Retardation factor determination

A clear demonstration of the relative importance of steric and resonance factors in radical additions to carbon-carbon double bonds can be found by considering the effect of (non-polar) substituents on the rate of attack of (nonpolar) radicals. Substituents on the double bond strongly retard addition at the substituted carbon while leaving the rate of addition to the other end essentially unaffected (for example, Table 1.3). This is in keeping with expectation if steric factors determine the regiospeeificity of addition, but contrary to expectation if resonance factors are dominant. 

The traditional method of determination of the numerical values of the retardation factor, Rjn quasi-automatically assumes the following preconditions ... 

With a growing implementation of the thin-layer chromatographic laboratories with densitometric scanners and particularly in the case of PLC, it seems quite important to reconsider definition of the Rp coefficient and the ways of its determination. Further, it seems strongly advisable to recommend the retardation factor in its i P(max) form as a more practical one for the preparative layer chromatographic usage. 

There are various parameters and assumptions defining radionuclide behavior that are frequently part of model descriptions that require constraints. While these must generally be determined for each particular site, laboratory experiments must also be conducted to further define the range of possibilities and the operation of particular mechanisms. These include the reversibility of adsorption, the relative rates of radionuclide leaching, the rates of irreversible incorporation of sorbed nuclides, and the rates of precipitation when concentrations are above Th or U mineral solubility limits. A key issue is whether the recoil rates of radionuclides can be clearly related to the release rates of Rn the models are most useful for providing precise values for parameters such as retardation factors, and many values rely on a reliable value for the recoil fluxes, and this is always obtained from Rn groundwater activities. These values are only as well constrained as this assumption, which therefore must be bolstered by clearer evidence. 

Krishnaswami S, Graustein WC, Turekian KK, Dowd F (1982) Radium, thorium, and radioactive lead isotopes in groundwaters application to the in-situ determination of adsorption-desorption rate constants and retardation factors. Water Resour Res 6 1663-1675 Krishnaswami S, Bhushan R, Baskaran M (1991) Radium isotopes and Rn in shallow brines, Kharaghoda (India). Chem Geol (Isot Geosci) 87 125-136 Kronfeld J, Vogel JC, Talma AS (1994) A new explanation for extreme " U/ U disequilibria in a dolomitic aquifer. Earth Planet Sci Lett 123 81-93... 

Empirically determined retardation factors (either partition coefficients or breakthrough curve measurements, which are the change in solute concentration measured over time in laboratory or field experiments) have been widely used because of their inherent simplicity.162 Modeling of specific geochemical partition and transformation processes is not necessary if the retardation factor can be determined empirically. 

In paper and thin-layer chromatography, the separation process is halted at a stage which leaves the separated components in situ on the surface in the form of spots. The rate at which a solute has moved is then determined by its retardation factor, R, which is defined as... 

T0 calculate the retardation factors for ionizable compounds such as acids and bases, the fraction of unionized acid (aj or base (at) needs to be determined (see Dissociation constant). According to Guswa et al. (1984), if it is assumed only the un-ionized portion of the acid is adsorbed onto the soil, the retardation factor for the acid becomes ... 

The characteristics of the experimental aquifer were independently determined from appropriate flowthrough column experiments or obtained directly from the literature. The dry bulk density of the sand ph= 1.61 kg/1, and the aquifer porosity 0=0.415 were evaluated by gravimetric procedures. The dimensionless retardation factor, R= 1.31, of the aqueous-phase TCE was determined from a column flowthrough experiment. The tortuosity coefficient for the aquifer sand was considered to be x =1.43 [75]. The molecular diffusion coefficient for the aqueous-phase TCE is D=0.0303 cm2/h [76]. The pool radius is r=3.8 cm. Bromide ion in the form of the moderately soluble potassium bromide salt was the tracer of choice [77 ] for the tracer experiment conducted in order to determine the longitudinal and transverse aquifer dispersivities a =0.259 cm and a-,— 0.019 cm, respectively. The experimental pool contained approximately 12 ml of certified ACS grade (Fisher Scientific) TCE with solubility of Cs=1100 mg/1 [78]. 

The index m stands for mobile and im for immobile. Mirn is the number of moles of a species in the immobile zone and Rim the retardation factor of the immobile zone, Cm and Om are the concentrations in mol/kg in the mobile and immobile zone respectively. The symbol a stands for an exchange factor (1/s). The retardation factor R = 1+ (dq/dc) is determined by chemical reactions. The integrated form of Eq. 96 is ... 

Table 2 also indicates that the nucleophiles effective for vinyl ethers are relatively mild, when compared with those for isobutene (cf., Section V.B.2). In fact, stronger bases lead to inhibition or severe retardation of polymerization [36,64] ketones aldehydes, amides, acid anhydrides, dimethyl sulfoxide (retardation) alcohols, aliphatic amines, pyridine (inhibition). The choice of nucleophiles is determined by their Lewis basicity (as measured by pKb, etc. [64,103]), and this factor determines the effic-tive concentrations of the nucleophiles. For example, the required amounts of esters and ethers decrease in the order of increasing basicity (i.e., a stronger base is more effective and therefore less is needed) [101,103] tetrahydrofuran < 1,4-dioxane ethyl acetate < diethyl ether. On the other hand, for amines not only basicity but also steric factors play an important role [142] thus, unsubstituted pyridine is an inhibitor, while 2,5-dimethylpyridine is an effective nucleophile for controlled/living polymerization, although the latter is more Lewis basic. 

In gas chromatography the analyte partitioning between mobile gas phase and stationary liquid phase is a real retention mechanism also, phase parameters, such as volume, thickness, internal diameter, and so on, are well known and easily determined. In liquid chromatography, however, the correct definition of the mobile-phase volume has been a subject of continuous debate in the last 30 years [13-16]. The assumption that the retardation factor, i /, which is a quantitative ratio, could be considered as the fraction of time that components spend in the mobile phase is not obvious either. 

Relyea J. E. (1982) Theoretical and experimental considerations for the use of the column method for determining retardation factors. Radioact. Waste Manage. Nuclear Fuel Cycle 3(3), 151-166. 

This retardation is caused by adsorption on clays and organic materials. The adsorption capacity is in turn a function of the surface area and availability of these materials to the adsorbing species in question. In real soils, and especially those of sedimentary origin, the adsorbant components are to be found as discrete lenses of low permeability rather than as an evenly distributed phase as is essentially assumed in the laboratory methodologies for determining the retardation factors. In these lenses penetration to the bulk of the adsorption sites is restricted to diffusion and the small residual convection fluxes. 

Since both these transport processes depend on the frictional coefficient of the molecule in the same way, this is a very reasonable assumption, and explains why under conditions of severe retardation zones remain extremely sharp. By application of Pick s laws, Richards and Lecanidou determine the evolution of the zone half-width with time, and derive the important result, confirmed by experiment, that the zone-width, provided the ratio of diffusion coefficient to retardation factor is of the right order, depends to only a slight extent on the width of the starting zone applied at one end of the gel. For typical conditions there is no detectable loss in resolution for an initial zone width of up to 2 mm, and even for a 1 cm column of RNA solution applied to the gel there is only a 25% increase in band width. 

The pore water velocity, v = 5 x 10 cm/s, and the retardation factor, R = 70, are given in the problem statement. Thus, the velocity of the naphthalene is determined... 

In gas chromatography (GC) the mobile phase is a gas (e.g. nitrogen, N ) and moves rapidly compared to the movement of the separated substances. Thus, R values (retardation factor, i.e. the distance travelled by an analyte divided by the distance travelled by the mobile phase (these values are routinely used in TLC)) would be very small and hard to determine. Retention times are therefore employed in preference to Revalues. The retention tune of a compound (also used in liquid chromatography) may be described simply as the length of time it takes a compound to be detected following injection. This quantity has units of time, that is, seconds or minutes. 

The magnitude of 1 + (pb/0) / is the retardation factor, estimating the degree to which the cation s movement is reduced. is simply the ratio of moles of Cs" adsorbed to moles in solution, and must be determined by an adsorption experiment using the soil material from the site of interest. The K4 can have a value of 100 or... 

The purpose for performing this procedure is to determine which amino acids, if any, are found in the food sample. This may be ascertained by simply comparing the known amino acid spots with the spots from the food sample. For example, if valine migrates 4.0 cm from the origin and the food sample shows an amino acid spot 4.0 cm from the origin, the logical conclusion is that the spot is valine. A more mathematical approach involves calculating retardation factors (Rf) values for all spots, with Rf defined as ... 

The factors determining the velocity V are, so far, unknown. The velocity, vx9 however, determines the limiting conductance, Ao, of an ion. The velocity which has an opposite sign to that of V is the retardation due to the electrophoretic effect. 

Rf is the retardation factor caused by the porous medium geometry and R the retardation factor due to physical or chemical interaction between the gas and porous medium. For the case of noble gases these latter interactions are usually insignificant and R = 1. Rf, determined for different rock varies over orders of magnitude but has not been correlated with physical rock properties to enable an assessment of all rock types. For example, Rf values of between 1.5 and 2.5 have been estimated for He and Ar in deep-sea sediments... 

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