preface to fifteenth edition

PRACTICAL LABORATORY INFORMATION 11.39
concentration distribution ratios are inversely proportional to the solution concentration of the resin
counterion.
To accomplish any separation of two cations (or two anions), one of these ions must be taken
up by the resin in distinct preference to the other. This preference is expressed by the separation
factor (or relative retention), K/Na , using K and Na as the example:
(D c) K kK/H
K/Na
K
(D ) k
c Na
The more deviates from unity for a given pair of ions, the easier it will be to separate them. If the
selectivity coefficient is unfavorable for the separation of two ions of the same charge, no variation
in the concentration of H (the eluant) will improve the separation.
The situation is entirely different if the exchange involves ions of different net charges. Now the
separation factor does depend on the eluant concentration. For example, the more dilute the counterion
concentration in the eluant, the more selective the exchange becomes for the ion of higher
charge.
In practice, it is more convenient to predict the behavior of an ion, for any chosen set of conditions,
by employing a much simpler distribution coefficient, D g , which is defined as the concentration
of a solute in the resin phase divided by its concentration in the liquid phase, or:
D g
Na/H
K/Na
concentration of solute, resin phase
concentration of solute, liquid phase
% solute within exchanger volume of solution
Dg
% solute within solution mass of exchanger
D g remains constant over a wide range of resin to liquid ratios. In a relatively short time, by simple
equilibration of small known amounts of resin and solution followed by analysis of the phases, the
distribution of solutes may be followed under many different sets of experimental conditions. Variables
requiring investigation include the capacity and percent cross-linkage of resin, the type of
resin itself, the temperature, and the concentration and pH of electrolyte in the equilibrating solution.
By comparing the ratio of the distribution coefficients for a pair of ions, a separation factor (or
relative retention) is obtained for a specific experimental condition.
Instead of using D g , separation data may be expressed in terms of a volume distribution coefficient
D v , which is defined as the amount of solution in the exchanger per cubic centimeter of resin bed
divided by the amount per cubic centimeter in the liquid phase. The relation between D g and D v is
given by:
Dv
Dg
where is the bed density of a column expressed in the units of mass of dry resin per cubic centimeter
of column. The bed density can be determined by adding a known weight of dry resin to a graduated
cylinder containing the eluting solution. After the resin has swelled to its maximum, a direct reading
of the settled volume of resin is recorded.
Intelligent inspection of the relevant distribution coefficients will show whether a separation is
feasible and what the most favorable eluant concentration is likely to be. In the columnar mode, an
ion, even if not eluted, may move down the column a considerable distance and with the next eluant
may appear in the eluate much earlier than indicated by the coefficient in the first eluant alone. A