Rotating disc method

The drug dissolution rate could be determined by dispersing the powder in a test medium under suitable agitation or by studying the dissolution for a constant surface area by using the rotating-disc method (Fig. 21.6). The latter method should be the technique of choice, except when studies of the effect of particle size are of... 

Fig. 21.6. Schematic diagram and photograph of the rotating-disc method.

While batch dissolution methods are simple to set up and to operate, are widely used, and may be carefully and reproducibly standardized, they suffer from the following disadvantages (1) the hydrodynamics are usually poorly characterized, with the notable exception of the rotating disc method, (2) a small change in dissolution rate will often create an undetectable and therefore an immeasurable perturbation in the dissolution time curve, and (3) the solute concentration cb may not be uniform throughout the solution volume V. 

Fig. 16 Lines of flow of liquid in the Levich rotating disc method of determining dissolution rates. There is a transition from (A) flow essentially normal to the surface to (B) flow parallel with the surface, pointing to the existence of a viscous boundary layer. (Reproduced with permission of the copyright owner, the Royal Society of Chemistry, from Ref. 101.)...

A reciprocal proportionality exists between the square root of the characteristic flow rate, t/A, and the thickness of the effective hydrodynamic boundary layer, <5Hl- Moreover, f)HL depends on the diffusion coefficient D, characteristic length L, and kinematic viscosity v of the fluid. Based on Levich s convective diffusion theory the combination model ( Kombi-nations-Modell ) was derived to describe the dissolution of particles and solid formulations exposed to agitated systems [(10), Chapter 5.2]. In contrast to the rotating disc method, the combination model is intended to serve as an approximation describing the dissolution in hydrodynamic systems where the solid solvendum is not necessarily fixed but is likely to move within the dissolution medium. Introducing the term... 

Fig. 5.3. Rapid-quenching device for investigating the process of dissolution of a solid in a liquid by the rotating disc method.309 1, electric-resistance furnace 2, solid specimen 3, protective tube 4, electric motor 5, rotating shaft 6, stopper 7, thermocouple 8, flux 9, liquid.

Table 5.9 characterise the diffusion in saturated solutions rather than in the pure solvent. It must therefore be quite clear that the rotating disc method cannot be employed to find out the concentration dependence of the diffusion coefficient of A in B. 

There are many classical methods to investigate the chemical reaction and kinetics at the liquid-liquid interface, which include a Lewis cell, a single drop method, and rotating disc method [22]. All of these methods however could not measure both the extraction rate and interfacial concentration of extractant, simultaneously. Modern experimental methods of interfacial reaction can determine the interfacial species, interfacial concentration, and interfacial chirality of an extractant or complex as a function of time. 

O ) polarography uses a dropping mercury cathode and the current is measured 00 the rotating-disc method uses a spinning platinum disc as the cathode and the current in the steady state situation at a series of rotation speeds is measured Hi) the potentiostatic method uses fixed electrodes and the fall of current with time is measured. 

Figure 12.1 Six configurations of simple dissolution test apparatus (a) the Wruble method in which the dose form is rotated in a container of solvent (b) a simple beaker method (c) an apparatus in which the dosage form is in a cradle which oscillates vertically (d) the rotating disc method to determine the intrinsic dissolution characteristics of a compressed powder (e) method in which the dosage form is placed in a medium which is shaken and (f) a method in which the decreasing size of suspensions is measured on dissolution.

The intrinsic dissolution rates were determined using the rotating disc method, which normally needs around 100 mg of compound. 

Figure 4.2b The rotating disc method experimental set-up for the rotating disc.

Size- and shape-controlled cubo-octahedral platinum(o) nanoparticles of 4nm average size stabilized by sodium polyacrylate showing 111 and 100 surfaces were used to prepare Vulcan supported electrocatalysts. Cyclic voltammetric CO oxidation studies carried out by the thin film rotating disc method show two different sites of CO oxidation (Fig. 2.19). This can be assigned to differences in the activity of the crystal surfaces and is in agreement with single crystal studies. TEM results after cyclic voltammetric characterization show a complete absence of agglomerations. 

The rotating disc method may be used to study the separate roles of diffusion and integration in crystal growth since it enables the mass transfer (diffusion) step to be isolated. A uniform hydrodynamic boundary layer of thickness... 

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