Liquid cylindrical sample cells

The instrumentation for dc conductivity measurements is dc power-source, sensitive electrometer (the currents are of microampere range and lower), and adequate sample cells. Such instrumentation is easily available and even the most sensitive kind is relatively inexpensive. Remote measurements are simple to perform, and parts exposed to irradiation, like reaction cells and cables, are cheap and can be easily replaced. Two types of sample cells for conductivity monitoring are most conunon flat and cylindrical coaxial. In flat cells the sample, usually solid, is sandwiched between electrodes and the interelectrode distance is kept by a spacer of chosen size. Cylindrical coaxial cells shown in Figure 13.1 are preferred for liquid samples, flow-through versions also exist (Battisti et al. 2009a). 

A standard type of sample cell for ordinary solutions is a vertically standing cylindrical glass tube sirrrounded by a refractive index-matching liquid to reduce surface scattering on the sample tube. The sample tube is placed in a thermally... 

It is especially useflil for liquid samples in flat cells, which may extend through tlie entire height of the cavity. In the cylindrical cavity a TEq mode is frequently used because of its fairly high g-factor and the very strong along the sample axis. 

Liquid samples may be sealed in ampules, tubing, or capillaries depending on the amount of the sample available (Fig. 2-20B). For microquantities ( 10 9 liter), capillaries as small as 0.5-0.1 mm bore and 1 mm in length have been used. Use of a large cylindrical cell, such as that shown in Fig. 2-20C, reduces local heating and allows more accurate determination of depolarization ratios. Strong spike noise may appear if the solution contains solid particles. 

Figure 2-20 (A) Gas cell with external resonating mirrors. (B) Capillary cell for liquids. (C) Cylindrical cell for liquids. (D) KBr pellet for solid samples.

Zhang et al. (1991) published a relatively simple transmission XAFS cell in 1991. Their design incorporated a cylindrical body with sealed Kapton windows at each end through which the reaction gas flowed. A heating coil was wrapped around the cylinder, and a liquid nitrogen vessel provided cooling of the sample. The sample was mounted on an insert and sealed on the holder with Kapton tape. This insert was placed inside the cylindrical cavity. It is not clear from the description provided how the gas in the cylindrical cavity made contact with the catalyst in the sample holder. 

Knudsen121 Effusion Gauge. To measure the vapor pressure of solids or liquids indirectly, a Knudsen cell is a cylindrical cell containing the sample. A small opening at the top of the cell allows molecules to evaporate at a fixed rate, proportional to the vapor pressure inside the cell. The mass loss of the cell is proportional to the pressure and is measured after a fixed time, for several temperatures. 

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