Concentration cell, potentiometric sensor

Figure 4 shows how the short circuit current depends on the concentration of H2 which is diluted with air or N2- It is noteworthy that for H2 in air the short circuit current is approximately in direct proportion to the H2 concentration. As mentioned before, this fact suggests that for practical purpose the amperometric sensor is more accurate than a potentiometric sensor. When H2 was diluted with N2, the sensor exhibited a very different behavior with far greater current values and a nonlinear dependence on H2 concentration. In this case, the cell is actually an H2 02 fuel cell which accounts for the greater current values. 

Another type of sensor is a high-temperature solid-state potentiometric sensor for oxygen (>400°C) in industrial processes. These are based on the measurement of the potential of a concentration cell of the type... 

Gas sensors — (a) Gas sensors with liquid electrolytes — Figure 2. can regarded as an Oxygen concentration Cell. They Scheme of a potentiometric gas sensor work depending on electrode material in a broad range... 

Because of the unavailability of the requisite solid electrolytes to form a concentration cell of gaseous oxides, it is impossible to fabricate type I potentiometric sensors for detecting gaseous oxides. 

Potentiometric sensors are based on the measurement of the voltage of a cell under equilibrium-like conditions, the measured voltage being a known function of the concentration of the analyte. Potentiometric measurements involve, in general, Nernstian responses under zero-current conditions that is, the measurement of the electromotive force of the electrochemical cell. 

The basic principle of the potentiometric ZrOz oxygen concentration cell is explained in Section 5.63.1, Fig. 5.6.2. The sensor consists of a Zr02 solid electrolyte body coated with two Pt electrodes, a measuring electrode exposed to the exhaust gas with an oxygen partial pressure of p 02, and a reference electrode exposed to a reference atmosphere with a constant oxygen partial pressure of p f2, typically ambient air. 

Different electrochemical sensors have been developed for cell concentration measurement. The most promising of these sensors are based on impedimetric measurements. A commercial version of a sensor that measures the frequency-dependent i)ermittivity is available from Aber Instruments Ltd [137-139]. Another type of electrochemical probe measures the potential changes in the cell suspension caused by the production of electroactive substances during cell growth [140-143]. To date, no on-line applications of these potentiometric sensors under real cultivation conditions have been reported. Other types of probes, such as amperometric and fuel-cell sensors, measure the current produced during the oxidation of certain compounds in the cell membrane. Mediators are often used to increase the sensitivity of the technique [143-145]. 

Most sensors are potentiometric and are based on concentration cells. A typical arangement is shown schematically in Figure 17. The solid electrolyte, b, is sandwiched between two electrodes, c. One is exposed to the test gas and the other either to a known concentration of gas or some reference half cell. The potential... 

When a redox reaction. Ox + Ze = Red, takes place at an electrode surface in an electrochemical cell, a potential may develop at the electrode-electrolyte interface. This potential may then be used to quantify the activity (or concentration) of the species involved in the reaction forming the fundamental of potentiometric sensors. 

The operation of potentiometric sensors is based on the measurements of concentration cell emf (see Chaps. 1 and 8), which makes it possible to extract the activity, concentration, or partial pressure of potential-determining species at the working electrode vs. RE. The WE potential may be established by a thermodynamic equilibrium or by a nonequdihrium steady state, whereas key requirements to the reference electrodes are related to their reversibility, stability, and, often, fast equilihratirMi on changing external conditions. The solid-state potentiometric sensors are used for a wide variety of technological applicatirms and probed species [2,3,5,15,18,86-91] their application for oxidic glass melts is addressed in Chap. 8. 

Potentiometric sensors (Fig. 19.2, left) are able to measure free oxygen and oxygen in an established thermodynamically equilibrium, e.g. the ratio of partial pressures of burnt and non-bumt components. They are oxygen concentration cells which can be symbolised by... 

Potentiometric ion-selective electrodes are passive probes, which in contrast to voltammetric sensors do not convert the analyte in the sample. The response of an ISE depends linearly on the logarithm of the activity (concentration) of a potential determining ion (primary ion) in the presence of other ions. The schematic layout of a complete potentiometric cell including an ion-selective electrode is shown in Figure 2. The electrochemical notation of the cell assembly is given as ... 

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