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18-7 - i-lr-t-t t H-H t-l-t-t-t-Hit-t t-t l y t r t t t t 1...

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Unformatted text preview: i-lr-t-t t H-H t-l-t-t-t-Hit-t t-t l y t r t t t t 1. ti r i i't-i r 'r 'r-h-i-t _i___ Paper and Liquid Chromatography Chapter IS Figure |3.2 The Cellobiose Repeating Unit in Cellulose The hydroxyl groups [-OH] in the cellulose are responsible for the hydrogen bonding of water. which makes up about 6% of the weight of the paper. It is this water layer, along with more water that is sometimes adsorbed during the chromatographic process. that forms the stationary phase in PC. Chromatography paper is unsizod and is carefully manufactured to produce a highly porous paper with relatively uniform fiber strucnu'e. If the edge of a sheet of chromatography paper is placed in a liquidt the liquid will be pulied through the paper by capillary action. The liquid moving through the pores in the paper constitutes a mobile phase. and chromatography can occur. The mobile phase may be pulled along, up, or donut. depending on where the liquid is initially applied. Let us now look at the factors that control the way in which components move in a paper chromatographic separation. Considers single nonelocn'olyte component applied to the paper at the start [see Figure 18.3). Volume: element in MP h'lob'ilvc phase-direction _— cellulose "’0 m PM" Figure Ill} Distribution of a Single Nonelectrolyte Component in PC The component molecules are distributed by partition between the aqueous stationary phase and the uonpolar nubile phase. The partition coefficient k is defined as the ratio ofthe concentration of the component in the mobile phase to that in the stationary phase. In the above diagram. k = 2 because I volume of the mobile phase contains 2 molecules. and an equal volume of stationary phase contains 1 molecule. In practice. however, the volumes of the two limlid phases are not equal. For many chromatography papers the mobile phase volume is 3 times the stationary phase volume, and the phase ratio r is 13-? ...
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