Clark sensor

Figure 19.6— The Clark sensor for oxygen determination. The Teflon membrane, permeable to gas, must be very close to the cathode so that the double diffusion process through the membrane and the liquid film will lead to a stable signal within a few seconds.

Amperometric gas sensors are - electrochemical cells that produce a - current signal directly related to the concentration of the - analyte by - Faraday s law and the laws of - mass transport. The schematic structure of an amperometric gas sensor is shown in Fig. 1. The earliest example of this kind of sensor is the - Clark sensor for oxygen. Since that time, many different geometries, membranes, and electrodes have been proposed for the quantification of a broad range of analytes, such as CO, nitrogen oxides, H2S, O2, hydrazine, and other vapors. 

Oxygen concentration is determined electrochemically using a membrane oxygen electrode (Clark sensor). Oxygen is consumed by micro-organisms, and the reduction in oxygen consumption with time is a measure of biological activity. 

Many types of Clark sensor have been prepared. An example of a small flow-through sensor fabricated from a printed circuit board is shown in Fig. 9. The gelatin layer contains a depolarizer (KCl) for the reference electrode and polystyrene functions as a gas permeating membrane [126]. 

Amperometry, Fig. 3 Schematic drawing of a membrane-covered amperometric Clark sensor... 

As an example, for a Clark sensor with a 12.5 pm thick PTFE membrane and a gold cathode with 1.5 mm diameter at the oxygen partial pressure P(02) = 21 kPa the resulting current would be Id= 1.5 pA. 

Stationary diffusion can also be achieved by interposing a diffusion barrier, e.g. a semipermeable membrane (Fig. 2.29). An instructive example is the Clark sensor, a chemical sensor for determining dissolved oxygen (see also Sect. 7.2.2). In this device, a gas permeable membrane is located between the cathode and the sample solution. 

Figure 7.20. Clark sensor. (A) classical design, (B) miniature shape...

The Clark sensor is equipped with a gas-permeable membrane located in direct contact with the working electrode surface (Fig. 7.20). The membrane separates the sensor from the sample solution. In this way the electrode surface is protected. Selectivity is increased since only gaseous components can reach the active area. Membranes of macroporous polytetrafluorethylene (PTFE) are used preferably. The material is strongly hydrophobic. Water in liquid state is unable to permeate the membrane pores. Unlike liquid water, gases easily diffuse through the pores. Membranes of silicon rubber have also been used with the Clark sensor. They are permeable for oxygen since it is dissolved homogeneously in the matrix. 

Figure 7.21. Clark sensor effect of diffusion barrier in front of active electrode area. Left-. current-potential diagram right concentration profile for working potential in limiting current region...

The miniature form of the Clark sensor depicted in Fig. 7.20 contains an anode and a cathode as well as a small electrolyte stock (chloride solution containing gel-forming agents) behind the membrane. The technology for manufacturing such sensors differs from standard microelectronic techniques. It had to be developed especially for sensor applications (Suzuki 2000). 

The oxygen electrode is based on voltammetric principles and depends on the diffusion and reduction of oxygen. It is also called the Clark sensor. 

---