Flow-through conductivity cell

Due to the contribution of the corrosion products to the electrical conductivity the accuracy of the static cells is 

In order to overcome the above-mentioned problems and to perform measurements on aqueous solutions near the critical point of water a flow-through conductance cell was developed by Wood and co-workers (Zimmerman et al., 1995 Gruszkiewicz and Wood, 1997 Sharygin et al., 2001). 

The solution flowed into the cup, through the sapphire rod and finally through the holes at the back of the inner electrode. This flow stream sweeps the contaminants dissolving from the sapphire insulator out of the measuring zone and eliminates adsorption effects on the wall of the cell. 

A cell constant close to 0.2 cm was determined from measurements on aqueous KCl at 25 C and the cell constant change with temperature from the known coefficients of thermal expansion of sapphire and platinum was 0.4% over the entire temperature range (306 C-400°C). 

A significant improvement in the speed and accuracy of the conductance measurement was achieved by the use of this flow cell. Zimmerman et al. (1995) have reported conductivity measurements with a precision of about 1% for concentrations as low as 10 mol kg at a water density of 300 kg m and 0.1 % or better for higher concentrations and water densities. The pressure upper limit of this cell is, however, restricted to 28 MPa. 

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Flow through conductance cells are useful detectors in ion-exchange chromatographic separations where the analytes are ionic when they enter the detector cell. At the low concentrations encountered, conductivity is proportional to the mobility of the ions involved as well as their concentration, and this, together with the background conductivity of the solvent, demands that standards are used. Conductivity increases with an increase in temperature and it is important in such measurements that temperature is monitored and appropriate corrections made when calculating the results. 

Fio. 6.47. Electrical conductivity cell (a) section through flow-through conductivity cell showing arrangement of electrodes (6) screw-in cell inserted in pipeline... 

GD-SIA system used for ammonium quantification. Detector two-electrode flow-through conductance cell HC holding coil IV injection valve SV selection valve. 

Figure 4.3 Alternating current flow-through conductivity cell (Reprinted with permission from Zimmerman, G.H., Gruszkiewicz, M.S. and Wood, R.H. J. Phys. Chem. 99, 11612. Copyright 1995 American Chemical Society).

Other flow-through conductivity cell have been described recently (Goemans et al, 1997 Ismail et al., 2003) having accuracy lower than the cells described above and were used over a restricted range of electrolyte concentrations. 

In order to measure the conductance of a solution, a conductance cell is used. It has two electrodes, often platinum, coated with platinum black, separated by a fixed distance. Connecting these to a conductance meter, or more simply to a Wheatstone bridge, the conductance may be measured directly. In order to prevent electrolysis (see Topics C2, C9) taking place, which would change the concentrations in the solution, the bridge uses alternating current. Plastic and flow-through conductance cells are also available... 

There is a potential drop V across the solution between the layer around the working electrode and the tip of the reference probe. This is related to the separation distance d by Equation 1.3 where i is the current flowing through the cell and K is the specific conductivity of the electrolyte. Tire reference electrode probe is... 

Flow-through conductivity sensors suitable for insertion in pipelines (see Fig. 6.47a) are now available for use at temperatures up to 480 K. and pressures up to 1700 kN/m2(64). As conductivity is temperature sensitive, a thermistor is usually included in the detector circuit as part of a temperature compensator. Screw-in cells (Fig. 6.476) will withstand higher pressures. More recently, electrodeless methods of measuring conductivity have become available. In this case the solution is placed between two energised toroids. The output voltage of the instrument (from the output toroid circuit) is proportional to the conductivity of the solution provided that the input voltage remains constant. This type of conductivity meter can be used under much more severe conditions, e.g. with highly corrosive or dirty systems 43 . 

The experiments were conducted as follows the sample was loaded into the sample loop of the injection valve and a valve at the cell outlet was opened to allow fluid from the pump to flow through the cell. The sample loop was then switched in line and the absorbance monitored to detect the appearance of the sample in the cell. The solute (probe) concentration was diluted by introduction of additional solvent to obtain the desired concentration (and... 

Waxman In a cell at resting potential the amount of current flowing through NaN is small, but it is flowing through the cell at rest and so may have a disproportionately large effect because membrane conductance is low. 

Other workers24 have used similar principles to enable continuous production of conducting polymer libers in a flow-through electrochemical cell. As with the hydrodynamic system described in the preceding text, polymer is produced at the anode and continuously removed from the cell in the form of a fiber. Alternatively, other fibers such as Kevlar or nylon can be coated using such hydrodynamically controlled polymerization systems. 

Commercial instruments for both contacting and noncontacting electrode types come in the form of meters, probes, sensors and flow-through cells. Top-of-the-line instruments will have temperature compensation (a thermistor) built in. It is a simple instrument with the following components a conductivity cell, electrodes and a voltage supply. The conductivity cell consists of two platinum plates with an AC voltage applied between them. The eluent flows through the cell and the current, which depends on the concentration and type of ions, is measured. A conductivity detector responds to all ions but not to molecular compounds like ethanol and water. 

Most studies today are conducted in one-chamber diffusion cells that hold receptor fluid beneath the skin. The top surface of the skin is exposed to the environment and is surroimded by a short wall. A tube extends upward from the receptor fluid for manual sample removal. The Franz cell is the most widely known cell of this type (Franz, 1975). A flow-through diffusion cell (Figure 2.1) is a modification of this design that should have a much smaller receptor fluid chamber to permit easy removal of contents with a moderate flow (1 to 2 ml/h) of receptor fluid (Bronaugh and Stewart, 1985). The continual replacement of the receptor fluid pomits maintenance of skin viability when a physiological buffer is used (Collier etal., 1989). This diffusion cell also has the advantage of automatic samphng with the use of a fraction collector. 

The methods reviewed in this chapter, electrochemical impedance and conductivity, have common features the measured object is a specialized electrochemical cell subjected to a periodic electrical perturbation signal (in most cases the sine-wave signal) and the resulting periodic current flowing through the cell is used for the evaluation of the overall cell impedance. This cell impedance consists of the individual impedance contributions of both electrodes and a solution placed in the cell. The techniques mentioned above are aimed at the determination of only one predominant contribution of the overall impedance by using experimental conditions that enable other cell impedances to be ignored. 

The specific electrical conductivity of pure Azoxy-compounds lies between 10 (12 cm)" and 10 ( 2 cm) h Various dopants are added to the Azoxy-compound to influence the conductivity and orientation. The effect of these additives on the variation with time of the electrical conductivity, of the switching times, and of contrast can be measured. It has been found that after roughly 500 hours of operation for instance the cell conductance when measuring the d.c. current flowing through the cell decreases by one to two powers of ten. As subsequent a.c. measurements revealed, this phenomenon results from the formation of double layers with lower electrical conductivity near the electrodes. These double layers disturb the ion injection from the electrode. As a result of low current densities the dynamic scattering disappears almost entirely. 

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