Chromatography A technique for separating and identifying components in a mixture. From the Greek: chroma = color and graph = writing. Involves the interaction of the components with 2 phases: Stationary phase: Does not move Mobile phase: moves through the stationary phase Separation and therefore identification depends on the relative attractions of each component for the 2 phases.

Strong attraction to the stationary phase = not move far. Strong attraction to the mobile phase = faster movement. Separation is a function of either partition or adsorption. Partition: Depends on differing solubilities of components in the mobile phase. Stationary phase is generally liquid. Adsorption: Relies on how different substances bond to the surface (adsorb) of the solid stationary phase. Components may be identified by determining their R f values. R f = Distance traveled by the component Distance traveled by the mobile phase Paper Chromatography - Based on partition (See Lab Handout – Introduction to Chromatography) In certain situations the separated components may not be visible and must be treated with a dye or UV light.

Thin Layer Chromatography (TLC) Similar to the idea of paper C except depends on adsorption. Stationary phase is a thin layer of silica (SiO 2 ) or alumina (Al 2 O 3 ) on glass or plastic. Depends on the extent to which the components bond to the stationary layer. Basically a function of polarity. Components still determined by R f, but since particles are finer than pores in paper, it usually gives better separation. This makes it more suitable than paper chromatography for collecting samples for further analysis. Common uses include detecting amino acids in a mixture & separating (identifying) components in dyes.

The mixture and solvent (eluant) are applied at the top of the column and allowed to flow down thru the packing and drip out through the opened tip. The components separate and each is collected as the eluate comes out of the tip. Each may be identified visually if colored or by chemical analysis if not visible. Also based on adsorption. A column (looks like a wide buret) is packed with alumina/silica powder. Column Chromatography

Heterogeneous(?) Aqueous Systems Two types: 1. Suspension - a mixture from which particles settle out -suspension differs from a solution because the particles of a suspension are much larger & do not stay suspended (> 1000 nm) - Particles are too large to pass through filter paper - Ex. – muddy water, gravy 2. Colloid -a mixture containing intermediate-sized particles larger than those in solutions but smaller than those in a suspension ( nm) - Particles are too large to pass through SP membrane. - Ex. – glue, paint, smoke, milk, fog

Tyndall Effect The scattering of visible light by colloidal particles is called the Tyndall effect. Good test to distinguish between a solution and a colloid.

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