Adalbert Wollrab
Justus Liebig University 63 Giessen, German Federol Republik
1
I
A Simple GPS Chromatoqroph - for koching P U ~ ~ O S S
Gas chromatography is one of the most efficient methods used for separation and identification of organic substances. The importance of this method in industry and research is so great, that it is not only dealt with in lectures and courses a t universities, but has also been integrated into chemistry classes a t secondary schools. A simple gas chromatographic apparatus is described and an experiment is given which is easy to execute with this equipment. The demonstration can be carried out at a university as well as at any secondary school. The simplicity of both the device and of the experiment itself makes the process quite plain. The necessary materials for this specific apparatus and the experiment were chosen because they can easily be ohtained. The functions of the apparatus and its parts are illustrated in Figure 1. Hydrogen is passed from a gas cylinder (A) through a reduction valve (B) into a thick-walled rubber tubing (C), whereupon the gas stream passes through a column (D)which is about 25-30 in. long, has a diameter of % in. and is packed with salt coated with a layer of paraffin oil. The gas then flows through a rubber connection joint (E) into a short glass tuhe with a capillary (F) made by drawing out a piece of tubing. The capillary has a diameter of about 0.02 in. The flow rate is regulated a t the reduction valve within the range of 150-250 ml/min. The hydrogen is lit' a t the outlet of the capillary. The hydrogen flame usually reaches a height of 0.3-1 in. A hypodermic syringe filled with 0.1 ml of pentanelhexane mixture (1:5) is injected into the vacuum tube, so that the needle punctures the tube close to the opening of the glass tuhe. The efflux of pentane and hexane after the completed separation enlarges and intensifies the flame. The time course of such a separation is shown schematically in Figure 2. The applied conditions were as follows: 69 g sodium chloride coated with 1% viscous paraffin oil (density 0.88), granular size of the sodium chloride 100-500 p , flow rate 240 ml/min, temperature 20°C. The measuring of the gas flow is done by introducing the gas into a graduated cylinder full of water, which is turned upside down in a pneumatic trough also filled with water. The exact timing is measured by means of a stop watch. Coating of the Sodium Chl6ride
Sodium chloride of 50-500 p granular size (salt consisting of larger particles should be ground in a mortar for 4-5 min) is coated with viscous paraffin (density 0.88). A better, but more expensive solid support is Chromosorb G 60180 mesh size (Fa. Varian aerograph, Walnut creek, California), instead of salt. When using sodium chloride, the coating is I%, in case of Chromosorb G it amounts to 3%. The coating procedure is as follows: 1g of viscous paraffin is mixed with 30 ml chloroform, the liquid is poured
200
/ Journal of Chemical Education
u Figure 1. Parts of thechromatograph
Figure 2. Gas chromatograficseparation of a pentane/hexane mixture
onto 100 g of sodium chloride, contained in a 250-ml beaker. The salt is thoroughly mixed with the liquid and placed in a large Petri dish (10-15 in. in diameter). The mixture is stirred and blended with a spatula in the fume cupboard until the salt no longer gets lumpy and looks dry. The dish is left in the fume cupboard overnight. The Packing of the Column
A glass tube with a diameter of approximately U in. and 25-30 in. long is closed with a cotton wool wad at one end; then the coated salt is filled into the glass tube with a funnel. In doing so the glass tube may be lightly tapped on a rubber surface to achieve a firmer packing. After filling the tube its other end is also provided with a cotton wool wad so that the material cannot drop out. 'Standard precautions (oxyhydmgen test) should he taken when lighting the hydrogen jet.
