Complexation in dilute solutions

LeVine H III. Soluble multimeric Alzheimer beta(l—40) preamyloid complexes in dilute solution. Neurobiol Aging 1995 16 755-764. 

Mercury-chloride complexes in dilute solutions. This slightly more difficult example will be useful in showing how to handle poorly conditioned systems of equations. It is assumed that mercury chloride HgCl2 is dissolved in pure water with a molality m = 10 5 mol kg-1. Given the equilibrium constants for chloride complex formation... 

Addition of methoxide ion in methanol For reactive substrates, such as 1,3,5-trinitrobenzene, which are converted into complex in dilute solutions of sodium methoxide, the equilibrium constant, Kx, is adequately expressed in terms of concentrations as [A]/[P][OMe-]. However, for less reactive substrates, such as dinitrobenzenes, significant conversion to complex only occurs at fairly high concentrations of sodium methoxide similarly for the higher equilibria of trinitro-substituted benzenes. In these solutions the basicity of the medium cannot be adequately described by the con-... 

Hem, J.D. and Roberson, C.E., Eorm and stability of aluminium hydroxide complexes in dilute solutions, in Chemistry of Aluminium in Natural Water, Geol. Survey Water-Supply, 1827-A, 1-55, U.S. Government Printing Office, Washington, D.C., 1967. 

Petrucci, S. (1971) Statistical Thermodynamics of Ionic Association and Complexation in Dilute Solutions of Electrolytes. In Ionic Interactions. From Dilute Solutions to Fused Salts S. Petrucci, Ed. Academic Press New York, Vol. 1 pp 117-177. 

The so-called dilution (5) effect illustrated by Table I indicates that chelate compounds are more stable than analogous complexes in dilute solution. If the Schwarzenbach model is used, a chelate may be considered to be more stable than the corresponding complex by about two log units per chelate ring formed. Thus the metal chelates and complexes in their standard state may be assigned the overall stability constants (p vdues) shown in Table I. If one assumes a value of p of 10 for the complex MAg, then the value of p for a didentate donor which has three chelate rings would be 10 , and with a hexadentate donor which forms five chelate rings p would be The formulations of the p values used and the units of p show that the metal complex is very sensitive to its concentration in solution, while the chelates are less sensitive to that factor. With the assumptions that have been made it is possible to determine the degree of dissociation in dilute solution. In very dilute solution (lO- M) the complex MA seems to be... 

The most common oxidation state of niobium is +5, although many anhydrous compounds have been made with lower oxidation states, notably +4 and +3, and Nb can be reduced in aqueous solution to Nb by zinc. The aqueous chemistry primarily involves halo- and organic acid anionic complexes. Virtually no cationic chemistry exists because of the irreversible hydrolysis of the cation in dilute solutions. Metal—metal bonding is common. Extensive polymeric anions form. Niobium resembles tantalum and titanium in its chemistry, and separation from these elements is difficult. In the soHd state, niobium has the same atomic radius as tantalum and essentially the same ionic radius as well, ie, Nb Ta = 68 pm. This is the same size as Ti ... 

Conductometric Analysis Solutions of elec trolytes in ionizing solvents (e.g., water) conduct current when an electrical potential is applied across electrodes immersed in the solution. Conductance is a function of ion concentration, ionic charge, and ion mobility. Conductance measurements are ideally suited tor measurement of the concentration of a single strong elec trolyte in dilute solutions. At higher concentrations, conduc tance becomes a complex, nonlinear func tion of concentration requiring suitable calibration for quantitative measurements. 

Standard potentials Ee are evaluated with full regard to activity effects and with all ions present in simple form they are really limiting or ideal values and are rarely observed in a potentiometric measurement. In practice, the solutions may be quite concentrated and frequently contain other electrolytes under these conditions the activities of the pertinent species are much smaller than the concentrations, and consequently the use of the latter may lead to unreliable conclusions. Also, the actual active species present (see example below) may differ from those to which the ideal standard potentials apply. For these reasons formal potentials have been proposed to supplement standard potentials. The formal potential is the potential observed experimentally in a solution containing one mole each of the oxidised and reduced substances together with other specified substances at specified concentrations. It is found that formal potentials vary appreciably, for example, with the nature and concentration of the acid that is present. The formal potential incorporates in one value the effects resulting from variation of activity coefficients with ionic strength, acid-base dissociation, complexation, liquid-junction potentials, etc., and thus has a real practical value. Formal potentials do not have the theoretical significance of standard potentials, but they are observed values in actual potentiometric measurements. In dilute solutions they usually obey the Nernst equation fairly closely in the form ... 

Chain-growth polymerizations are diffusion controlled in bulk polymerizations. This is expected to occur rapidly, even prior to network development in step-growth mechanisms. Traditionally, rate constants are expressed in terms of viscosity. In dilute solutions, viscosity is proportional to molecular weight to a power that lies between 0.6 and 0.8 (22). Melt viscosity is more complex (23) Below a critical value for the number of atoms per chain, viscosity correlates to the 1.75 power. Above this critical value, the power is nearly 3 4 for a number of thermoplastics at low shear rates. In thermosets, as the extent of conversion reaches gellation, the viscosity asymptotically increases. However, if network formation is restricted to tightly crosslinked, localized regions, viscosity may not be appreciably affected. In the current study, an exponential function of degree of polymerization was selected as a first estimate of the rate dependency on viscosity. 

In the case of polynuclear metal cluster SCO complexes in the solid state, there will be intra-cluster, as well as inter-cluster cooperativity. To eliminate inter-cluster effects totally, studies must be made in dilute solutions. Williams et al. have done just this for a dinuclear [Fe(II)2L3] helicate complex which does not contain a good superexchange pathway between the Fe(II) centre but, rather, three flexible bis-bidentate ligands. A very broad, two step, SCO was observed (LS-LS<->LS-HS<->HS-HS) and fitted to a model for negative cooperativity in which subtle structural changes around each Fe oc-... 

Smith, R. W., and J. D. Hem (1972), "Effect of Aging on Aluminum Hydroxide Complexes in Dilute Aqueous Solutions", U.S. Geological Survey Water-Supply Paper 1827-D, 51. 

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