الفهرس 
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Abstract
In the present work,, the mechanism and the rate of dissolution of COM crystals in aqueous solution were investigated in absence and presence of’ some additives. The thesis consists of three chapters. The first chapter contains a general introduction. The nucleation, crystal growth and cluster formation as well as crystal dissolution and reaction mechanism. Inhibition and effect of additives were discussed. ’The. effect of the presence of some cations and anions on the rates of dissolution and crystallisation of calcium oxalate and some sparingly soluble salts were discussed. The-second chapter includes the experimental part concerned with the preparation of solutions, preparation of COM seeds crystals, measurements of surface area, X-ray diffraction and the dissolution procedures. The third chapter is devoted to the results and their discussion. It is subdivided into the main sections. A , B and C : A) Studying the dissolution of COM crystals in absence of additives: The dissolution of calcium oxalate crystals was carried out at different relative degree of undersaturation (σ = 0.04 - 0.15) and different amounts of seeds under all experimental conditions of t= 37°C, ionic strength = 0.15 mol dm-3 and stirring rate 200 r.p.m using potentiometric method. Constant composition method was used. The process appear to follow normal surface controlled mechanism. The suggestion of predominantly surface-controlled mechanism was supported by the observed low value of the activation energy and independence of the rates of dissolution of COM crystals by solution dynamics. B) The influence of additives It was noticed that the presence of some organic solvents retard the rate of dissolution of COM crystals. It can be seen from the data obtained that concentrations as low as 10-8 mol dm-3, For each additive markedly reduced the dissolution rates. As the concentration of additive molecule increases, the active sites on the crystal surface are blocked through adsorption and rates of dissolution decreases. The values of adsorption affinity constants reflected the high adsorption at the same value of relative undersaturation in the order: PA> Acetone > DMSO> IPA> DMF In order to investigate the adsorption of some organic solvents and the nature of interaction between the additive and COM surface, plotting Langmuir isotherm. The validity of application of Longmuir isotherm rulled out adsorption of additive on the surface of COM crystals. The values of affinity constant , KL, in the presence of PA , acetone , DMSO , IPA ,and DMF were found: 6.34, 3.42, 1.16, 1.07 and 0.33 X 107 dm3 mol-1 which pointed to surface controlled mechanism. C) The effect of change of ionic strength the, pH, σ and temperature on the mechanism of dissolution of COM crystals in the presence of DMSO (as an example of the additives) are studied. It was found that pH values had no influence on the rates, but increasing ionic strength decrease the inhibitory of DMSO. The order was found equal (2) in case of the presence of DMSO and (Ea) was found had small value indicating surface- controlled mechanism.