Rotating-disk method

The intrinsic dissolution rate is usually evaluated by using a rotated disk method (Fig. 7). The pure powdered solute is compressed in a die under high pressure, in the absence of any excipients. The resulting nondisintegrating disk is then transferred to a dissolution cell which has sufficient volume to maintain sink conditions. The die is rotated at a certain speed, and the rate of drug dissolution is then measured. 

An alternative to the rotating disk method in a quiescent fluid is a stationary disk placed in a rotating fluid. This method, like the rotating disk, is based on fluid mechanics principles and has been studied using benzoic acid dissolving into water [30], Khoury et al. [31] applied the stationary disk method to the study of the mass transport of steroids into dilute polymer solutions. Since this method assumes that the rotating fluid near the disk obeys solid body rotation, the stirring device and the distance of the stirrer from the disk become important considerations when it is used. A similar device was developed by Braun and Parrott [32], who used stationary spherical tablets in a stirred liquid to study the effect of various parameters on the mass transport of benzoic acid. 

Rotating disk method (free boundary method) Diffusion coefficient determination from solids mass transport studies 25-29... 

The release of timolol from isopropyl PVM-MA matrices used the rotating disk method [5]. The dissolution medium was 100 ml of pH 7.42 mM phosphate buffer at 32°C. Ionic strength was adjusted to 0.5 with sodium chloride. Samples of volume 3. 0 ml were withdrawn and replaced by dissolution medium. Timolol concentrations were analysed with a UV spectrophotometer at 294 nm and the pH was measured. 

The dissolution rate is directly proportional to the equilibrium solubility if the appropriate experimental conditions such as the ones used for intrinsic dissolution rate measurements are selected. The rotating-disk method is the most useful and most widely used technique for measuring intrinsic dissolution rates. The theoretical considerations and experimental details of this method will be considered later in this chapter in the discussion dealing with dissolution. 

M.-C. Aubert, M.-P. Elluard and H. Barnier, Shear stress induced erosion of filrtration cake studied by a flat rotating disk method. Determination of the critical shear stress of erosion. /. Membr. Sci., 84 (1993) 229. 

Figure 19.6 A schematic drawing and photo of the rotating-disk method.

Barton, A. F. M., and N. M, Wilde. 1971. Dissolution rates of polyctystalline samples of gypsum and orthorhombic forms of CaS04 by a rotating disk method. Trans. Farad. Soc. 67 3590-97. 

In the rotating-disk method the thickness of the diffusion and reaction layers at the electrode surface is made less than at the dropping mercury electrode surface (Orsega et al., 1982). This reduces the extent of 07... 

There are many elassieal methods for investigating extraetion kineties, whieh include a Lewis cell, a single-drop method, and a rotating-disk method [11]. None of these methods, however, could be used to measure the extraction rate and interfacial concentration of extractant simultaneously. A modern experimental method for interfacial reactions has to be able to determine the interfacial concentration of an extractant or complex as a function of time through the extraction. 

The dissolution rate of amorphous silica in 0.01 /V NaOH solution has been measured at 23-88 C by a rotating disk method by Anatskii and Ratinov (221, 222). 

De Keyser and Wollast (223a) have been able to use the constant of the rate of dissolution of a solid, obtained by the rotating disk method, to calculate the rate of solution of powders of which the particle size has been measured by the cumulative sedimentation method. 

A rotating disk method has also been used to study effects of high-velocity corrosion, and the potential for damage from erosion mechanisms [83], A rotating dissolution ceD has also been used for measurements of corrosion in liquid mercury [84]. 

In addition to the two categories of catalyst described above, Ross et al. [11] developed a non-platinum based alloy, Au-Pd/C, which showed promising CO tolerance. They reported a three-fold improvement in electrooxidation of CO/H2 with their catalyst as compared to a PtRu catalyst. Schmidt et al. [12] also prepared a PdAu/C electrocatalyst, and the CO/H2 oxidation measurements made by the thin-film rotating disk method showed the superior activity of PdAu as compared with a PtRu catalyst. However, further no further reports on that catalyst-based MEA are available. 

Dissolution rate studies of cholesterol monohydrate in bile acid-lecithin solutions using the rotating-disk method, J.Pharm.Sci., 65 685 (1976). 

Jahn, D. Vielstich, W. (1962). Rates of electrode processes by the rotating disk method. Journal of Electrochemical Society 109 849-852. 

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