Ilkovic constant, equation

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. 

The original IlkoviC equation neglects the effect on the diffusion current of the curvature of the mercury surface. This may be allowed for by multiplying the right-hand side of the equation by (1 + ADl/2 t1/6 m 1/3), where A is a constant and has a value of 39. The correction is not large (the expression in parentheses usually has a value between 1.05 and 1.15) and account need only be taken of it in very accurate work. 

Resolving this equation for c at the electrode surface, i.e., at x = 0, Ilkovic assumed the following initial and final conditions c — C for t — 0 and c = c0, the constant value obtained at t > 0, dependent on the electrode potential thus he obtained the expression... 

The derivation of the Ilkovic equation assumes that the drops of mercury are spherical the rate of flow of mercury is constant, the way analyte diffuses toward the DME obeys the Cottrell equation, and the diflusion current is truly limiting, i.e. that Csurfacc = 0-... 

In practice, this pseudo-mathematical expression, known as the Ilkovic equation, which takes into account several factors, is replaced by the simplified formula below where K is a graphical constant that includes several parameters related to the method and the instrument used. 

Both m and t in the Ilkovic equation [Eq. (74)] depend on the height of the mercury reservoir h, and since m is equal to c h and t is equal to c"/h, where c and c" are constants, we have the relationship between id and h given by Eq. (82). Thus the magnitude of a purely diffusion-controlled polarographic wave is proportional to the square root of the height of the mercury reservoir. 

If the average diffusion current is measured instead of the maximum, the constant 708 in the Ilkovic equation becomes 607 because = 6/7... 

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 ... 

For a given electroactive substance under a given set of experimental conditions (temperature, supporting electrolyte, potential of the DME, etc.), should actually be a constant according to the Ilkovic equation it should be independent of the capillary characteristics and reproducible in different laboratories or in the same laboratory with different capillaries. Cadmium ion exhibits a reversible two-electron reduction wave at -0.64 V in 1 M HCl. A 0.50 mM Cd + solution gave a wave with average id = 3.96 fiA at the En the capillary characteristics were m = 2.50 mg/sec, t = 3.02 sec. (a) Calculate h for Cd +. (b) Calculate the diffusion coefficient for Cd in 1 M HCl. 

Note that the Ilkovic equation contains a term m, the mercury flowrate, which reflects the growth of the drop during the measurement process. In contrast the Parry-Osteryoung equation contains a term A for the surface area of the electrode as if this were a constant. A is for practical purposes a constant since the growth of the drop is negligible during the very short pulse. 

This is called the Ilkovic equation. For a particular capillary and pressure head of mercury, is a constant. Also, the value of n and that of the diffusion coefficient for... 

What is the upper limit of molecular weight and concentration at which the Ilkovic equation can be applied for a direct determination of diffusion constants of linear macromolecules or their electron uptake ... 

This is called the Ilkovic equation. For a particular capillary and pressure head of mercury, is a constant. Also, the value of n and that of the diffusion coefficient for a particular species and solvent conditions are constants. Thus, (l is proportional to the concentration C of the electroactive species, and this is the basis for quantitative analysis. The Ilkovic equation is accurate in practice to within several percent, and routinely 1% precision is possible. It is commonplace to use standard additions to obtain a calibration curve or an internal standard. Internal standards are useful when chemical sampling and preparation procedures involve the possibility of losses. The principle is that the ratio of the diffusion currents due to the sample and the added standard should be a constant for a particular electrolyte. 

Ilkovic equation), S denotes the drop surface at the time t. The relationships for i dpp b found in [120]. The shapes of simulated curves for different forward rate constants are illustrated in Fig. 33. As may be expected, the peak currents increase in increasing kf at constant... 

Here, n is the number of electrons exchanged in the electrode reaction, D is the diffusion coefficient of the depolarizer (cm s ), m is the rate of mass flow of the mercury (mg s ), t is the drop time (s) and C is the depolarizer concentration. With the last quantity expressed as millimole litre", and the numerical constant 706, the maximum current during the drop-life is given in microamps. If, as is more usual, mean currents are measured, the numerical constant becomes 607, and the practical form of the Ilkovic equation becomes... 

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