Slow precipitation

Total concentration of metal species note that Fe(III) is slightly oversaturated with respect to Fe(OH)3(s) Cu(II) is oversaturated with respect to malachite but because formation is slow, precipitation of the solid has not been allowed. All other metals are thermodynamically soluble at the concentrations specified. 

The clear solution, obtained by centrifuging a solution of the oxide in aqueous ammonia which had been treated with silver nitrate until precipitation started, exploded on two occasions after 10-14 days storage in closed bottles in the dark. This was ascribed to slow precipitation of amorphous silver imide, which is very explosive even when wet [1], When silver oxide is dissolved in ammonia solution, an extremely explosive precipitate (probably Ag3N4) will separate. The explosive behaviour is completely inhibited by presence of colloids or ammonium salts (acetate, carbonate, citrate or oxalate). Substitution of methylamine for ammonia does not give explosive materials [2],... 

Slow precipitation tends to give large crystals with resulting inclusion of colored impurities. 

Finally, even if these criteria are satisfied, there remains the question of whether the product will adhere to form a film or just precipitate homogeneously in the solution. This is the most difficult criterion to answer a priori. The hydroxide and/or oxy groups present on many substrates in aqueous solutions are likely to be quite different in a nonaqueous solvent (depending on whether hydroxide groups are present or not). Another factor that could conceivably explain the general lack of film formation in many organic solvents is the lower Hamaker constant of water compared with many other liquids this means that the interaction between a particle in the solvent and a solid surface will be somewhat more in water than in most other liquids (see Chapter 1, van der Waals forces). From the author s own experience, although slow precipitation can be readily accomplished from nonaqueous solutions, film formation appears to be the exception rather than the rule. The few examples described in the literature are confined to carboxylic acid solvents (see later). 

Details of these and other slow precipitations are given in Ref. 31. 

Probably the least-known aspect of the CD process is what determines the nncle-ation on the substrate. Why do adherent films grow under some conditions and poorly adherent films or even no film at all nnder others, even when slow precipitation occurs in solution In considering this aspect, the two basic mechanisms— hydroxide and ion-by-ion— may behave very differently, althongh there are also featnres in common. When considering nncleation, the anion-mediated mechanisms and the complex-decomposition mechanisms will behave similarly in most cases. Some basic featnres of nncleation will first be considered, followed by issues specific to each. 

Most of the compounds deposited by CD have been sulphides and selenides. Apart from a very few examples of tellurides (and some related teUuride experiments) and with a very few exceptions, discussed at the end of this chapter, what is left is confined to oxides (including hydrated oxides and hydroxides and two examples of basic carbonates.) This chapter deals mainly with these oxides. In addition, as noted in Chapter 3, there are a nnmber of slow precipitations that resnlt in precipitates, rather than films, of varions other componnds, not necessarily semiconductors in the conventional sense. These potential CD reactions, briefly discnssed in Chapter 3, will be somewhat expanded on in this chapter. 

These processes have been described for rapid precipitation reactions. However, they should also be valid in general for slow precipitation—i.e., for CD— with possible differences due to the very different kinetics involved. Thus, if free sulphide is involved, since it is always present in very low concentration, the lower-solubility product metal sulphide is more likely to deposit first, compared to rapid precipitation. Solid-state diffusion processes have much more time to occur in CD (although they may occur in rapid precipitation after the precipitation itself), increasing the probability of solid solution formation. 

If ammonium carbamate be treated with less than its eq. of calcium chloride, it soon gives a precipitate, and the filtered soln. gives no further precipitation when heated. The reaction is the same as when calcium chloride is added in excess. An ammoniacal soln. of calcium chloride gives an instantaneous but slight precipitate when treated with an excess of a soln. of ammonium carbamate. The precipitate increases slowly on standing, and next day, when boiled, the supernatant liquor gives a copious precipitate. The immediate precipitation is presumably due to the presence of ammonium carbonate, and the subsequent precipitation is due to the slow conversion of carbonate to carbamate in the presence of free ammonia. Hence, an ammoniacal soln. of calcium chloride gives an immediate precipitation with an excess of ammonium carbonate and a very slow precipitation with an excess of the carbamate. 

A These downward-growing, icicle-shaped structures called stalactites and the upward-growing columns called stalagmites are formed in limestone caves by the slow precipitation of calcium carbonate from dripping water. 

Glutamic acid has been prepared from the hydrochloride by treating a water solution with strong alkalies,2 alkali carbonates,3 and ammonium hydroxide.4 Hopkins 5 has shown that the addition of 6-8 volumes of alcohol to a concentrated water solution of glutamic acid hydrochloride will cause a slow precipitation of the free amino acid. 

Until velocity coefficients and radical concentrations are known with greater certainty one cannot be sure how closely the true state of affairs is approximated by an algebraic treatment. Further effort to describe these heterogeneous systems by formal kinetics does not appear warranted at present. Progress is more likely to result from detailed investigations into the physical state of these systems. It seems quite possible that polymerization is occurring simultaneously on the particles and, because of slow precipitation, in the liquid phase as well. This would correspond to the situation described in a later section for aqueous polymerization. 

One simple means of causing slow precipitation is to add denaturant to an aqueous solution of protein until the denaturant concentration is just below that required to precipitate the protein. Then water is allowed to evaporate slowly, which gently raises the concentration of both protein and denaturant until precipitation occurs. Whether the protein forms crystals or instead forms a useless amorphous solid depends on many properties of the solution, including protein concentration, temperature, pH, and ionic strength. Finding the exact conditions to produce good crystals of a specific protein often requires many careful trials and is perhaps more art than science. I will examine crystallization methods in Chapter 3. 

Crystals of an inorganic substance can often be grown by making a hot, saturated solution of the substance and then slowly cooling it. Polar organic compounds can sometimes be crystallized by similar procedures or by slow precipitation from aqueous solutions by addition of organic solvents. If you work with proteins, just the mention of these conditions probably makes you... 

Fast precipation generally gives a light brown, fine-grained powder, while slow precipitation gives a dark brown, glasslike, brittle product. Some authors have found two products at the same time, for example Emons and coworkers found both a yellow and a brown compound in different parts of the apparatus used. 

J This compound is a colloidal hydroxyapatite, containing about 5% water, which is formed by slow precipitation from calcium chloride and disodium hydrogen orthophosphate solutions at 100° and pH of 9 to 10. 

Boron trifluoride etherate (2.5 equiv. with respect to the alkenylsilane) was added dropwise to a stirred suspension of the appropriate alkenylsilane and iodosylbenzene (2.5 equivalent with respect to the alkenylsilane), in dichloromethane, under nitrogen. A yellow colour developed, while the mixture was stirred for 15 min to 7 h, at 0°C or room temperature, according to the substrate. A saturated aqueous sodium tetrafluoroborate solution was added and the mixture was stirred vigorously for 0.5 h. The reaction mixture was poured into water and extracted with dichloromethane. The organic layer was concentrated to give an oil which was washed several times with hexane and/or ether by decantation. Further purification was accomplished by repeated dissolution of the salt in dichloromethane or ethyl acetate followed by slow precipitation with hexane or ether. 

After activation of Li with a small amount of CH3I, the reaction occurs exothermically with slow precipitation of LiH. 

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