Solute capacity factor experiment

The choice of variables remaining with the operator, as stated before, is restricted and is usually confined to the selection of the phase system. Preliminary experiments must be carried out to identify the best phase system to be used for the particular analysis under consideration. The best phase system will be that which provides the greatest separation ratio for the critical pair of solutes and, at the same time, ensures a minimum value for the capacity factor of the last eluted solute. Unfortunately, at this time, theories that predict the optimum solvent system that will effect a particular separation are largely empirical and those that are available can be very approximate, to say the least. Nevertheless, there are commercially available experimental routines that help in the selection of the best phase system for LC analyses, the results from which can be evaluated by supporting computer software. The program may then suggest further routines based on the initial results and, by an iterative procedure, eventually provides an optimum phase system as defined by the computer software. 

Three experiments are in principle sufficient to establish the three coefficients in eqn.(3.70) for a given solute. In practice this is only true if the three experiments are taken at such values of the pH (relative to pKJ that a sensible estimate of all three coefficients can be made. This implies one experiment within one pH unit of the pKa value, one experiment at a higher and one at a lower pH. If the pKa value of a solute is known, then the retention behaviour can be estimated from a minimum of two experiments. Another way to reduce the minimum number of required experiments is to assume a negligible capacity factor for the charged species. Of course, once more experimental data points become available initial assumptions about the value of any of the coefficients in eqn.(3.70) can be abandoned. 

It is essential to start a series of such scouting experiments under conditions at which very low retention may be anticipated for all solutes. In this way, no late eluting peaks will be overlooked. It is much more practical to increase short capacity factors than it is to decrease large ones. 

Fig. 7,2. Solvent effects in reversed phase chromatography. The individual terms of the solvophobic equation are plotted as the function of the composition of the water-acetonitrile eluent. The solute is undissociated toluic acid. The data was gathered using an ODS column at ambient temperature. The logarithm of the capacity factor on the ordinate can be considered as a dimensionless energy appropriate at the temperature of the experiment. Reproduced from Horvath and Melander (1978), with permission.

The effects of liquid and gas flow rates for this process are given by Equation 3-13 using the exponents for liquid-film-controlled systems. For an unpromoted 25 wt % K2CO3 solution at a liquid rate of 40 gpm/ft with a gas capacity factor (Fg) of 0.94 Ib - /ft s, industrial experience indicates that the overall K a values will be approximately one-sixth of the values for the standard C02/Na0H system given in Tables 3-3 through 3-6. These values apply for 20% conversion to bicarbonate in the solvent at a temperature of 235""F. The K a value decreases as the percent conversion to bicarbonate increases at the rate of K a oc 1.207 — 1.08 B, where B is the mol fraction of K2CO3 present as bicarbonate. 

The time taken to add Al2(S04)3 is a very important factor in process scale-up. To find this relationship, four experiments were run with four different dosage times 2, 10, 20 and 40 sec. Statistical analysis of the data showed that the dosage time, while the solution is not mixed, does not influence the exchange capacity of precipitated aluminium hydroxides. 

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