Ionic strength dependence

If the rate equation contains the concentration of a species involved in a preequilibrium step (often an acid-base species), then this concentration may be a function of ionic strength via the ionic strength dependence of the equilibrium constant controlling the concentration. Therefore, the rate constant may vary with ionic strength through this dependence this is called a secondary salt effect. This effect is an artifact in a sense, because its source is independent of the rate process, and it can be completely accounted for by evaluating the rate constant on the basis of the actual species concentration, calculated by means of the equilibrium constant appropriate to the ionic strength in the rate study. 

This composite rate constant is predicted to have a different ionic strength dependence for the two schemes. According to the Br0nsted-Debye-Hiickel equation, the composite rate constant for Eq. (9-76) will be independent of ionic strength if Scheme ... 

The ionic strength dependence of intrinsic viscosity is function of molecular structure and protein folding, ft is well known that the conformational and rheological properties of charged biopolymer solutions are dependent not only upon electrostatic interactions between macromolecules but also upon interactions between biopolymer chains and mobile ions. Due electrostatic interactions the specific viscosity of extremely dilute solutions seems to increase infinitely with decreasing ionic concentration. Variations of the intrinsic viscosity of a charged polyampholite with ionic strength have problems of characterization. 

Manz, B Stilbs, P Jonsson, B Soderman, O Callaghan, PT, NMR Imaging of the Time Evolution of Electroosmotic Flow in a Capillary, Journal of Physical Chemistry 99, 11297, 1995. Matthew, JB Hanania, GIH Gurd, FRN, Electrostatic Effects in Hemoglobin Bohr Effect and Ionic Strength Dependence of Individual Groups, Biochemistry 18, 1928, 1979. 

A more recent examination confirmed the existence of an unusual ionic-strength dependence of the reaction rate , which features a minimum at /r 0.83, but it was noted that the initial kinetics differ from those occurring later in the reaction. Consideration of the equilibria prevailing in aqueous solutions of Cr(VI) produced a simplified rate law... 

This is an example of a reversible reaction the standard electrode potential of the 2PS/PSSP + 2c couple is zero at pH 7. The oxidation kinetics are simple second-order and the presence of a radical intermediate (presumably PS-) was detected. Reaction occurs in the pH range 5 to 13 with a maximum rate at pH 6.2, and the activation energy above 22 °C is zero. The ionic strength dependence of 2 afforded a value for z Zg of 9 from the Bronsted relation... 

Raman and Brubaker have also examined reduction by chelates of Fe(II) and explain the ionic strength dependence in terms of a pre-equilibrium ionpairing. 

Hasan MN, Kwakemaak C, Sloof WG, Hagen WR, Heering HA. 2006. Pyrococcus furiosus 4Fe-ferredoxin, chemisorbed on gold, exhibits gated reduction and ionic strength dependent dimerization. J Biol Inorg Chem 11 651-662. 

Austin et al. [132] measured the ionic strength dependence of the liposome-water distribution of several acidic and basic drugs and modelled the data with a combination of electrostatic and ion pair models. They concluded that the increased apparent Dmw values at higher ionic strength were due primarily to the reduction in surface potential and not to ion pairing. Ion pairing was also excluded because the apparent Dmw varied at fixed ionic strength with the... 

It has been shown elsewhere (26) that in natural waters the degree of enhancement of Mn(II) oxidation predicted on the basis of model calculations is as follows y-FeOOH > a-Fe00H > silica > alumina. It has also been shown that the rate of Mn(II) oxidation is strongly influenced by pH, y-FeOOH concentration, temperature and ionic strength. Depending on the conditions, the predicted half-life 1/2 = ln 2/ki ) f°r Mn(II) oxidation may vary from a few days to thousands of years. By way of example, at pH 8, p02 0.21 atm, 25°C in waters containing 4(iM y-FeOOH and 0.2uM Mn(II), the half-life for oxidation is about 30 days. 

Recognizing the ionic strength dependence of the effect of short range forces in binary interactions, Pitzer was able to develop an empirical relation for B,-a(I). The expression for systems containing strong electrolytes with one or both ions univalent is... 

Marshall s extensive review (16) concentrates mainly on conductance and solubility studies of simple (non-transition metal) electrolytes and the application of extended Debye-Huckel equations in describing the ionic strength dependence of equilibrium constants. The conductance studies covered conditions to 4 kbar and 800 C while the solubility studies were mostly at SVP up to 350 C. In the latter studies above 300°C deviations from Debye-Huckel behaviour were found. This is not surprising since the Debye-Huckel theory treats the solvent as incompressible and, as seen in Fig. 3, water rapidly becomes more compressible above 300 C. Until a theory which accounts for electrostriction in a compressible fluid becomes available, extrapolation to infinite dilution at temperatures much above 300 C must be considered untrustworthy. Since water becomes infinitely compressible at the critical point, the standard entropy of an ion becomes infinitely negative, so that the concept of a standard ionic free energy becomes meaningless. 

Fluorescence resonance energy transfer has also been used for ionic strength measurements.(95) Fluorescein labeled dextran (donor) and polyethyleneimine-Texas Red (acceptor) were placed behind a dialysis membrane. The polymer association is ionic strength dependent and the ratio of intensities (F o/Fw) was used as the measured parameter. Since both the donor and acceptor are fluorescent, this kind of sensor may allow expand the sensitive ionic strength range by shifts in observation wavelength, as was discussed for pH probe Carboxy SNAFL-2 (see Section 10.3). 

Histone HI from the fruit fly Ceratitis capitata has two tyrosine residues. Jordano et al.(<)2> have observed two differences from calf thymus HI (1) the apparent quantum yield does not increase on protein folding and (2) there is a pH- and conformation-dependent shoulder at 340 nm in the emission spectrum. This group has attributed this 340-nm emission to tyrosinate.(97) Their studies demonstrate that the folding of histone HI from C. capitata is pH and ionic strength dependent. The possibility of tyrosinate formation at neutral pH is discussed in greater detail in Section 1.5.2. 

If Y depends on the ionic strength, and the ionic strength depends on the number of ions in solution, then what happens if the analyte is present in low concentration but the solvent is itself an ionic solution ... 

The rate constants for the reaction of a pyridinium Ion with cyanide have been measured in both a cationic and nonlonic oil in water microemulsion as a function of water content. There is no effect of added salt on the reaction rate in the cationic system, but a substantial effect of ionic strength on the rate as observed in the nonionic system. Estimates of the ionic strength in the "Stern layer" of the cationic microemulsion have been employed to correct the rate constants in the nonlonic system and calculate effective surface potentials. The ion-exchange (IE) model, which assumes that reaction occurs in the Stern layer and that the nucleophile concentration is determined by an ion-exchange equilibrium with the surfactant counterion, has been applied to the data. The results, although not definitive because of the ionic strength dependence, indicate that the IE model may not provide the best description of this reaction system. 

Less is known about the interaction of the nucleosomes between themselves or with free DNA. The nucleosome-nucleosome interaction has recently been parameterized by using the surface charge density of the known crystal structure [39] in a point-charge model [51]. While in that work only electrostatic interactions were considered and the quantitative influence of the histone tails on the interaction potential still remains obscure, simulations based on this potential allowed to predict an ionic-strength dependent structural transition of a 50-nucleosome chromatin fragment that occurred at a salt concentration compatible with known experimental data (Ref. [65], see below). 

The Debye-Hiickel term, which is the dominant term in the expression for the activity coefficients in dilute solution, accounts for electrostatic, nonspecific long-range interactions. At higher concentrations, short-range, nonelectrostatic interactions have to be taken into account. This is usually done by adding ionic strength dependent terms to the Debye-Hiickel expression. This method was first outlined by Bronsted [5,6], and elaborated... 

The term Baj in the denominator of the Debye-Hiickel term has been assigned a value of Baj = 1.5, as proposed by Scatchard [9] and accepted by Ciavatta [10]. This value has been found to minimize, for several species, the ionic strength dependence of (y, /j between / = 0.5m and 4i = 3.5 m. It should be mentioned that some authors have proposed different values for... 

Ion interaction coefficients can be described more accurately with an ionic strength-dependent function, listed in Table 6.5. 

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