Hydrolysis precipitation equilibria

The preparation, properties, and usefulness of copolymers of vinyl pyrrolidone and acrylamide will be discussed in this paper. Although neither homopolymer appears economically viable for large scale use in hostile oil recovery environments (hard brines at temperatures in excess of 170 F), the various copolymers were found to produce solutions which resist precipitation on aging. The inclusion of vinyl pyrrolidone in copolymers of acrylamide apparently protects and limits the equilibrium degree of hydrolysis. The equilibrium level is less that the total available acrylamide, and it is related to the amount of vinyl pyrrolidone in the copolymer. The importance of these findings will also be discussed. 

Equations (141) and (142) describe the equilibrium between the hydrolysis of complex fluoride acids (shift to the right) and the fluorination of hydroxides (shift to the left). Near complete precipitation of hydroxides can be achieved by applying an excessive amount of ammonia. Typically, precipitation is performed by adding ammonia solution up to pH = 8-9. However, the precipitate that separates from the mother solution can be contaminated with as much as 20% wt. fluorine [490]. Analysis of niobium hydroxides obtained under different precipitation conditions showed that the most important parameter affecting the fluorine content of the resultant hydroxide is the amount of ammonia added [490]. Sheka et al. [491] found that increasing the pH to 9.6 toward the end of the precipitation process leads to a significant reduction in fluorine content of the niobium hydroxide. 

The amount of precipitated bismuth decreased as the concentration of bismuth salt increased (Table 9.16) and the duration of sonication required to bring about hydrolysis also increased. The initial reaction was spontaneous as per Eq. (9.111), which, however, seemed to be facilitated by ultrasonic cavitation at high concentration of bismuth. Since the H+ ions were also produced during the formation of bismuthyl ion, at the point where the sum of concentration of H+ ions present initially and formed by Eq. (9.110) was equal to the concentration required to shift the equilibrium of Eq. (9.111) towards left side, the hydrolysis did not occur even after sonication. 

The fluoride ion is the only inorganic ligand to form a complete substitution series, Be(H20)4 flFJ(2 1 (n = 1-4), though there is considerable variation in the equilibrium constants that have been reported. The most reliable values are probably those of Anttila et al. (117) who used both glass and fluoride-ion selective electrodes and also took account of the competing hydrolysis reactions. They did not, however, make measurements in the conditions where BeF2 would have been formed. A speciation diagram based on reported equilibrium constants is shown in Fig. 12. It can be seen that the fluoride ion competed effectively with hydroxide at pH values up to 8, when Be(OH)2 precipitates. 

Equilibrium composition of solutions in contact with freshly precipitated, Al(OH)3 and Fe(OH)3, calculated, using representative values for the equilibrium constants for solubility and hydrolysis equilibria. Shaded areas are approximate operating regions in water treatment practice coagulation in these systems occurs under conditions of oversaturation with respect to the metal hydroxide. 

Therefore, when an anhydrous aluminium salt is dissolved in water initially, the octahedral ion [A1(H20)6]3 + is formed by hydration of the Al3 ion. However, since some hydrolysis occurs, the solution will contain H30+ and be acidic. Addition of any molecule or ion which removes H30+ for example alkali, or even sodium carbonate will cause the equilibrium to be displaced to the right and hydrated aluminium hydroxide is precipitated. 

This appears not to be one specific compound or mixture, but is of variable composition, depending on the method of preparation. Prepared from gold chloride and aqueous ammonia, the explosive precipitate is largely (C1AuNH2)2NH, but on washing with ammonia hydrolysis to the more explosive (HOAuNH2)2NH occurs, and the equilibrium is reversed by washing with chloride. 

A review of iron(III) in aqueous solution covers hydrolysis and polymerization, the formation and dissociation of binuclear species, and kinetics and mechanisms of water exchange and complex formation. " Kinetic and equilibrium data for hydrolytic reactions of iron(III) have been conveniently assembled. Reviews of hydrolysis of Fe aq, and subsequent precipitation of hydrated oxide-hydroxide species, cover a very wide range of media, from geochemistry to biology to human metabolism. Added anions or pH variation can affect which form... 

Metastability of Hydrolyzed Iron (III) Solutions The low solubility of ferric hydroxide has been alluded to in the Introduction. Feitknecht and Michaelis (29) have observed that aU ferric perchlorate solutions to which base has been added are unstable with respect to eventual precipitation of various forms of hydrated ferric oxides. In 3 M NaC104 at 25° C the two phase system reaches an apparent equilibrium after 200 hours, according to Biedermann and Schindler (6), who obtained a reproducible solubility product constant for ferric hydroxide at varying degrees of hydrolysis. It appears that many of the solutions used in the equilibrium studies of Hedstrom (9) and Biedermann (22) were metastable, and should eventually have produced precipitates. Nevertheless, since the measured potentials were reversible, the conclusions reached about the species present in solution remain valid. 

Hedstrom s equilibrium measiurements were limited (P) to solutions containing fewer than 0.5 base equivalent per mole of iron. At higher degrees of hydrolysis he observed drifting potentials, and attributed them to incipient precipitation of ferric hydroxide. However, addition of bicarbonate to ferric hydroxide produces clear brown solutions containing up to 2.5 base equivalent per mole of iron, and no observable precipi-... 

With regard to a solubility equilibrium, the fact that vitreous silica behaves like a precipitate of polymeric silicic acid must be caused by the similarity between polymeric silicic acid and the hydrated surface of vitreous silica. Both forms can release silicic acid by hydrolysis and desorption, and likewise both forms are able to adsorb and condense silicic acid by means of silanol groups randomly distributed on their surfaces. Thus, in order to explain equal final states, the only assumption necessary is that the condensates will not attain the degree of dehydration of the bulk of the vitreous silica. The resulting equilibrium then relates to the two-phase system silicic acid—polymeric precipitate, and strictly speaking, this system is in a supersaturated state with respect to vitreous silica, which can be considered as an aged form of silica gel. 

Gallium. In the earlier quoted study of gallium hydrolysis (36), solutions at hydroxyl number 1.5 deposited small amounts of precipitate in days, those at hydroxyl number 1.75 in 2 weeks, those at hydroxyl number 2.00 in 4 months. Certainly the results reported cannot at all represent the equilibrium state. 

As a chemical phenomenon, weathering can be viewed as the result of the tendency of the rock-water-mineral system to attain equilibrium. This occurs through the usual chemical mechanisms of dissolution and precipitation, acid-base reactions, complexation, hydrolysis, and oxidation-reduction. 

A well-washed suspension of the precipitate shows a slight alkaline reaction owing to the hydrolysis equilibrium ... 

Silver borate, AgB02.—A solution of borax reacts with one of silver nitrate to precipitate the white metaborate, AgB02. It is also produced by dissolving silver monoxide in boric acid, an equilibrium being attained. Conversely, water causes partial hydrolysis of silver borate to silver monoxide and boric acid.6... 

Figure 14.18. Equilibrium composition of solutions in contact with freshly precipitated A1(0H)3 and Fe(OH)3, calculated using representative values for the equilibrium constants for solubility and hydrolysis equilibria. Shaded areas are approximate operating regions in water treatment practice coagulation in these systems occurs under conditions of oversaturation with respect to the metal hydroxide. Oversaturation is induced by adjusting the pH of incipiently acidic Fe(Ill) and Al(IU) solutions. After initiation of the oversaturation, the hydrolysis of Al(III) and Fe(III) progresses and charged multimeric hydroxo Al(III) and Fe(IIl) species, the actual coagulants, are formed as intermediates ultimately, these intermediates polymerize to solid Al(III) and Fe(III) (hydr)oxides, respectively. (From Stumm and O Melia, 1968.)...

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