THE DISSOLUTION PROCESS

Use colligative properties to determine molecular weights of compounds 

Describe how dissociation and ionization of compounds affect colligative properties 

Recognize and describe colloids the Tyndall effect, the adsorption phenomenon, and hydrophilic and hydrophobic colloids 

Sodium reacts with water to form hydrogen gas and a solution of sodium hydroxide. A small amount of phenolphthalein indicator added to this solution turns pink due to the presence of sodium hydroxide. 

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Reaction conditions depend on the composition of the bauxite ore, and particularly on whether it contains primarily gibbsite, Al(OH)2, or boehmite [1318-23-6] AlOOH. The dissolution process is conducted in large, stirred vessels or alternatively in a tubular reactor. The process originated as a batch process, but has been converted to a continuous one, using a series of stirred tank reactors or a tubular reactor. 

Due to high oxidation ability of Ce(IV), special attention is to be paid to retaining its valence state unchanged in the dissolution process of the analyzed samples which contain microquantities of the detenuined component. 

However, it should be mentioned that the dissolution process of a solid, crystalline complex in an (often relatively viscous) ionic liquid can sometimes be slow. This is due to restricted mass transfer and can be speeded up either by increasing the exchange surface (ultrasonic bath) or by reducing the ionic liquid s viscosity. The latter is easily achieved by addition of small amounts of a volatile organic solvent that dissolves both the catalyst complex and the ionic liquid. As soon as the solution is homogeneous, the volatile solvent is then removed in vacuo. 

Since the hydroxyl anion is involved in the mechanism given before, the implication is that other anions may also take part in the dissolution process, and that the effect of various chemicals may be interpreted in the light of the effect of each anion species. Most studies have been in solutions of sulphuric and hydrochloric acids and typically the reaction postulated for active dissolution in the presence of sulphuric acid is ... 

Majima, Awakura, Mashima and Hirato [451] investigated in detail the kinetics of the dissolution process of columbite and tantalite in aqueous solutions of HF, HF - HC1, NH4F - HC1 and NH4F - H2S04, in the 60-80°C temperature range. 

Hydrofluoric acid, at relatively high concentrations and at elevated temperatures, dissolves columbite-tantalite concentrates at a reasonable rate. The dissolution process is based on the fluorination of tantalum, niobium and other metal oxides and their conversion into soluble complex fluoride acids yielding complex fluoride ions. 

The dissolution process of plated lithium may be the reverse of the plating proc-... 

Calcium/magnesium carbonate/hydroxide and calcium phosphate can be removed by using 5 to 15% hydrochloric acid at 140 to 150 °F, by recirculating tetrasodium EDTA at 200 to 300 °F, or by 7 to 10% sulfamic acid at 140 to 150 °F. The temperature may need to be a little higher to start the dissolution process. 

Between 1865 and 1887, Dmitri 1. Mendeleev developed the chemical theory of solutions. According to this theory, the dissolution process is the chemical interaction between the solutes and the solvent. Upon dissolution of salts, dissolved hydrates are formed in the aqueous solution which are analogous to the solid crystal hydrates. In 1889, Mendeleev criticized Arrhenius s theory of electrolytic dissociation. Arrhenius, in turn, did not accept the idea that hydrates exist in solutions. 

Taking all the aspects considered thus far, the dissolution process of an ionic salt MX(s) in plain water categorized as a physical process may be represented in the manner presented below ... 

The dissolution process is associated with an increase in entropy due to the increase in disordering. This increase may be taken to be equal in a series of salts such as the alkali halides. Therefore, the solubility of a salt is approximately proportional to the heat of solution, AHS, which is provided by (given earlier) ... 

To provide a kinetics side of the dissolution process with water, it may be added that the rate equation for the process goes by the following equation ... 

The dissolution process in presence of an alkali, on the other hand, occurs because aluminum hydroxide can behave tike an acid and the overall process in that case may be represented as ... 

Figure 5.3 Galvanic action facilitating the dissolution process ...

The comparison of I —> N and N —> I may also be explained by the buffered pH in the diffusion layer and leads to an interesting comparison between a process under kinetic control versus one under thermodynamic control. Because the bulk solution in process N —> I favors formation of the ionized species, a much larger quantity of drug could be dissolved in the N —> I solvent if the dissolution process were allowed to reach equilibrium. However, the dissolution rate will be controlled by the solubility in the diffusion layer accordingly, faster dissolution of the salt in the buffered diffusion layer (process I—>N) would be expected. In comparing N—>1 and N —> N, or I —> N and I —> I, the pH of the diffusion layer is identical in each set, and the differences in dissolution rate must be explained either by the size of the diffusion layer or by the concentration gradient of drug between the diffusion and the bulk solution. It is probably safe to assume that a diffusion layer at a different pH than that of the bulk solution is thinner than a diffusion layer at the same pH because of the acid-base interaction at the interface. In addition, when the bulk solution is at a different pH than that of the diffusion layer, the bulk solution will act as a sink and Cg can be eliminated from Eqs. (1), (3), and (4). Both a decrease in the h and Cg terms in Eqs. (1), (3), and (4) favor faster dissolution in processes N —> I and I —> N as opposed to N —> N and I —> I, respectively. 

H Sunada, A Yamamoto, A Otsuka, Y Yonezawa. Changes of surface area in the dissolution process of crystalline substances. Chem Pharm Bull 36 2557, 1989. 

When the relative flux is 1, reaction of the basic species with the acid does not contribute to the dissolution of HA since the concentration of B is too low relative to the saturated solubility of HA to impact the dissolution rate (CB < Cha )-As the concentration of the basic species in bulk increases (CB CHa ), the relative flux will approximately double and the reaction of B with HA will begin to dominate the dissolution process. The utility and elegance of Eq. (8) is that one can estimate the increase in dissolution of an acid due to reaction with a base by simply knowing the base concentration in the dissolution media and the... 

Mechanisms of dissolution kinetics of crystals have been intensively studied in the pharmaceutical domain, because the rate of dissolution affects the bioavailability of drug crystals. Many efforts have been made to describe the crystal dissolution behavior. A variety of empirical or semi-empirical models have been used to describe drug dissolution or release from formulations [1-6]. Noyes and Whitney published the first quantitative study of the dissolution process in 1897 [7]. They found that the dissolution process is diffusion controlled and involves no chemical reaction. The Noyes-Whitney equation simply states that the dissolution rate is directly proportional to the difference between the solubility and the solution concentration ... 

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