Tank Chromatography.

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Lee Bali - Chromatography

Tank Chromatography

Introduction:

Chromatography is a method used to separate both organic and inorganic compounds so that they can be analysed. Scientists often use chromatography to figure out which basic constituents makes up a specific mixture. Chromatography relies on the principle of selective absorption.

A botanist named M.S. Tswett in 1906 first discovered chromatography. Tswett used a method called ‘column chromatography’ to separate chlorophylls by pouring petroleum-ether extract of green leaves into a column of powdered calcium carbonate. The compounds slowly move downwards through the glass column and each compound of the chlorophyll is absorbed into beds at different heights depending on their individual absorption affinity.

Forensic scientists heavily rely on chromatography currently when solving crimes. It is also used for various purposes such as determining the presence of cocaine in urine, alcohol in blood and how much lead there is in water. Each of these are very important uses and chromatography is the method chosen as it is very reliable and easy to perform.

Aim:

Our aim was to use paper chromatography to separate various inks (both permanent and water based) into their core pigments. We could use the formula Rf = the distance moved by each solute (measured to the centre of each solute stain) / the distance moved by the solvent. This would give us the Rf values for each pigment in the ink. These Rf values should be constant for each individual pigment.

Apparatus:

  • Chromatography Paper
  • Three permanent pens (Red, Green, Blue)
  • Three water based pens (Red, Green, Blue)
  • Chromatography tank pre-filled with acetone
  • Drying Rack
  • Pencil (for various markings)
  • Scissors (used for serration)
  • Ruler (used for all measurements)
  • Bag Ties

Method:

The base of the chromatography paper was marked and then cut to form a serrated edge. This edge was made to ensure that there was a controlled flow of acetone up the chromatography paper. If a full edge was placed in the acetone, the chromatography paper would absorb the acetone too quickly and we would run the risk of the different pigments blending and smudging.

After the edge was serrated, a line one cm. up was marked across the paper and twelve ink spots were placed across the line at equal intervals. These spots were placed above the ‘points’ of serration

(see Dia. 1) and were in the order of:

Permanent Red, Water Based Red, Permanent Green, Water Based Green, Permanent Blue, Water Based Blue and then a repeat of these colours in the same order.

The repeat was made due to the width of the paper and therefore gave us a repetition of results, enabling us to make averages of the two to give more reliable end results.

Once the initial ink spots were placed, these were allowed to dry for 10 seconds and then another spot of the same colour was placed on top and was allowed to dry once more. This was repeated several times to build up the concentration of the ink at the specific point and saturate the paper, which would inevitably improve the visibility of the results. The spots were continually allowed to dry between dabbings so that the paper would not absorb the ink and cause the size of the ink spot to increase greatly which would make the results anomalous.   

Once the line of ink spots was complete, the chromatography paper was tied to another piece of identically set up chromatography paper and the glass lid of the chromatography tank was removed.  The two sheets of paper were then hung over the frame inside the chromatography tank (see Dia. 2). The tips of the serrated edges were allowed to hang inside the acetone in the bottom of the tank and left to absorb the acetone. A great deal of care was used to make sure the chromatography paper wasn’t touching the inside of the chromatography tank, as this would spoil the experiment. The edges of the lid were covered in petroleum jelly to form an airtight seal with the ground glass edge of the tank, the lid was then replaced to ensure a constant environment and to enable the vapours of the acetone to saturate the air within the tank.

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The tank is then left for twenty-five minutes/so that the acetone has reached roughly ¾ of the way up the paper and the pigments have been separated. At this point we had to extract the paper from the tank and leave it until it had completely dried.

Once the paper was dry, a series of measurements (see Dia. 3) was then taken. Using a ruler to measure from the starting line, the distance of the solvent front and then various distances of the centre of each pigment stain were then noted.

As the solvent front was not ...

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