Potentiometric setup

Scheme of a potentiometric setup with an anode (electrode on the right) at which oxidation reactions occur and a cathode (left electrode) at which reduction reactions take place. 

Scheme of a potentiometric setup, showing reference electrode (RE) and working electrode (WE). 

A potentiometric setup for measuring Rfis shown in Fig. 5. The detector is usually a high-resolution, high-impedance digital voltmeter. Voltage drops are measured across and across a series standard resistance f 5that has a stable constant value, usually 100 O. The value of R, is then given by... 

Figure 20 Potentiometric setup for EMF measurements using a liquid membrane.

An important characteristic of a potentiometric indicator electrode is the response time. The response time is the time required for the establishment of an equilibrium electrode potential. According to an lUPAC recommendation [22], the response time is the time interval from the moment of inserting the potentiometric setup into the test solution to the moment when the potential deviates from the equilibrium potential by 1 mV. This time interval can span from milliseconds to minutes or hours and depends on many conditions, e.g. concentration of the measuring ion (small exchange currents), speed of stirring, temperature, history and pretreatment of the indicator electrode, and so on. 

It is possible to monitor the course of a titration using potentiometric measurements. The pH electrode, for example, is appropriate for monitoring an acid-base titration and determining an end point in lieu of an indicator, as in Experiment 10 in Chapter 5. The procedure has been called a potentiometric titration and the experimental setup is shown in Figure 14.11. The end point occurs when the measured pH undergoes a sharp change—when all the acid or base in the titration vessel is reacted. The same... 

FIGURE 2.5 A flow injection system for the potentiometric determination of ammonium in Kjeldahl plant digests used in Brazil in 1976. The flow setup is supported by LEGO blocks and includes a needleless syringes for manual sample injection, an air-gap ammonium electrode (in white), a model 8511 Polymetron peristaltic pump and a model 64 Radiometer pH meter (recorder not shown). For experimental details, see Ref. [43],... 

Principle of setup of a biosensor. B = bioactive layer, containing recognition molcules (enzymes, antibodies, receptor proteins) T = transducer, a probe sensitive to the primary signal produced by the recognition process (potentiometric or amperometric electrode, FET (Field-effect transistor), piezoelectric crystal) A amplifier, R = recorder. 

Figure 14>U. Schematical setup for measurements with potentiometric enzyme electrodes.

Trace amounts of cyanide are usually determined by flow injection spectrophotometric procedures. The target species is first halogenated with chlora-mine-T, after which it reacts with a mixture of pyrazolone or barbituric acid and isonicotinic acid or pyridine to form a bluish-violet polymethine dye. The implementation of gas-diffusion modules in the flow setup for hydrogen cyanide separation avoids matrix interferences and enables the adaptation of inherently nonselective detectors, such as metallic silver-wire electrodes for potentiometric measurements. Total inorganic cyanide, including free and complexed species, such as iron-cyanide complexes, may be determined by sample decomposition with UV irradiation and further photometric or amperometric analysis. 

The selectivity of the glass electrode for NH3 and CO2 may be improved over that of pH measurement by inclusion of a gas permeable membrane between the enzyme layer and the pH electrode. At constant solution, pH a defined relation exists between the potential of the glass electrode and the concentration of the gas-forming ions HC03 or NH4 which are formed in the enzyme reaction. Maximum sensitivity of the electrode is reached when the H+ concentration in the solution is sufficient to ensure maximum conversion of the weak electrolyte into its undissociated form, that is CO2 or NH3. With NH3 this occurs at pH > 10 and with CO2 at pH > 5. Generally, these pH values differ substantially from the pH optima of deaminase and decarboxylase enzymes therefore, for the respective enzyme electrodes a compromise pH has to be found. To obtain optimal conditions for both steps, the enzyme reaction is often separated from the potentiometric indication and a pH change is included between these stages. This setup is termed a reactor electrode. 

Fig. 1 Typical potentiometric sensor setup incorporating an ion-selective membrane electrode [1] (Reprinted from Electroanalysis, 1999. 11(13) p. 915-933 with permission from John Wiley Sons)...

Early gas titrations based on soUd electrolyte devices for end point detection were carried out by Mobius [1], using air as the gaseous titrant and a potentiometric ceU made of stabilized zirconia for the end point detection. The setup is schematically outlined in Fig. 1. 

Fig. 1 Experimental setup for potentiometric gas titration of combustibles with air [1]...

Gas Titration with Solid Electrolytes, Fig. 2 Results of the potentiometric gas titration with the setup given in Pig. 1, according to [1]... 

A schematic drawing of a possible setup for continuous titration is given in Fig. 4. It contains the electrolysis cell, which can be controlled galvanostatically [15] or potentiometrically [7] by the end point potential of a downstream potentiometric cell. 

Flow setup used for the potentiometric detection of cyanides. EC electrochemical cell containing two similar silver amalgam electrodes HC holding coil SV selection valve. 

Potentiometric detection is based on the detection of a membrane potential across the membrane of a probe electrode. Potentiometric probes measure a membrane potential linearly dependent on the logarithm of the activity of a primary ion. Experimentally, a typical setup would be... 

Therefore the setup where the dissolution reaction is followed by potentiometric measurements of the pertinent H -concentration within the galvanic cell (glass electrode solution salt bridge reference electrode) is an optimal system for solubility measurements. This method is called potentiometric method of solubility measiuements ( Potentio in Table 1.1). An example of such measurements could be tiiat of solubility of metal oxides at temperatures up to 300°C performed in ORNL using simultaneously sampling method and the hydrogen-electrode concentration cell (HECC). The details of such measurements and experimental setup can be found in Chapter 3 in this Book. 

Potentiometry has been used for electroanalytical purposes for many years. In biosensing in particular, this technique is based on ion-selective electrodes and ion-sensitive field effect transistors. Basically, the setup of a potentiometric measurement presents an indicator and a reference electrode (connected to the two terminals... 

Fig. 14 Setup of a novel potentiometric non-Severinghaus concept. The pC02 is determined by the potential difference between a carbonate ISE and a pH electrode. Both electrodes are directly immersed in the sample solution without the use of a separating membrane. Adapted from [46] with permission...

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