Cobalt solubility constant

Temperature Dependence There are a few studies that have obtained solubility constants for -Co(OH)2(s) or pink cobalt hydroxide at zero ionic strength and for reaction (2.13) (M = Co ", x = 0). Three studies have presented data for the solubility constant at 25 °C and these values are in reasonable agreement (Nasa-nen, 1942b Makovskaya and Spivakovskii, 1974 Ziemniak, Goyette and Combs, 1999). In addition, solubility constants can also be derived from the work of Ziemniak et al. for higher temperatures up to 300 °C. Figure 11.57 illustrates that the accepted solubility constants are a linear function of the reciprocal of absolute temperature. 

Solubility constant data for the two cobalt(II) phases, OoO(s) and Oo(OH)2(s) (pink), are listed in Table 11.38. For OoO(s), the solubiUty constant from Robie and Hemingway (1995) was derived from their thermodynamic data and the Gibbs energy for Co " given by Wagman et al. (1969). 

The stability and solubility constants derived at 25 C for zero ionic strength have been used to create a predominance speciation diagram for cobalt(ll). The... 

The organic solvent should feature a low solubility in water and a high dielectric constant. Numerous studies have been reported for liquid-liquid junctions involving DCE [43,62,70,71,73], nitrobenzene [67,68,74,75], and nitrophenyloctylether (NPOE) [56]. Various hydrophobic electrolytes have been employed in these solvents. Tetraphenylarsonium (TPAs+) [[71,75,76], bis-triphenylphosphoranylidene (BTPPA+) [43,50], and hydrophobic tetra-arylammonium [77,78] are among the cations used in the organic phase. The choice for anions has been mostly restricted to borate derivatives, tetraphenylborate (TPB ) [70,79,80], tetrakis(4-chlorophenyl)borate (TPBCH) [43,81,82], and tetrakis(penta-fluoro)phenylborate (TPFB ) [49,83], as well as dicarbollyl-cobaltate [75]. 

Exactly the same problem arises with the recent studies of NiO solubility by Tremaine and Leblanc (25) and again the thermodynamic data on the aqueous anionic species at 300 C are likely to be more reliable than on the Ni + ion. There is good spectroscopic evidence for complex formation in chlorides of nickel (II), (26) cobalt (II) (27), and copper (II) (28) at 300°C and above. Most of the work was done at rather high Cl concentrations but qualitatively the effects of dielectric constant and concentration are as expected. A noteworthy feature (which estimation procedures will have to allow for) is the change from 6 to 4 coordination at the lower pressures (150-300 bar) and the higher Cl concentrations. This change appears to take place with only 2 or 3 Cl ions coordinated to the metal (at least in the case of Ni(II)). 

Hexammino - cobaltic Chloride, or Luteo-cobaltic Chloride, [Co(NH3)8]Cl3.—Several methods of preparation have been described. The best method is that of Jorgensen,1 whereby the salt is prepared by warming chloro-pentammino-cobaltic chloride, [Co(NH3)5C l]Cl2. in a pressure flask with 20 per cent, aqueous ammonia and ammonium chloride for several hours with constant shaking. After cooling, the mixture is removed from the flask and ammonia allowed to evaporate. The liquid is then diluted, hydrochloric acid added, and the whole heated on the water-bath, thus changing any aquo-pentammino-chloride into ehloro-pentammino-ehloride. More concentrated hydrochloric acid is added and the mixture cooled and filtered. The residue on the filter consists of ammonium chloride, chloro-pentammino-chloride, and hexammino-cobaltic chloride. Ammonium chloride is removed by treating with a 20 per cent, solution of hydrochloric acid, and the residue is then treated on a filter with cold water in which chloro-pentammino-cobaltic chloride is insoluble and hexammino-cobaltic chloride soluble. The salt is precipitated from its warm solution by the addition of half its volume of concentrated hydrochloric acid. 

The salts of the series are bright red crystalline bodies. They are soluble in water, neutral in reaction, and dilute mineral acids do not transform them into aquo-salts. The least soluble member of the series is the sulphate [(NH3)4Co (OH)2 Co(NH3)4](S04)a.2H30, which is prepared by heating hydroxo-aquo-tetrammino-cobaltic sulphate at 100° C. till it is constant in weight. The mass is extracted with water and the sparingly soluble sulphate collected and dried. The crude product so obtained is converted into the chloride and an aqueous solution of this then treated with a solution of sodium sulphate, when a crystalline precipitate of the diol-sulphate is obtained. It is collected, washed with water, alcohol, and finally with ether. It forms small red needle-shaped crystals which contain two molecules of water of hydration. 

Cobalt Naphthenate. Cobalt naphthenate behaved in much the same way as cobalt chloride, but at equivalent metal concentrations the soluble salt gave much faster rates. As the triethylamine concentration was increased, the 1 2 contents increased from 21 to 39% at the expense of cis structure. The trans contents remained constant at 30 3% (Table VII) until a ratio of NEt3/Co of about 4 was reached (dependent on the cobalt concentration (Table VIII)) when high trans polymer was obtained. 

The inertness of the dinuclear complexes is greatest in slightly acidic solutions, which therefore have been employed for the reprecipitation reactions. Apparently the chromium systems are much more labile toward bridge breaking than are the cobalt systems. In aqueous solution the meso-[(en)2Cr(OH)2Ci(en)2] cation (I) enters into a rapidly established (t 1 min. at room temperature) equilibrium with the mono-ju-hydroxo complex [(OHXen)2Cr(OH)Cr(en)2-(HaO)] (n) The equilibrium constant K = [II]/[I] is 0.83 in 1 Af NaC104 at 0°. The salts (dithionate, bromide, chloride, and perchlorate) of the di-p-hydroxo cation are less soluble than the respective salts of the mono-/i-hydroxo cation. It is therefore possible to precipitate the pure salts of the di-/i-hydroxo cation from the equilibrium mixture following the procedure given above. 

Since ammonia forms stable, water-soluble complexes with many metals, leaching can be carried out under alkaline conditions to give these metals in solution. Of particular interest are the metals copper, nickel and cobalt, which form particularly stable amines lliat have been well characterized as having the following approximate stability constants (at high ionic strength) Cu, 2 = Cu , 4 = 13 Ni , 6 = 9 = 5 Fe ,j52 < 2. 

Pyatnitskii and Durdyev [66PYA/DUR] attempted to determine the stability constants of cobalt-selenite complexes from solubility measurements in selenite solution. As discussed in Appendix A, their equilibrium model is likely to be incorrect. The proposed equilibrium constant of the reaction ... 

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