Black inks
Draw a line in pencil 1cm from the bottom of the chromatography paper.

 

 

Place a thick dot of ink at the centre of this line.
Select a beaker and pour in solvent liquid (methylated spirits) to a depth of 0.5cms. Place the chromatography paper into the beaker taking care not to submerge the dot in the solvent.
A number of chromatograms can be developed in one beaker as shown on the right.

The chromatorgram formed is unique for each type of ink. The individual dye components can be identified by their unique Rf values. The Rf value can be calculated as shown below.

Rf = distance travelled by the solvent("A") / distance travelled by the dye("B").

 

Example: The chromatogram of a black ink is shown on the right. Calculate the Rf value for the purple dye.

The distance travelled by the solvent("A") is 8 cm while the distance travelled by the purple dye("B") is 7 cm.

Rf = 7/8 = 0.87

The chromatograms formed can be very spectacular and provide a unique fingerprint for the type of ink used. Chromatogram "A" is that of an orange coloured ink while "B" is of a blue ink and "C" is the chromatogram of a black ink.
A letter was forged with black ink. Chromatograms revealed that the ink from the authentic letter did not match that of the forgery. Five suspects were arrested and samples of black ink taken from their premises and examined. View the results and decide which suspect needs to be questioned in more detail.
One of the five suspects claims to be the author of the original letter. Is any suspect likely to be the author? Explain.
Sample of ink from the forged letter.

Sample of ink from the authentic letter

Continue with an exercise on plant pigments

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