Separation optimum range

Thirty grams of sodium. pentacyanonitrosylferrate (II) 2-hydrate (No. 74) are covered with 120ml of ice-cold water in a 250ml suction flask with a gas-inlet tube and a thermometer. The flask is cooled in an ice-salt bath while a steady stream of ammonia (3 bubbles/sec) is led in under the hood. Csre must be taken that the temperature does not rise above 20°C during this time because decomposition would occur the optimum range is 8-12°C. When no more gas is absorbed at this temperature, (indicated when the level of the liquid fails to rise in the inlet tube as the current of gas is interrupted), the dark yellow-brown solution is allowed to stand at 0°C for about 2 days. The amber-colored crystalline product separates with attendant evolution of gas,... 

Because of the occurrence of the excess quantities hEand sE in eqn.(3.9), the coefficients in eqn.(3.10) for the temperature dependence of the retention are a function of the stationary phase. Hence, every stationary phase may be expected to yield a different optimum temperature, at which the capacity factors of all sample components fall in the optimum range. Therefore, to make a fair comparison between two different stationary phases for a given separation problem, the (potentially different) optimum temperature should be established for each of them and the resulting chromatograms should be compared. The common practice of characterizing (and consequently comparing) stationary phases at a standard temperature is a very convenient one. Nevertheless, it may give rise to erroneous conclusions in some cases. 

The next most important factor is to bring the capacity factors into the optimum range. At the same time or immediately thereafter, we should try to optimize the selectivity (a). Both are very important stages in the method development process, because no separation will be obtained if either k=0 or a= 1 (see section 1.5), no matter how efficient the column and how good the instrument. Very large k values should also be eliminated at this stage, because of both time and sensitivity considerations (see e.g. figure 6.1b). 

Most separation processes are efficient only under certain conditions of feed solution concentration and required product quality. Therefore various separation processes are often combined, each operating its optimum range of application. A typical example for the combination of different processes is the combination of conventional ion-exchange process with electrodialysis. There are however, other combinations such as electrodialysis and reverse osmosis which are of growing technical and commercial interest. 

Water contents of 1-20% in the feed and 0.1-1% in the product define the optimum range for pervaporation in its present development (Fig. 7.10). Care must be taken to avoid feeding solutions which deposit material on the membrane surface when water is separated from the solvent. 

The optimum range of initial oxidation potential of the solution is 480-560 mV (versus SCE). The holding time of solution before casting may be associated with the degree of polymerization of pyrrole and the extent of phase separation. 

---