Diffusion Currents constants

Figure 3.2. Variation of halt-wave potential (Ek) and diffusion current constant (I) for the two polarographic waves of cyclohexenone in aqueous buffers. Data from ref. [84].

This generic term may denote either the measured value of the independent variable under the experimental conditions employed or some function of that value as with the characteristic potential, the nature of the response constant depends on the technique used, and in addition it depends on the behavior of the system being studied. For a diffusion-controlled process the preferred polarogrephic response constant is the diffusion current constant iH/Cm2 3t1 6. but the ratio id/ is often given instead when a value of nj2 3 1 could not be obtained from the or glnal, and even values alone are sometimes quoted for want of anything better values... 

Column 19 identifies the response constant whose value is given in column 20. Units are given in the list of abbreviations a polarographlc diffusion current constant identified by an "I" in column 19 Is always in microamperes-millimole 1- iter-mg"2 3-s1 2 a calculated diffusion coefficient identified by a "D" in column 20 is always 10 times the reported value in cm2-s 1, and so on. 

Belal et al. [50] used an anodic polarographic method for the determination of omeprazole and lansoprazole in pure form and in pharmaceutical dosage forms. The study was carried out in Britton-Robinson buffer over the pH range 4.1—11.5. In Britton-Robinson buffer of pH 7, well-defined anodic waves were produced with diffusion-current constant (ft]) of 1.7 0.01 (n = 6) and 1.66 0.01 (n = 8) for lansoprazole and omeprazole, respectively. The current-concentration plots were rectilinear over the ranges of 4—24, 2—16 /using direct current (DCt) mode for lansoprazole and omeprazole, respectively, and over the range 2—18,... 

E1/2 (volts vs mercury pool anode) -1.44 Id (diffusion current constant) 1.3... 

Table 1. Diffusion-Current Constant, I, for [(1,2,-CjB,H )jNi] in Various Non-aqueous Solvents (The Supporting Electrolyte is 0.1 M n-Bu N PF J at 298 K)...

Polarographic data (half-wave potentials, diffusion current constants, etc.) on several oxazole derivatives have been obtained by Bezuglyi... 

This diffusion current constant, (/)max is independent of the specific values of m, / ax and Cq used in the measurement. Since it depends only on n and Dq, it is a constant of the electroactive substance and the medium in much the same way that molar absorptivity, e, is a constant of the system for optical measurements. Given (/)max for the system at hand, one can evaluate Cq simply by measuring t, and m. No standards are needed. The method is not wholly accurate, because (7.1.7) is itself an approximation. Many workers have reported diffusion current constants and large tabulations exist (21-25). 

Because the DME is now in declining use, diffusion current constants are rarely reported in new literature however the data in older literature remain useful in characteriz-... 

Since the older work was often based on average limiting currents, many reported diffusion current constants are defined from the version of the Ilkovic equation for average currents, which gives ... 

This equation applies to the totally mass-transfer-limited condition at the RDE and predicts that //c is proportional to Cq and One can define the Levich constant as which is the RDE analog of the diffusion current constant or current function in voltammetry or the transition time constant in chronopotentiometry. 

The relative methods mentioned above, based on comparison with standards, are to be preferred over so-called absolute methods where concentrations are calculated using predetermined and tabulated diffusion-current constants or diffusion coefficients, both known to be dependent on experimental conditions. 

The diffusion current constant Ja is used to correct polarographic diffusion currents for differences in capillary characteristics. For average currents... 

The half wave potential (E1/2 versus standard calomel electrode) of the acetone derivative of phenelzine was reported (7) as -1.33V in pH 5.9 phosphate buffer (0.1 M KH2PO1, Na2HPO 2H2O) employing a 0.01% gelatin solution as a maximum suppressor. The diffusion current constant (I ) was approximately 2.5. Four electrons were transferred for the reduction of the hydrazone derivative. Waveheight was proportional to concentration in the range of 2.5 x 10 to 2.5 X 10 M in pH 5.9 phosphate buffer. 

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