Polarographic cells

Polarographic cells. Numerous types of polarographic cells have been described and various forms are available commercially the choice may well be dictated by the electrode stand in use. 

Cells. The cell employed can be a suitable polarographic cell, or can be specially constructed to fulfil the following requirements. Efficient reproducible stirring of... 

Electrochemical analytical techniques are a class of titration methods which in turn can be subdivided into potentiometric titrations using ion-selective electrodes and polarographic methods. Polarographic methods are based on the suppression of the overpotential associated with oxygen or other species in the polarographic cell caused by surfactants or on the effect of surfactants on the capacitance of the electrode. One example of this latter case is the method based on the interference of anionic surfactants with cationic surfactants, or vice versa, on the capacitance of a mercury drop electrode. This interference can be used in the one-phase titration of sulfates without indicator to determine the endpoint... 

Fig. 3.42 represents the symmetric bell shape curve of 7, i.e., the genuine fundamental harmonic ac polarogram, which means the curve of only 7F discriminated for 7C, e.g., by means of phase-selective ac polarography. The term "fundamental is related to the character of the polarographic cell as a non-linearized network whose response is not purely sinusoidal but consists of the sum of a series of sinusoidal signals at first harmonic (o>) response, besides that of the second harmonic (2a>), the third harmonic (3a>), etc. 

In the ac circuit of the polarographic cell there is such an external ohmic resistance that via the alternating voltage (300 V) together with a superimposed dc the voltage over the cell alternates from 0 to -2V vs. an SCE within these limits oxidation of Hg and reduction of Na+ (electrolyte) to Na(Hg) remains sufficiently restricted. 

CC, Capillary cell (stagnant diffusion) DS, diffusion to spherical electrode ICT. from mobility measurements (International Critical Tables) LFA. laminar-flow annular cell (Leveque relation) LM, from limiting mobility at infinite dilution POL. polarographic cell RDE, rotating-disk electrode. 

Stolzberg [143] has reviewed the potential inaccuracies of anodic stripping voltammetry and differential pulse polarography in determining trace metal speciation, and thereby bio-availability and transport properties of trace metals in natural waters. In particular it is stressed that nonuniform distribution of metal-ligand species within the polarographic cell represents another limitation inherent in electrochemical measurement of speciation. Examples relate to the differential pulse polarographic behaviour of cadmium complexes of NTA and EDTA in seawater. 

When the potential applied to a polarographic cell exceeds the decomposition potential of an electroactive species, its concentration at the surface of the mercury drop is immediately diminished. A concentration gradient is thereby established and more of that species diffuses from the bulk solution to the electrode surface (Fick s law of diffusion). The resulting current flow is proportional to the rate of diffusion which in turn is determined by the concentration gradient, i.e. 

Residual or condenser current flowing in a polarographic cell is the sum of the Faradic current (if) and the charging current... 

Figure 19.3—Polarographic cell and diffusion current. Dissolved oxygen, which leads to an interfering double wave, has to be removed from the sample solution by degassing. On the right features of the diffusion current are shown. These increase with time for every drop of mercury in a static (unstirred) solution. Direct polarography is a slow method of analysis. More than 100 droplets are needed to record the voltammogram.

Kalousek, Mirko — Figure. Polarographic cell designed by M. Kalousek... 

Figure lj. Polarographic cell used for rapid radiochemical detection of short-lived daughter nuclide (163). 

The polarographic cell used is shown in Fig. 13- The active solution is placed above the carbon tetrachloride in contact with the mercury pool electrode or with the calomel electrode through agar gel. The dropping mercury capillary is placed in the solution. The purpose of the large volume of carbon tetrachloride in the other compartment is to keep the solution - carbon tetrachloride interface at a constant height. 

When only the inert electrolyte is present in the polarographic cell a residual current will still flow. This current, which is non-faradaic, is attributable to the formation of an electrical double layer in the solution adjacent to the electrode surface (Fig. 3). At all applied potentials, a current flows to develop this double layer, and the process may be considered analogous to the charging of a parallel plate capacitor. Therefore, the charging current is a capacitance current and varies during the drop lifetime, i.e., with the size of the mercury drop. When the drop surface area is increasing rapidly from the start of the drop lifetime, the capacitance current is a maximum, falling to a minimum near the end of the drop lifetime when the drop size is at a... 

Most of the industrial cells discussed in Chapter 31 have been developed from laboratory prototypes mentioned in this chapter. Polarographic cells and other cells for electroana-lytical purposes discussed in different monographs [1,2] have been omitted however, some electron spin and some electroanalytical cells can be found in Chapter 2. The problems connected with the scale-up of organic electrode processes and electrochemical engineering have been treated [3-5] and are discussed in Chapter 31. Fuel cells have also been omitted here they are discussed in a special issue of Electrochimica Acta (nr 24, 1998). 

Richter, P., Total, M. I., Quiroz, G., and Jaque, P. Flow-through polarographic cell for flow-injection analysis. Determination of nifedipine in pharmaceutical formulations. Lab. Rob. Autom. 9(5) 255-262, 1997. 

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