Retardants of Dissolution

As discussed earlier there has been much study of the effect of impurities, especially aluminum, on the solubility of silica, but only limited research on the effect on the rate of dissolution. 

The rate of dissolution of fused silica from a flask was measured by Sasaki (228) the rate was proportional to the base concentration. Zinc and aluminum both greatly diminished the rate of solution. Complete inhibition in 0.1 N NaOH was observed at concentrations of 3.4 x 10 N aluminum and 5 x 10 N zinc. 

Hudson and Bacon (229a) examined inhibitors to prevent the attack of 3% NaOH solution on glass at 125 F. By far the best was beryllium, which almost stopped attack at a concentration of 10 mg atoms per kilogram of solution (about 2 g l BeS04 4H20). Catechol, then zinc, were next best at this concentration. Surprisingly, aluminum in this strong alkali was little better than barium, lead, and other heavy metals, none of which were very effective because they were precipitated from solution. 

Porous glass membranes used for hyperfiltration were stabilized against dissolution by having 0.3 g l of AlClj-6HjO in the feed solution (229b). 

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Turning now to the acidic situation, a report on the electrochemical behaviour of platinum exposed to 0-1m sodium bicarbonate containing oxygen up to 3970 kPa and at temperatures of 162 and 238°C is available. Anodic and cathodic polarisation curves and Tafel slopes are presented whilst limiting current densities, exchange current densities and reversible electrode potentials are tabulated. In weak acid and neutral solutions containing chloride ions, the passivity of platinum is always associated with the presence of adsorbed oxygen or oxide layer on the surface In concentrated hydrochloric acid solutions, the possible retardation of dissolution is more likely because of an adsorbed layer of atomic chlorine ... 

Manikandan, R. Studies on the Retardation of Dissolution of Gelatin Containing Formulations on Aging. M. Pharm. (Formulation) Thesis, NIPER, S.A.S. Nagar, India, 1999.. 

Acceleration of dissolution by lower silicon concentration and retarding of dissolution by higher silicon concentration [54]. 

In tenns of an electrochemical treatment, passivation of a surface represents a significant deviation from ideal electrode behaviour. As mentioned above, for a metal immersed in an electrolyte, the conditions can be such as predicted by the Pourbaix diagram that fonnation of a second-phase film—usually an insoluble surface oxide film—is favoured compared with dissolution (solvation) of the oxidized anion. Depending on the quality of the oxide film, the fonnation of a surface layer can retard further dissolution and virtually stop it after some time. Such surface layers are called passive films. This type of film provides the comparably high chemical stability of many important constmction materials such as aluminium or stainless steels. 

Copper anodes for use in acid copper plating solutions preferably contain a small amount of phosphoms [7723-14-0] usually 0.03—0.04 wt %, which retards chemical dissolution of the copper and thus the subsequent copper build-up. Typically, acid copper plating solutions increase in copper and require periodic dilution. Additionally, additives for brightening acid copper baths tend to last longer in plating tanks using phosphorized copper anodes. In cyanide copper solutions, phosphorized copper anodes should not be used. 

Thus inhibitive anions can retard the dissolution of both the T-FejO, and the magnetite layers of the passivating oxide layer on iron. This has the dual effect of preventing breakdown of an existing oxide film and also of facilitating the formation of a passivating oxide film on an active iron surface, as discussed in the previous section. 

Although it presents an obstacle in practical applications, the photoanodic corrosion of colloids has often been used to obtain information about the interaction of dissolved compounds with the photo-produced charge carriers, as it was found that solutes can influence the rate of the dissolution. Both promoting and retarding effects were observed The rate of dissolution is readily followed by recording the decrease in the intensity of the absorption spectrum of the colloid upon illumination, or more precisely, by determining the yields of metal and sulfate ions in solution. 

It is found that the dissolution of zinc sulfides occurs more rapidly when they are in contact with copper sulfide or iron sulfide than when the sulfides of these types are absent. This enhancement is brought about by the formation of a galvanic cell. When two sulfide minerals are in contact, the condition for dissolution in acidic medium of one of the sulfides is that it should be anodic to the other sulfide in contact. This is illustrated schematically in Figure 5.3 (A). Thus, pyrite behaves cathodically towards several other sulfide minerals such as zinc sulfide, lead sulfide and copper sulfide. Consequently, pyrite enhances the dissolution of the other sulfide minerals while these minerals themselves understandably retard the dissolution of pyrite. This explains generally the different leaching behavior of an ore from different locations. The ore may have different mineralogical composition. A particle of sphalerite (ZnS) in contact with a pyrite particle in an aerated acid solution is the right system combination for the sphalerite to dissolve anodically. The situation is presented below ... 

As shown by Grandstaff (1976) the cations, thorium, lead and the rare earths, associated with uraninite retard the dissolution of UO2 significantly. 

It is interesting to note that many crystal poisons not only interfer with nucleation and the growth of crystals but may also retard their dissolution. As we have seen (Chapter 6), precipitation and dissolution of solids proceed by the attachment or detachment of ions most favorably at kink sites of the crystalline surface. Solutes such as organic substances, or phosphates may upon adsorption immobilize kinks and thus retard dissolution. 

Insulin suspensions. When the hormone is injected as a suspension of insulin-containing particles, its dissolution and release in subcutaneous tissue are retarded (rapid, intermediate, and slow insulins). Suitable particles can be obtained by precipitation of apolar, poorly water-soluble complexes consisting of anionic insulin and cationic partners, e.g the polycationic protein protamine or the compound aminoqui-nuride (Surfen). In the presence of zinc and acetate ions, insulin crystallizes crystal size determines the rate of dissolution. Intermediate insulin preparations (NPH or isophane, lente or zinc insulin) act for 18 to 26 h, slow preparations (protamine zinc insulin, ultralente or extended zinc insulin) for up to 36 h. 

Arsenious oxide exhibits considerable catalytic activity, which may act either positively or negatively. The effect on many oxidation and reduction reactions has been mentioned above. Other examples are the increase in the rate of dissolution of zinc by dilute acids and the retardation of the dissolution of marble. In the latter case a concentration of 0-005N As303 reduces the velocity constant for the dissolution of marble in 0-1N HC1 by 12 per cent.5... 

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