Nickel ionic strength dependence

The corrected complexation constants of pyrophosphate with nickel show a marked ionic strength dependence that would correspond to Ae values of (2.5 0.7) kg-mol" and (1.7 0.7) kg-mol. The former, at least, is inconsistent with any reasonable selection of e values (though the same is not true for the Ae values for the first protonation constant for pyrophosphate, which also involves highly charged ions). Based on these calculations, only the results from the higher ionic strength solutions (7=0.1 M and 0.2 M) are used in the present review. 

The ionic strength dependence of the nickel bisulphide stability constants is depicted in Figure A-39. By applying the SIT concept [97GRE/PLY2] the respective stability constants are extrapolated to zero ionic strength. As well as the stability constants, the optimisation procedure also yields the SIT interaction parameters Ae in chloride media, r= 298.15 K ... 

An additional factor was found to influence the rate of reaction in the experiments involving tetrakis ( -mercaptoethylamine) trinickel (II) ion. The addition of nickel chloride retarded the process. Methanol was used as the solvent to demonstrate that the dependence was actually due to the presence of nickel ion and not an ionic strength effect. Magnesium chloride accelerates the rate slightly, while nickel ion greatly retards the rate of reaction. This effect was studied in greater detail, but solubility requirements necessitated the use of a water-methanol mixed solvent. A solution of 5.5M water in methanol was found to be satisfactory to obtain the necessary solubilities of complex and nickel chloride. 

Nickel may be transported into streams and waterways from the natural weathering of soil as well as from anthropogenic discharges and runoff. This nickel can accumulate in sediment, with the adsorption of the metal to the soil depending on pFI, redox potential, ionic strength of the water, concentration of complex-ing ions, and the metal concentration and type. 

Like [Ni(cyclam)], the above dinuclear nickel complex exists in aqueous solution as a mixture of low-spin, four-coordinated and high-spin, six-coordinated species, with the equilibrium between the two being dependent on the ionic strength. In the presence of excess per-... 

The thermodynamic parameters for the formation of several metal-cyanide complexes, among others those of Ni(CN)4 , have been determined using pH-metric and calorimetric methods at 10, 25 and 40°C. In case of nickel(II), the thermodynamic data were determined by titration of Ni(C104)2 solutions with NaCN solutions. The ionic strength of the solutions were 1 < 0.02 M in all cases. The Debye-Huckel equation, related to the SIT model, was used to correct the formation constants to thermodynamic constants valid at 7 = 0. Since previous experiments indicated that the dependence of A,77° in the ionic strength in dilute aqueous solutions is small compared to the experimental error, the measured heats of reaction (A,77 = - 189.1 kJ mol at 10°C A,77 ,= -183.7 kJ mol at 40 C) were taken to be valid at 7 = 0, but the uncertainties were estimated in this review as 2.0 kJ moT. From the values of A,77 , as a function of temperature, average A,C° values were calculated. 

The dependence of rate constant on ionic strength is still widely used, often in conjunction with the dependence of rate constant on dielectric constant, as an indicator of substitution mechanism. Recent instances of this classical approach include the reaction of /ra j-[Co(dmgH)2(SCN)(tu)] with thiourea (tu), aquation of trans-[Rh(dmgH)2Cl(tu)], aquation of the [Co(02CCHaCl)(NH3)6]"+ cation, and substitution at the [Fe(CN)6(OH2)] anion by nitrite, thiocyanate, sulphite, or nitrosobenzene. Salt effects on observed rate constants for the reaction of nickel(n) with pyrophosphate operate via the outer-sphere association constant rather than via the interchange rate constant. ... 

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