Response slope

In addition, most devices provide operator control of settings for temperature and/or response slope, isopotential point, zero or standardization, and function (pH, mV, or monovalent—bivalent cation—anion). Microprocessors are incorporated in advanced-design meters to faciHtate caHbration, calculation of measurement parameters, and automatic temperature compensation. Furthermore, pH meters are provided with output connectors for continuous readout via a strip-chart recorder and often with binary-coded decimal output for computer interconnections or connection to a printer. Although the accuracy of the measurement is not increased by the use of a recorder, the readabiHty of the displayed pH (on analogue models) can be expanded, and recording provides a permanent record and also information on response and equiHbrium times during measurement (5). 

FIGURE 4.5 Typical ISE response curves to monovalent cations (solid circles) with a response slope of 59.2mV decade-1 and to divalent cations (open circles) with a Nemstian slope of 29.6mV decade-1. Intercepts of the linear ranges of sensor responses define the lower and higher detection limits. 

Controlled potential methods have been successfully applied to ion-selective electrodes. The term voltammetric ion-selective electrode (VISE) was suggested by Cammann [60], Senda and coworkers called electrodes placed under constant potential conditions amperometric ion-selective electrodes (AISE) [61, 62], Similarly to controlled current methods potentiostatic techniques help to overcome two major drawbacks of classic potentiometry. First, ISEs have a logarithmic response function, which makes them less sensitive to the small change in activity of the detected analyte. Second, an increased charge of the detected ions leads to the reduction of the response slope and, therefore, to the loss of sensitivity, especially in the case of large polyionic molecules. Due to the underlying response mechanism voltammetric ISEs yield a linear response function that is not as sensitive to the charge of the ion. 

Calibration curve and linear response slope of pH microelectrodes... 

The properties of a pH electrode are characterized by parameters like linear response slope, response time, sensitivity, selectivity, reproducibility/accuracy, stability and biocompatibility. Most of these properties are related to each other, and an optimization process of sensor properties often leads to a compromised result. For the development of pH sensors for in-vivo measurements or implantable applications, both reproducibility and biocompatibility are crucial. Recommendations about using ion-selective electrodes for blood electrolyte analysis have been made by the International Federation of Clinical Chemistry and Laboratory Medicine (IFCC) [37], IUPAC working party on pH has published IUPAC s recommendations on the definition, standards, and procedures... 

It is clear from the Nemst equation that the temperature of the solution affects the response slope (2.303A7//0 of the calibration curve. The electrode voltage changes linearly in relationship to changes in temperature at a given pH therefore, the pH of any solution is a function of its temperature. For example, the electrode response slope increases from 59.2mV/pH at 25°C to 61.5 mV/pH at a body temperature of 37°C. For modem pH sensing systems, a temperature probe is normally combined with the pH electrode. The pH meter with an automatic temperature compensation (ATC) function automatically corrects the pH value based on the temperature of the solution detected with the temperature probe. 

Accuracy is a measure of how close the result is to the true value while reproducibility or precision is a measure of how close a series of measurements on the same sample are to each other. The accuracy and reproducibility of pH measurements can be highly variable and are dependent on several factors electrode stability (drift and hysteresis), response slope/calibration curve, and accuracy of the pH meters. While some of these factors are determined by the properties of electrodes, some measures can be taken to improve measurement accuracy and reproducibility. 

Several factors need to be considered to reduce the pH measurement error. First, an electrode with a high response slope should be used. Second, it is important to use a meter that is capable of measuring the millivolts or microvolts accurately and precisely. With modem meter technology, this is not normally a limiting factor. The... 

It is reported that the greatest interference that affects selectivity of an oxide-based pH electrode is from redox couples. For practical applications, some oxide-based sensors are covered with a thin layer of a size-exclusive protection membrane such as Nafion or polyphenol [43], However, the improvement in selectivity is at the expense of the response slopes and, sometimes, the response times. 

Table 2, Response Slope and Selectivities of ISEs Based on Receptor 12 and...

The agreement between the calculated and observed response slopes indicates that the primary factor determining the EMF slope is the surface-charge density which is governed by the ionophore concentration in the membrane. As a result. 

Most pH meters will display the electrode response (slope) as a percentage of the theoretical value (59.16 mV/pH unit at 25°C). The electrode response should not be less than 95% or more than 105% of the theoretical value at a given buffer temperature. Contamination of the electrode or changes in the liquid junction potential will typically lower the electrode response to below 95%. Some common procedures to regenerate the electrode are listed below. Replacement of... 

In addition, most devices provide operator control of settings for temperature and/or response slope, i.sopolential point, zero or standardization, and function (pH. mV. or monovalent-bivalent cation-nrioni Microprocessors are incorporated in advanced-design meters to facilitate calibration, calculation of measurement parameters, and automatic temperature compensation. 

Figure 27.21 Establishing detection limits in LCEC or FIA requires measurement of both the response slope (sensitivity) and noise. Sensitivity alone does not tell the story.

This resulted in the development of robust sensors for the highly charged blood proteins heparin and protamin, which normally have very low theoretical response slopes [89,90]. 

The acid-base properties of polyaniline can be utilized to produce solid-state pH sensors where polyaniline works both as the pH-sensitive material and as the ion-to-electron transducer. An excellent example is the electrodeposition of polyaniline on an ion-beam etched carbon fiber with a tip diameter of ca. 100-500 nm resulting in a solid-state pH nanoelectrode with a linear response (slope ca. — 60mV/pH unit) in the pH range of 2.0-12.5 and a working lifetime of 3 weeks [104]. The response time vary from ca. 10 s (around pH 7) to ca. 2 min (at pH 12.5). 

Fig. 2.2. Potentiometric calibration curves obtained for PVC-DOS electrodes based on Cs I and UIC (see text for membrane composition). After the preliminary conditioning steps (0.1-M TMAC1 followed by 0.1-M LiOH), the electrodes were conditioned in 0.01-M NaCl and contained 0.01-M NaCl as the inner filling solution. Data are fitted with Nernstian response slopes for monovalent (solid line) and divalent (dashed line) ions, respectively.

Chapter 2. Moreover, the observation of Nernstian response slopes indicates excellent functionality of UIC as an ion exchanger. The importance and advantages of UIC as an ion exchanger are described elsewhere [1,2]. 

A major problem in practical applications of electrochemical sensor systems stems from the adsorption of surface-active materials onto the working electrode. This adsorption causes electrode fouling, which can have a very negative impact on sensor characteristics like baseline stability and response slope. Such surface fouling... 

Earlier sensors were made of plasticized PVC with neutral carriers TDDA (tridodecylamine) for H+ and valinomycin for K+. Accelerated deterioration tests of the proton sensor have been performed by heating TDDA in nitrogen, air and oxygen (9). Partially destroyed TDDA carriers have then been incorporated into the electrodes, and their responses were tested. Results demonstrated a deleterious effect of air oxidation on the response slopes. 

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