الفهرس 
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Abstract
In the present work, the mechanism and the rates of dissolution of COM crystals in aqueous solution were investigated in the absence and the presence of’ some additives. The thesis consists of three chapters:- 1- The first chapter contains general introduction. The nucleation, crystal growth and cluster formation as well as crystal dissolution and reaction mechanism. Inhibition and effect of additives on the rates of dissolution and crystallization of calcium oxalate and some sparingly soluble salts were discussed. 2- 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. 3- 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 the absence of additives:
The dissolution of calcium oxalate crystals was carried out at different relative degree of undersaturation and different amounts of seeds under experimental conditions of t = 37°C, ionic strength = 0.15 mol dm-3 and stirring rate 200 r.p.m using pH-meteric method. Constant composition method was applied. The process appear to follow normal surface controlled-mechanism. The order of reaction n2, which suggest surface-controlled mechanism. The suggestion of predominantly surface-controlled mechanism was supported by the observed low value of the activation energy and the independence of the rates of dissolution of COM crystals on the solution dynamics.
B) The influence of additives:- The dissolution mechanism of COM crystals was studied in the presence of Hydrochlorothiazide (Hz) and Furosemide (FM) as additives. from the present work it was found that :- 1- Hz and FM are good inhibitor for dissolution of COM crystals at the same experimental conditions. . 2- Increasing the concentration of additives decreased the rates of dissolution of COM crystals . 3- The order of inhibition of additives was: FM < Hz . 4- On studying the effect of experimental conditions on the mechanism of dissolution of COM crystals in the presence of Hz was studied. from which it was found that the increasing of ionic strength of the medium decreased the inhibitory effect of Hz . The change of pH also affects on the degree of inhibition of COM crystals. It was investigated that the degree of inhibition of Hz decreased by increasing the pH of the medium till pH = 8 and after this value of pH, the inhibition increased by increasing the pH value . The order of dissolution rates of COM crystals in the presence of Hz was also equal 2 Which suggests the physical adsorption of Hz molecules on the active sites of COM crystals. Since the ionic strength affected on the dissolution rates so, the type of interaction between Hz molecules and surface of COM crystals surface was electrical in its nature, in which the negative parts of Hz molecules adsorbed on Ca2+ ions active sites . 5- For comparison, the drug of FM called lasix was used at the same experimental conditions. It was found that the order of Inhibition of COM crystals was FM < lasix.
Langmuir-isotherm can be applied, which confirms the suggestion of surface-controlled mechanism . The values of (KL) were: 5.263×106 and 5.6 × 105 dm3 mol-1 in case of the presence of FM and Hz, respectively. 7- ΔG values were: – 44.3 and – 49 kJ / mole in the presence of Hz and FM, respectively. 8- At concentration of 8 × 10-7 mol dm-3 of Hz inhibition of COM crystals was completed at which only 6.7% of the surface covered by Hz molecules which investigate at the small number of dissolution active site on COM crystals and ruled out monolayer adsorption of Hz molecules and supported the surface-controlled mechanism. 9- Kads value in case of the presence of Hz and FM were 5.28×105 and 3.3×10 6 respectively which suggested that they are good inhibitors . 10- The seed of COM collected from experiments of dissolution using 10-7 mol dm-3 dried at 40°C and exposed to IR,SEM and XRD analyses . from which physical adsorption of Hz molecules on the surface of COM crystals was confirmed . C) Concluding Remarks.