Capacity measurements cations

Standard cations used for measuring cation exchange capacity are Na+, NHJ, and Ba2+. NH is often used but it may form inner-sphere complexes with 2 1 layer clays and may substitute for cations in easily weathered primary soil minerals. In other words, one has to adhere to detailed operational laboratory procedures these need to be known to interpret the data and it is difficult to come up with an operationally determined "ion exchange capacity" that can readily be conceptualized unequivocally. 

Lobacz et al. [52] have described partial adsorption ofTl+-cryptand (2,2,2) complex on mercury electrode. From voltocoulom-etry, cyclic voltammetry, and chrono-coulometry, it has been deduced that electroreduction of this complex proceeds via two parallel pathways from the solution and from the adsorbed states, which are energetically close. Also, Damaskin and coworkers [53] have studied adsorption of the complexes of alkali metal cations with cryptand (2,2,2) using differential capacity measurements and a stationary drop electrode. It has been found that these complexes exhibit strong adsorption properties. Novotny etal. [54] have studied interfacial activity and adsorptive accumulation of U02 " "-cupferron and UO2 - chloranilic acid complexes on mercury electrodes at various potentials in 0.1 M acetate buffer of pH 4.6 and 0.1 M NaCl04, respectively. 

Because so much of the behavior of suspensions is determined or modified by charge associated with the solid phases, ZPC may be inferred from a wide variety of experiments involving pH as a master variable. For example, coagulation and sedimentation rates are maximum at the ZPC, and anion and cation exchange capacities (measured with nonspecific, symmetrical electrolytes) are equal and minimum at the ZPC. More direct and less ambiguous are electrophoresis and streaming potential, in any of their modifications. One can estimate the IEP(s) by measuring adsorption of H+ and OH" if one is certain that no specific adsorption of other species occurs. 

Equivalent The amount of a substance that will react with one mole of H+ or OH-. The related term equivalent mass ( weight ) refers to a molar mass of a substance divided by the absolute value of its valence state. For anion exchange capacity and cation exchange capacity measurements, the results are usually reported in milliequivalents (1/1000 of an equivalent) per 100 g of material. 

TABLE 10.5 Measured cation exchange capacities, in situ soil pH values and percentages of major exchangeable ions of some soils at pH = 7... 

Wada, K., and Y. Harada. 1971. Effects of temperature on the measured cation-exchange capacities of Ando soils. J. Soil Sci. 22 109-117. 

K, when it decomposes peritectoidally into 72 and P2. There have not been any heat capacity measurements reported for 71. Ball et al. [92BAL/DIC] obtained estimates of C (NiTeo.775, cr, 298.15 K) = 45.8 (48.7) J-K -moT and 5 (NiTeo.775, cr, 298.15 K) = 71.0 (67.6) J K mol by taking a weighted average of heat capacities of the P and NiTe2 v(cr) phases. The values in brackets have been calculated with the cationic and anionic contributions to the heat capacity at 298 K and the revised Latimer entropy contributions [93KUB/ALC]. If reasonable uncertainties of 5.0 J K mol are chosen for and A5°, both methods of estimation lead to results that are consis-... 

Anions are strongly adsorbed from alcohol solutions in the same general order as from water. Frumkin s early results for methanol shown in Fig. 7.2.1 are typical. Anion adsorption appears to be somewhat weaker in methanol than in water according to the relative lowering of the interfacial tension. As in water, no significant specific adsorption of fluoride ions can be detected from capacity measurements. On the other hand specific adsorption of certain cations, e.g. Cs and NH, which is difficult to detect in water, is evident in methanol. ... 

There is no significant specific adsorption of fluoride ions from either formamide or NMF. However, specific adsorption of alkali metal cations, especially Cs+, is evident both from electrocapillary and capacity measurements in most of these solvents. " A study of specific adsorption of Cs+ and 1 ions in DMF solutions was reported in a recent paper. ... 

In experiments at trace level or very low loading of one ion, the isotope dilution procedure is not required. All that is necessary is that the exchangeable ions on the solid be those of the salt present in macro concentration. The computation of results, which followed that conventionally used in the determination of distribution coefficients, ignores possible effects of colon exclusion from water in the clay pack it is implicitly assumed that the water in the pack has the same coion concentration as the supernatant solution. It is possible that coion exclusion has an effect which may be significant on distribution coefficients (particularly when they are low) and on capacities computed from the results. We shall discuss this interference later in connection with the capacities measured by displacement of both cations and anions from the clay pack and associated water. Coion exclusion has been further discussed in references 1 and 7 (see also 8, 9, and 10). 

Ion-exchange capacities Measurements of distribution coefficients of both cations combined with knowledge of the concentration in the equilibrium aqueous solution should be sufficient information for computation of ion-exchange capacities. Values obtained in this way are expected to be less precise than those obtained by the more conventional method of converting the clay to a given ionic form, displacing these ions by ions of another type, and analyzing the displaced ions ("displacement"). 

Other important characterization studies are ion-exchange capacity, especially cationic, and specific surface measurement. CEC is an important parameter in clay characterization and soil behavior, because it affects nutrient availability, among other soil properties. Some clay minerals (smectites and vermiculites) have a high... 

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