PAPER CHROMATOGRAPHY
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Stationary Phase

Paper chromatography uses paper as the stationary phase. The exact type of paper used is important. Filter paper is one of the best types, although paper towels and even newspaper can also be used. Writing paper is coated so that ink does not run and because of this is less satisfactory. Of course, wax paper, not being absorbant, is unsatisfactory.

Mobile Phase

The mobile phase used in this experiment is called "eluting solution" and consists of a mixture containing Ethyl alcohol, Butyl Alcohol, 6 M HCl and acetone. It is introduced at the bottom of the chromatogram and allowed to move upwards by capillary action.

Spotting the Chromatogram

You will use solutions of Ag+, Co+2, Cu+2, Fe+3, Hg+2, a KNOWN mixture (containing all five ions) and an UNKNOWN.

Using a pencil draw a line 1.5-2 cm from the bottom.
Divide this into eight segments and label each. You should have one each for the five single ions, one for the KNOWN mixture of the fivew ions and two for the UNKNOWN.

Paper ready for spotting


Using a glass capillary tube, introduce a spot of each chemical about .5 cm in diameter.
These eight spots are put on the spotting line in the properly labeled segment.
 

Using Capillary tube in spotting

Double Spotting.... The KNOWN and the UNKNOWN probably contain less of each ion than the corresponding single ions. This is because they were prepared from the single ion solutions. Therefore these spots are "double spotted".
That means, a spot of each of these is applied and allowed to dry. Then a second spot of the same material is applied directly on top of the first now dry spot.
Allowing the first spot to dry before respotting allows the concentration of the spot to be increased without the spot size increasing.

Duplicate SPOTS.... The UNKNOWN is spotted in two separate places. Duplicate spots allow confirmation. Note the single ion spots are confirmed by the spots in the KNOWN

ELUTING
The separation of the mixtures takes place at this step.

A small amount of Eluting Solution is added to a covered container and the chromatogram dipped into this.
In our experiment, the chromatogram is curled into a cylinder and taped at the top. The cylinder formed in this way has a small gap between the left and right sides of the chromatogram. This prevents solvent or compound from crossing over.
The chromatogram is placed in a 600 mL breaker containing about 15 mL of the eluting solution. The top is sealed off with plastic wrap held in place with a rubberband. Sealing the container is important to prevent evaporation of the eluting solution from the paper.
Once elution starts, do not shake or disturb the system.
Stop the elution when the solution has come to within 2-4 cm from the top of the chromatogram.

DEVELOPING..... Immediately after removing the paper from the eluting tank, mark the furthest advance of the solvent. This is the Solvent Front line.
Allow the solvent to dry for a few minutes and mark any spots which you can see. At this point, marking can be done with either pen or pencil.
Label these as the spots you see on the "dry" chromatogram.

Every time you see a single ion spot, or see it change, check the KNOWN and UNKNOWN for a corresponding spot or change.

Chromatogram after removal from eluting tank. Note spotting line

NH3..... Ammonia is a basic gas which forms colored ammine complexes with several of the ions.
The purpose of exposing the chromatogram to ammonia is two fold.
  1. To neutralize the excess HCl.
  2. To develop characteristic colors.
H2S.... Hydrogen sulfide is a foul smelling gas which forms sulfides with metallic ions. Most of these are colored. The colors tend to be dark or dirty looking.
After you chromatogram comes out of the H2S developing tank, all known ions should be visible. At this point check to make sure that all spots of the known are visible.

MEASURING the Retardation Factor Rf

RETARDATION FACTOR Rf

Rf= distance spot traveled/distance solvent traveled

Distance is always measured from the spoting line to the spot and again from the spotting line to the solvent front line.
In this chromatogram, the copper(II) ion moved 3.05 cm while the solvent moved 5.33 cm.

Rf = 3.05 cm/ 5.33 cm = .572

Note the distances traveled have units of length. In this experiment you will use cm.
You will measure the distances using a ruler and reading to between the smallest markings on the ruler. On most metric rulers the smallest units are mm. Remember sig figs are not arbitrary but reflect the accuracy of the measurement or calculation.

Several questions.....

  RWK 1997

  1. What are the units of Rf?
  2. What ions does the UNKNOWN contain?
  3. What do green egg yokes and tarnished silver have to do with this experiment?

These questions, like SCIENCE itself, have....    ANSWERS

 

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