Exchangeable cations measurement methods

Roots have cation exchange properties that correspond closely to those exhibited by soils. They are believed to act as amphoteric colloids and absorb cations from the plant culture solution or soil. Cations are held by the roots when they are placed in water but may be set free when other cations are added to the culture medium. They are also readily removed by electrodialysis, and this serves as a convenient way of measuring root CEC. The accuracy of this method has been questioned because of the uncertainty as to whether this procedure removes all of the exchangeable cations, and only these. Furthermore, some organic substances may also be removed, leaving hydrogen ions adsorbed on the root surface. 

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"). 

The measurements of the rates of these reactions are carried out by a variety of methods, some very sophisticated, depending on the magnitude of the exchange rate. Such methods demonstrate that these reactions are first order in the concentration of the original hydrated cation, M(H20)6 that is, the rate law for these reactions is as given in Equation (5.30) ... 

Some properties of the rock used in this study were measured The cation exchange capacity (cec) was determined by the barium sulfate method as described by Mortland and Mellor (33). Surface area was measured by using a Digisorb Meter (Micromeritics Instrument Corporation) through nitrogen adsorption. Estimation of mineral composition and indentification of the rock were performed by X-ray diffraction. 

This method does not measure exchangeable protons attached to cation exchange sites therefore, it is also common to use a salt solution (either KC1 or CaCl2) instead of distilled water in determining soil pH. The K+ or Ca2+ in the solution exchanges with exchangeable hydronium, thus bringing it into... 

Finally, another possibility often discussed in the literature is that cation dopants from the electrode may enhance the electronic conductivity of the gas-exposed surface of the electrolyte in the vicinity of the TPB, thereby extending the reduction zone along the electrolyte surface via mixed conduction. The surface exchange rate of oxygen on both YSZ- and rare-earth-doped ceria (as measured by isotope methods) is only about 1 order of magnitude lower than on LSM at 700 Thus, if there were sufficient... 

The TEB value may be obtained by either the sum of the individually measured cations or by evaporating and igniting a portion of the ammonium ethanoate leachate to convert the metallic cations to oxides and carbonates, followed by addition of excess acid (to convert carbonates to chlorides) and back-titration with alkali. The latter method is difficult if the soil is insufficiently base-rich to provide an adequate amount of bases for the titration. On the other hand, the calcium carbonate in calcareous soils may be partially leached by the ammonium ethanoate at pH 7.0 in addition to the exchangeable bases and thus give an exaggerated TEB value and a percentage base saturation in excess of 100%. The TEB by ignition/titration can serve as a check on the values from the summation method. 

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