Direct current polarography diffusion

Quantitative analysis using dc polarography amounts to making use of the linear relationship between the diffusion current and the bulk analyte concentration. It may seem attractive to use the llkovic equation directly. However the diffusion coefficient (Dox) is usually not known accurately for the particular solution conditions and more important, the constant 708 emerges from the less than perfect model of the diffusion layer used in the derivation. The errors introduced are usually < 109c but this is not satisfactory for serious analytical work. 

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.

As Equation 3.28 shows, the diffusion current id is directly proportional to the concentration of the electroactive species. Consequently, polarography has been used extensively for the analysis of solutions containing electroactive materials. The optimum concentration range for determinations by conventional polarography as described here is lO -lO-4 M. Reproducibility is generally 3%. 

Of the diarylarsinic acids only the unsubstituted diphenylarsinic acid has been studied in detail by polarography in buffered aqueous solution" . The reduction is associated with adsorption as indicated by a prewave and a polarographic maximum on the main wave which, however, could be suppressed by addition of Triton X-100" . The height of the reduction wave is directly proportional to the concentration of Ph2 AsO(OH) in the range lO -lQ- M, and the limiting current (ij) is almost entirely diffusion-controlled". Similar conclusions were drawn for a series of bis(aminophenyl)arsinic acids based on measurements of ij as a function of the concentration and the height of the mercury column ... 

The current thus depends on the area of the electrode, on the concentration of the electroactive species, and on its diffusion coefficient. Apart from the difference in the value of the proportionality constant k, the peak current ip shows a dependence on the number of electrons transferred (n) different from that observed in polarog-raphy in DC polarography, the diffusion current is directly proportional to n, whereas in linear-sweep voltammetry the peak current is proportional to Finally, the essential difference between the currents obtained by the two techniques is in the dependence of the peak currents on the rate of scanning, v, which becomes an important variable, whereas polarographic diffusion currents are not dependent on v. 

This is the Ilkovic equation, which was developed by solving the diffusion equations directly very early in the history of polarography. It shows that the diffusion current varies with (Obviously the maximum current will depend on r/, where is the duration of drop life.) It also shows that the current depends on the rate of flow of mercury. However, the quantity m is not very easily determined, although it can be measured if needed. The parameter that can be measured easily is the height of the surface of the mercury in the reservoir (Fig. 12) above the tip of the capillary, h, Poiseuille s equation states that m is proportional to h and that is inversely proportional to h. Thus // is proportional to h. Thus, in order to determine whether a current measured with the d.m.e. is diffusion controlled, all that is necessary is to check whether I is proportional to both and The current actually measured and reported in polarography is not the instantaneous one but an average current... 

Characteristics of diffusion-controlled polarographic waves The magnitude of diffusion-controlled currents are direct functions of depolarizer concentration and in this fact lies the quantitative analytical significance of polarography. All quantitative analysis by the technique is based on the direct proportionality between the limiting diffusion controlled current and depolarizer concentration as expressed in the Ilkovic equation viz.,... 

Zn + in the molten NaSCN-KSCN eutectic by conventional and linear sweep polarography. T1+ showed reversible behavior at sweep rates up to 12 V/sec, whereas Cu+, Cd -, Zn +, and Pb + showed increasing departures from reversible behavior at sweep rates >1.5 V/sec. The diffusion or peak currents were found to be directly proportional to concentration. The standard electrode potentials of Ag+/Ag, Cd +/Cd, Co lCo,... 

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