understood. Common to all potentiometric ion selective sensors, a pH sensitive membrane is the key component for a sensing mechanism. When the pH sensitive membrane separates the internal standard solution with a constant pH from the test solution, the potential difference E across the membrane is determined by the Nemst equation ... 

Ciosek et al. (2005) used potentiometric ion-selective sensors for discriminating different brands of mineral waters and apple juices. PC A and ANN classification were used as pattern recognition tools, with a test set validation (Ciosek et al., 2004b). In a subsequent study, the same research group performed the discrimination of five orange juice brands, with the same instrumental device. A variable selection was performed, by means of strategies based on PCA and PLS-DA scores. The validation was correctly performed with an external test set. 

How analytical methods deal with interferences is one of the more ad hoc aspects of method validation. There is a variety of approaches to studying interference, from adding arbitrary amounts of a single interferent in the absence of the analyte to establish the response of the instrument to that species, to multivariate methods in which several interferents are added in a statistical protocol to reveal both main and interaction effects. The first question that needs to be answered is to what extent interferences are expected and how likely they are to affect the measurement. In testing blood for glucose by an enzyme electrode, other electroactive species that may be present are ascorbic acid (vitamin C), uric acid, and paracetamol (if this drug has been taken). However, electroactive metals (e.g., copper and silver) are unlikely to be present in blood in great quantities. Potentiometric membrane electrode sensors (ion selective electrodes), of which the pH electrode is the... 

The GECE sensors were used for lead determination in real water samples suspected to be contaminated with lead obtained from water suppliers. The same samples were previously measured by three other methods a potentiometric FIA system with a lead ion-selective-electrode as detector (Pb-ISE) graphite furnace atomic absorption spectrophotometry (AAS) inductively coupled plasma spectroscopy (ICP). The results obtained for lead determination are presented in Table 7.1. The accumulation times are given for each measured sample in the case of DPASV. Calibration plots were used to determine the lead concentration. GEC electrode results were compared with each of the above methods by using paired -Test. The results obtained show that the differences between the results of GECE compared to other methods were not significant. The improvement of the reproducibility of the methods is one of the most important issues in the future research of these materials. 

While most gas sensors rely on potentiometric detection, the important oxygen probe is based on amperometric measurements. In particular, membrane-covered oxygen probes based on the design of Clark et al. (105) have found acceptance for many applications. The sensor is based on a pair of electrodes immersed in an electrolyte solution and separated from the test solution by a gas-permeable hydrophobic membrane (Fig. 6.22). The membrane is usually... 

---