Formation of Insoluble Substances

When anodic oxidation of a metal is followed by a chemical reaction with a component of the solution, insoluble substances are often formed. Considerable attention has been devoted to all the phases of such a process, since this involves a number of electrochemical systems of great practical importance, as, e.g., the dissolution of lead to form PbS04, the formation of calomel in reference electrodes, etc. The formation of insoluble oxides can also be a cause of passivity. 

Electrocrystallization of a nonmetallic phase can be a secondary effect, arising when saturation of the electrolyte by the product of the electrode reaction is reached. It can, however, also result from direct formation of the product by a solid state reaction. 

A detailed study of each particular system is needed to distinguish between the two mechanisms. However, as far as the kinetics of the process is concerned, the dissolution-precipitation mechanism could be treated along the same lines of reasoning as used in the previous parts of this chapter, while a direct formation of insoluble product has a number of specific features which are treated elsewhere (Volume IV, Chapter 10, Volume VIII, Chapters 3 and 4) and are outside the scope of this review. 

A good example of the dissolution-precipitation mechanism is found in the case of formation of calomel by anodic dissolution of mercury in chloride solutions. It has been realized that the low-solubility product of calomel in aqueous solutions gives far too low a concentration of mercurous ions in 

A generalization seems to be justified based on this and other similar experience with systems with insoluble reaction products not leading to passivation and those leading to fast or practically instantaneous passivation  

---

Ozonolysis produces peroxide intermediates that pose a potential explosion hazard. Formation of insoluble substances on the wall of the reaction vessel during ozonolysis points to solid peroxides and special care must be taken during workup. The solvent chosen should be able to dissolve not only the starting material, but also the ozonide and any peroxide substance formed. 

The problem of estimating aerosol transport is thus focused on the study of watershed media effects on the transport of substances in solution rather than in insoluble aerosol particles. Thus, sorption by watershed media is a prime factor in determining the hydrologic transport of soluble radioaerosols, such as cesium and strontium. Removal of soluble radioaerosol elements from runoff by formation of insoluble compounds is considered unlikely. 

Antifoams can be divided into two groups. The first group includes substances, for which the principle of defoaming action is based on the interaction with the foaming agent resulting in formation of insoluble or poorly soluble compounds. For instance, if soluble calcium or aluminium salts (chlorides) are added to the foaming solution of sodium or potassium salts of fatty acids, or cationic surfactants to anionic surfactants solution, insoluble compounds are formed and the foam is destroyed [6]. The less soluble compound formed, the more efficient the antifoam is. 

The quantity of insoluble substance which can be solubilised in micelles depends to a considerable extent on the chemical structure of the surfactant and is influenced by the presence of other components, which may influence either the micelle formation concentration (CMC) or the micelle geometry (aggregation number, shape). The transition from solubilisation to another important phenomenon, the formation of a micro-emulsions, is continuous. Microemulsions form spontaneously, whereas typical solubilisation systems attain their equilibrium state often only after extreme long periods of intensive mixing of both phases. 

On the other hand, favoring low absorption are observations such as the following (1) food and liquid mixed with the toxic substance not only provide dilution but also reduce absorption because of the formation of insoluble material resulting from the combinatory action of substances commonly contained in such food and liquid (2) there is a certain selectivity in absorption through the intestine that tends to prevent absorption of unnatural substances or to limit the amount absorbed and (3) following absorption into the blood stream the toxic material goes directly to the hver, which metabolically alters, degrades, and detoxifies most substances. 

Amphiphilic Components. This category includes a variety of substances that are activated by either temperature increases that create insolubility near the cloud point (low HLB nonionic ethoxylates, fatty acids, fatty amines) or in situ formation of insoluble calcium salts [22, 23],... 

An active substance, although initially released from its dosage form (and dissolved), may become unavailable for absorption due to reactimis with other medicines or food components [4]. An example is the formation of insoluble complexes of tetracycline with calcium or aluminium ions from antacids or milk products. Interaction (chelation or binding) with iron ions leads to a reduced absorption for a variety of active substances such as doxycycline, penicillamine, methyldopa and ciprofloxacin. The absorption of active substances showing pH-dependent dissolution behaviour may be influenced by medicines that influence the gastric pH, such as H2-antagonists, proton pump inhibitors and antacids. Antimycotic active substances such as ketoconazole or itraconazole dissolve better in acidic fluids. Therefore their bioavailability may be increased by the concomitant use of an acidic drink like cola, whereas the concomitant use of antacids or proton pump inhibitors is likely to reduce the bioavailability. Concomitant use of milk may increase the dissolution of acidic active substances, whereas fats from food may increase the bioavailability of lipophilic active substances like albendazole and griseofulvin. 

In small-scale preparation the quaternary ammonium compounds containing an alkyl chain are the most important. Cetrimide (Cetrimonium bromide, see Fig. 23.8) is used in a concentration of 0.5-2 % as emulsifier in creams. Quaternary ammonium compounds of this type are used from a concentration of 0.004 %. They also exhibit antiseptic and preservative properties. Notably benzalkonium chloride, as well as being a surfactant of the oil-in-water emulsifying type, is also important as a preservative in for example eye and nose drops. Catioiuc-active compounds are often incompatible with anionic substances such as sulfobituminose ammonium due to the risk of the formation of insoluble ion pairs. Since it is often difficult to estimate this risk, it is better to avoid these combinations. 

Reaction 1, as well as reaction 2, is enzymatically catalyzed, since no reaction between o-quinone and monophenol occurs in the absence of enzyme. The o-quinones are net end products of enzymatic action but undergo further oxidations and condensations with the ultimate formation of insoluble polymers like melanin. Among the phenolic substances oxidized by these enzymes are some which occur naturally, such as tyrosine dopa (II),and urushiol (III), a phenolic substance which occurs in plants of the genus Rhus. 

Phenolic substances of tea are probably the most powerful fartor that reduces iron resorption. Even in the presence of ascorbic acid, the resorption of iron is diminished due to the formation of insoluble complexes with tannins. Non-haem iron resorption is reduced by up to 62 and 35% when food is administered simultaneously with tea or coffee, respectively. In contrast, orange juice increases the resorption of iron by up to 85%. 

Very weak and very strong mineral acids are less aggressive. Different concentrations initiate different conversions. In addition, complexing and the formation of insoluble salts on the glass surface create substances that may act as diffusion barriers. In terms of reaction kinetics, the ion substitution reaction, which causes acid corrosion of individual fibers is determined by a diffusion process ( /f-kinetics). 

---