Polarographic currents

Mishra and Gode developed a direct current polarographic method for the quantitative determination of niclosamide in tablets using individually three different buffer systems, namely Mcllraine s buffers (pH 2.20 8.00), borate buffers (pH 7.80—10.00), and Britton Robinson s buffers (pH 2.00—12.00) as well as 0.2 M sodium hydroxide. The drug was extracted from the sample with methanol, appropriate buffer was added to an aliquot and the solution then polarographed at the dropping-mercury electrode versus saturated calomel electrode at 25°C [36], The resultant two-step reduction waves observed were irreversible and diffusion-controlled. For the quantitative determination, the method of standard addition was used. Niclosamide can be determined up to a level of 5—10 pg/mL. 

The direct current and alternating current polarographic response of aspirin in an a aprotic organic solvent system (acetonitrile - 0.1M tetrabutyl ammonium perchlorate) has been studied. 00 The following values were obtained ... 

Takahashi, T., and E. Niki An improved alternating current polarograph. 

Hanekamp, H.B. Voogt, W.H. Frei, R.W. Bos, P. Continuous flow alternating current polarographic detection of nitrazepam in liquid chromatography. Anal. Chem. 

For direct current polarographic determination of TcOj in the concentration range 0.1-1.1 ppm in fission product solutions, a phosphate buffer of pH 7 was recommended. 12 of the wave used was -0.68 V V4- SCE. Neither rhenium nor ruthenium nor other fission products interfered. However, [AsPh4]Cl, present in certain fission product solutions, must be separated out [97], A rapid method was developed for the determination of " Tc in fission product mixtures. It consists of a selective reduction of Tc04 at a dropping mercury electrode at -1.55 V v.f SCE in a medium of I M... 

Kinetic Currents. Polarographic currents whose magnitudes are controlled by the rates of chemical reactions are called kinetic currents. For instance, one may have a reaction sequence... 

Alternating current polarographic (ACP) limits are set by the frequencies generally applied (f= 10-2000 Hz). Similarly, the limitations of the rotating disc electrode (RDE) are given by the angular rotation rates (6-6000 rad s" ). The slowest chemical reactions may be followed in coulometric analysis (cf. subsection 6.1) lasting usually 10 min and... 

The polarognqihic bdiaviours of a variety of deoxy-sugars has been conqKued with those of their parent D-pentoses and -hexoses. The direct current polarographic determination of sugars as their aldimines proved suitable for kinetic measurements and the monitoring of syntheses. ... 

Hayes JW, Reilley CN (1965) Operational-amplifier, alternating-current polarograph with admittance recording. Anal Chem 37 1322-1325. doi 10.1021/ ac60230a009... 

Of the other types of organo-metallic stabilisers, Mal kova and co-workers (a.l7) have described an alternating current polarographic method for the determination of cadmium, zinc and barium stearates or laurates in PVC. Other techniques used on PVC extracts include, column chromatography for barium, cadmium and zinc salts of fatty acids, paper chromatography for cadmium, lead and zinc salts of fatty acids (a. 18) and polarography for cadmium, lead and zinc salts (a. 19). 

FIG. 24-14 Dissolved-oxygen electrodes a) polarographic (impress breakdown voltage for oxygen measure current) (h) voltametric (measure electromotive force). 

Now at some pH comparable to pK, two waves are observed, corresponding to the reduction of both HA and A. The currents are proportional to the concentrations of the electroreducible species. Because the pH and pK are known, the concentrations of HA and A in the bulk solution can be calculated. It is then found that the observed polarographic currents cannot be accounted for on tbe basis of the known bulk concentrations. It is concluded that the ratio of the concentrations at the electrode surface is different from the ratio of bulk concentrations, and this is a consequence of the coupling between the chemical and electrode processes. In the pyruvic acid system, HA can be converted to the hydroxy acid by the electrode... 

The method, as so far developed, is limited by the condition that the hydration-dehydration reaction must proceed at a rate that is slow compared with the time needed to obtain a polarogram. In principle, the method is capable of much wider application to covalent-hydration studies if use is made of oscillographic polarographic techniques or of chronopotentiometry. These refinements are currently being investigated. 

If — during this process — the Cu2+-concentration decreases, the mixed potential will shift along the cathodic partial current density curve (like a polarographic curve in this example) toward the equilibrium potential of the zinc amalgam, in case the amalgam reservior is large enough. 

The constant 607 is a combination of natural constants, including the Faraday constant it is slightly temperature-dependent and the value 607 is for 25 °C. The IlkoviC equation is important because it accounts quantitatively for the many factors which influence the diffusion current in particular, the linear dependence of the diffusion current upon n and C. Thus, with all the other factors remaining constant, the diffusion current is directly proportional to the concentration of the electro-active material — this is of great importance in quantitative polarographic analysis. 

Polarographic maxima. Current-voltage curves obtained with the dropping mercury cathode frequently exhibit pronounced maxima, which are reproducible and which can be usually eliminated by the addition of certain appropriate maximum suppressors . These maxima vary in shape from sharp peaks to rounded humps, which gradually decrease to the normal diffusion-current curve as the applied voltage is increased. A typical example is shown in Fig. 16.3. Curve A is that for copper ions in 0.1 M potassium hydrogencitrate solution, and curve B is the same polarogram in the presence of 0.005 per cent acid fuchsine solution. 

Here Ee is the standard potential of the reaction against the reference electrode used to measure the potential of the dropping electrode, and the potential E refers to the average value during the life of a mercury drop. Before the commencement of the polarographic wave only a small residual current flows, and the concentration of any electro-active substance must be the same at the electrode interface as in the bulk of the solution. As soon as the decomposition potential is exceeded, some of the reducible substance (oxidant) at the interface is reduced, and must be replenished from the body of the solution by means of diffusion. The reduction product (reductant) does not accumulate at the interface, but diffuses away from it into the solution or into the electrode material. If the applied potential is increased to a value at which all the oxidant reaching the interface is reduced, only the newly formed reductant will be present the current then flowing will be the diffusion current. The current / at any point... 

The potential at the point on the polarographic wave where the current is equal to one-half the diffusion current is termed the half-wave potential and is designated by 1/2. It is quite clear from equation (9) that 1/2 is a characteristic constant for a reversible oxidation-reduction system and that its value is independent of the concentration of the oxidant [Ox] in the bulk of the solution. It follows from equations (8) and (9) that at 25 °C ... 

This equation represents the potential as a function of the current at any point on the polarographic wave, it is sometimes termed the equation of the polarographic wave. 

With a well-defined polarographic wave where the limiting current plateau is parallel to the residual current curve, the measurement of the diffusion current is relatively simple. In the exact procedure, illustrated in Fig. 16.6(a), the actual... 

The essential requirements for producing polarographic current-voltage curves are ... 

Commercial polarographs are also available in which the voltage scan is carried out automatically while a chart recorder plots the current-voltage curve. A counter-current control is incorporated which applies a small opposing current to the cell which can be adjusted to compensate for the residual current this leads to polarograms which are better defined. Most of these instruments also incorporate circuits which permit the performance of alternative, more sensitive types of polarography as discussed in Section 16.9... 

The resultant peak current is greater than the dilfusion current recorded with a conventional d.c. polarograph by a factor of ten or even more. The method thus shows enhanced sensitivity and it can be used to make measurements with solutions having concentrations as low as 10 6 — 10 7M and with a resolution of the order of 0.05 V. 

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