Illustrated Guide to Home Chemistry Experiments

110 DIY Science: Illustrated Guide to Home Chemistry Experiments

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As you are waiting for the strips to develop, you can

start work on Part II. Keep a close eye on the strips,

though. If the solvent front reaches the top of the strip,

that strip is ruined, because an accurate Rf value can no

longer be calculated.

concentrated as possible. Choose one of the pens and

touch it to the center of the pencil line on strip A. Allow

the paper to absorb ink until there is a spot 2 to 3 mm in

diameter. Remove the pen, and allow the ink spot to dry

completely while you spot strips B and C with the same

pen (one strip for each of the three solvents). Return to

strip A and place the tip of the marker pen in the center

of the existing ink spot. Allow the paper to absorb more

ink without significantly increasing the size of the spot.

Retreat strips B and C similarly, and repeat this process

until each strip has been treated at least five times.

Repeat this procedure using the second marking pen for

strips D, E, and F, the third marking pen for strips G, H,

and I, and the fourth marking pen for strips J, K, and L. At

the end of this procedure, you have 12 strips, 3 marked

with each pen: the black water-soluble pen, the brown

water-soluble pen, the black permanent pen, and the

brown permanent pen. Record the analyte (pen type)

used for each strip in Table 6-5.

5. Attach a paper clip to the labeled end of each strip.


6. Thread the stiff wire through the paper clips for strips A,
   D, G, and J (one strip of each marking pen type). Suspend
   those four strips in the first chromatography jar. The
   bottom of each strip should be immersed about 2 cm
   into the solvent. Make sure that the spot on each strip
   remains well above the level of the solvent. Repeat this
   procedure to suspend strips B, E, H, and K in the second
   chromatography jar, and strips C, F, I, and L in the third
   chromatography jar.


7. Observe the rate at which each solvent is drawn up the
   paper by capillary action. Different solvents and different
   types of paper progress at different rates. Typically, it
   might take from five minutes to one hour for the solvent
   front to approach the top of the strip. If the solvent front
   stops climbing before it reaches the top of the strip, that
   means that the capillary action and evaporation rate have
   reached equilibrium. Stop the process for strips using
   that solvent when the solvent front reaches that level.
   Otherwise, allow the solvent front to approach (but not
   reach) the top of the strip.


8. When the solvent front approaches the top of a set of
   strips (or stops climbing), immediately remove those
   strips from the chromatography jar. Use the pencil to
   make a small mark at the highest point that the solvent
   front reached on each strip. Suspend the strips and allow
   them to dry thoroughly.


9. For each strip, measure the distance between the pencil
   line and the maximum height reached by the solvent front.
   Record that value on the strip itself and in Table 6-5. Also
   measure the distance between the pencil line and any
   significant color spots on the strip—measure from the line
   to the middle of the spot—recording their descriptions,
   distances, and Rf values in Table 6-5.

Figure 6-6 shows five example chromatograms. None of these

show particularly good separation of the components, but all

illustrate some important issues.

From the left, the first two chromatograms are of black and blue

permanent markers, using acetone as the solvent. (Neither

dye was at all soluble in water.) No separation is evident, which

almost certainly means that these pens use a single dye. (It’s

just possible, although very unlikely, that two or more dyes were

used that had identical characteristics with this substrate and

solvent, in which case there would also be no separation.) Clearly,

the blue dye bonds very tightly to the acetone solvent, and quite

loosely to the paper substrate, which accounts for its very sharply

delineated spot. Conversely, the black dye, although readily

soluble in acetone, clearly also bonds more tightly to the paper

substrate, accounting for the strung-out spot it produces.

The third and fourth strips were spotted with green food coloring

dye, which is clearly a mixture of dyes. The third strip used

the nonpolar acetone as a solvent. The blue component of the

green food coloring dye obviously binds tightly to acetone and

reasonably tightly to the paper substrate. The yellow component

FIGURE 6-6: These chromatograms show the separation of

the ink and food coloring components with polar (water) and

nonpolar (acetone) solvents