Limiting equivalent conductivities

Good agreement of the observed limiting equivalent conductances with the predicted values indicates that the component ions exist in DMSO without significant deterioration under argon. It was also shown that [l 2 ] and [24+2 ] are dissociated to more than 99% in DMSO over a concentration range 10-" -10- m. 

Selvaratnam, M. Spiro, M. (1965). Transference numbers of orthophosphoric acid and the limiting equivalent conductance of the HgPO ion in water at 25 °C. Transactions of the Faraday Society, 61, 360-73. 

It may be added that Kohlrausch s law does not lead to any method of deducing the contributions of the individual ions. The immediate practical application of Kohlrausch s law of independent contributions of the ions at infinite dilution is a method for deducing the limiting equivalent conductance, A0, of weak electrolytes. This will be illustrated by taking a specific example of a weak electrolyte. 

Hence the decrease of AgN03 concentration within the catholyte is exactly equal to its increase within the anolyte, which for this symmetrical type of cell is to be expected therefore, only one of the two needs to be measured in order to determine the transference numbers. From the transference numbers and the limiting equivalent conductivity A0, one obtains the equivalent ionic conductivities Aq = tg A0 and Aq = tg A0. 

The ratio of the equivalent conductivity at a given concentration to the limiting equivalent conductivity then is... 

The equivalent conductivity of an electrolyte solution decreases with increasing concentration due to interionic attractions described mainly by the electrophoretic and relaxation field effects 2-35>. This decrease is more pronounced if in addition the electrolyte is associated. Association of ionic salts by ion-pairing is commonly observed in solvents of low or moderate dielectric constant. The immediate goals in the analysis of conductance data are the. determination of the limiting equivalent conductance at infinite dilution, A0, and the evaluation of the association constant, KA, if ion-pairing occurs. 

The limiting equivalent conductance X0 is equal to the sum of cation and anion limiting conductances, Xj, and 0. These quantities are related to the limiting transference numbers, tj and to, of the electrolyte by the equations... 

Table 2 lists limiting equivalent conductance and association constant values for a number of 1 1 electrolytes in the solvents of Table 1, and Table 3 gives single ion mobility values. The data include results that appear to have sufficient precision to give meaningful values when treated by the Fuoss-On-sager conductance equation. In a few cases data of somewhat lower precision have been included to indicate the magnitude of the association constants, which can often be determined with fair accuracy from such data. 

Table 8.1 Limiting Equivalent Conductance of Ions in Water at 25 °C...

Disadvantage of the conductometric signal transduction originates from its additivity. For instance, interferants adsorption adds to the conductivity measured as the detection signal. Hence, conductivity is not so widely used for transduction as other electroanalytical techniques. Moreover, any similar difference in the limiting equivalent conductance is insufficient for discrimination between the analyte and its interferants. 

Table 1. Limiting equivalent conductivities (X °cation (T=25 °C) ) of quaternary ammonium ions [39]...

X °cation=34.8 S-cm2-mol 1 was obtained for the cation C8H16N+. Data in Table 1 show that this limiting equivalent conductivity fits onto a single line with those of the other quaternary ammonium salts of the homologues series. 

Table 2.6 The limiting equivalent conductivities, A.00, and the Walden products, X°°r, of tetrabutylammonium and sodium ions in various solvents at 25°C (Kratochvil and Yeager 1972, Marcus 1997)... 

Thus, it is worthy of consideration to try to acidify a solution with a view to a better detection of negative ions, and vice versa. Indeed, both II301 and OH have high limit equivalent conductivities, as shown in Table 1.3. 

Also, in this case, conductivity data were analyzed by Fuoss-On-sager-Skinner equations and limiting equivalent conductances A<> and association constants KA are collected in Table II together with physical properties of solvent mixtures. Furthermore, Table III shows ethanol and tert-butanol concentration in the mixture [ROH], the relevant dielectric constant c, and the pK of picric acid. 

Table II. Limiting Equivalent Conductances and Association Constants"...

It is seen that at the higher concentrations the equivalent conductance is very low, which is the characteristic of a weak electrolyte, but in the more dilute solutions the values rise with great rapidity the limiting equivalent conductance of acetic acid is known from other sources to be 390.7 ohms cm. at 25 , and so there must be an increase from 131.6 to this value as the solution is made more dilute than 10 equiv. per liter. The plot of the results for acetic acid, shown in Fig. 20, may bo regarded as characteristic of a weak electrolyte. As mentioned in Chap. I, it is not possible to make a sharp distinction between electrolytes of different classes, and the variation of the equivalent conductance of an intermediate electrolyte, siu h as trichloroacetic, cyanoacetic and mandelic acids, lies between that for a weak electrolyte, e.g., acetic acid, and a moderately strong electrolyte, e.g., nickel sulfate (cf. Fig. 20). 

The adoption of an eluent having a very low conductivity, which can be passed directly through the conductometric detector. Typical eluents used are benzoate, phthalate, or other aromatic acid salts, with low limiting equivalent conductances (leading to direct detection) or potassium hydroxide eluent, with high conduc-... 

In order for us to have flexibility in our modeling of natural water chemistry we need a way to obtain individual ion activity coefficients from mean values. To do so requires that we make an assumption, called the Macinnes convention (Macinnes 1919), which states = 7c - The convention is based on the observation that and Cr ions are of the same charge and nearly the same size, have similar electron structures (inert gas), and similar ionic mobilities. In support of this assumption, tracer diffusion coefficients, D°, of K+ and Cl" at infinite dilution are nearly equal at 19.6 and 20.3 X 10" cmVs (Lerman 1979). Also, limiting equivalent conductances, A°, of and Cl" are comparable at 73.50 and 76.35 cmV(ohm) (equiv.) at 25°C (Robinson and Stokes 1970),... 

With a conductivity detector with a known cell constant, the conductances of various solutions of known concentration can be calculated from a table of equivalent conductances, with Eq. 4.6. The limiting equivalent conductances of some common ions are given in Table 4.1. The equivalent conductances of ions generally decrease with increasing concentration because of inter-ionic effects. For dilute solutions (10 to 10" N) the equivalent conductances are not greatly different from the values listed in the table. 

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