Analysis of Mr. Sketch Ink by Paper Chromatography

Background  

A very approximate, but useful, description of white light such as the sunlight reaching the earth, is that the light is a mixture of light of several colors. Specifically, we can construct white light by overlapping three beams of light, one each of red, green, and blue. These are the additive primaries as shown in the figure below. Where all three overlap we see white light.

Those colors formed at the places where two light beams are overlapped are called the subtractive primaries. They are yellow (red + green), magenta (red + blue), and cyan (blue + green). Dyes of these colors are frequently used to make colored photographs and inks such as are found in Mr. Sketch pens. If a piece of yellow cellophane, for example, is placed in front of a white light source, such as a flashlight or slide projector, a beam of yellow light is formed. Since white light can be described as a mix of red, green and blue and since yellow is a mixture of red and green, but no blue, we can say that the yellow cellophane has allowed red and green light to pass through, but subtracted or absorbed the blue light.

Mixtures of subtractive primary dyes can be used to make inks of different colors. For example, it is common to prepare green ink by mixing yellow and cyan subtractive primary dyes. The origin of this color can be visualized by considering the effect of placing cyan and yellow cellophane filters in front of a white light source.

When dyes are mixed in the ink on a paper surface, light reflecting from the paper after passing through the dyes is now colored. In this example the cyan dye absorbs the red light and the yellow dye, the blue light. Only the green light is not absorbed and so the ink is seen as green.

Paper Chromatography Paper chromatography is a method chemists use to separate compounds in a mixture from one another without changing them.   In this section we shall explore how this separation is made for mixtures of different inks.  Molecules with similar arrangements of their atoms or molecular structures are attracted to each other. This is the basis for chromatography and solubility.  Water molecules have the structure shown below in which the two hydrogen atoms form a 104^o angle with the oxygen at the vertex.

Because of this structure the oxygen end of the molecule has a small negative electrical charge and the hydrogen end has a small positive charge. Liquid water is held together by the attraction between the charges on different molecules. This is shown below for a small cluster of water molecules.

A molecule with these charged regions is called a polar molecule. Methanol (CH₃OH) has a similar structure (see below), and the methanol molecules are very soluble in water because of the mutual attraction between the two polar molecules.

A more complex, yet still similar molecule is cellulose, a molecule which is the basic component of paper. It is a very long molecule (a polymer) in which thousands of rings of six atoms each are linked together like beads. A portion of a cellulose molecule is shown below.

The polar -OH regions of these molecules are attracted to OH groups on adjacent cellulose chains helping to hold the fibers together in paper. Not surprisingly, water molecules, being polar, are also attracted to these regions and when paper is wet it loses strength because the water molecules get between the cellulose chains and weaken the attraction between them.

When the end of a piece of paper is dipped into water the water molecules keep finding new places (polar regions) to stick to and so the water molecules climb up the paper being replaced by new water molecules below. Other molecules which might be dissolved in the water will also be carried along up the paper. This is applied to the separation of dyes in a technique known as paper chromatography.

A spot of dye is placed on the paper above the level of the water. As the water moves up, the dye molecules will move with the water if they are more strongly attracted to the water molecules than to the paper molecules. On the other hand, if the dye molecules are more strongly attracted to the paper than to the water, they will move more slowly than the water or perhaps, not mot even move not at all. What if the dye is a mixture? If two or more dyes have been mixed to form an ink, then they may move at different rates as the water moves up the paper. If this happens, they will separate and we can identify them . This is shown in the sketches below.

After running the chromatogram, each separated "spot" can be assigned a Retention Factor (R_F) which is characteristic of the specific dye(s) associated with it. The R_F is a ratio of the distance the "spot" travels relative to the distance the solvent (water in this case) travels. The R_F is calculated by dividing the "spot" distance, at the leading edge, by the solvent distance. This ratio should be a constant that is characteristic of the dye(s) in a particular spot under a particular set of chromatographic conditions (i.e. paper, solvent, etc.).  See below for a sample calculation.

We can use this as a tool to help answer several questions about the inks. These are listed in the Purpose section which follows.

Purpose

1. What is the origin of the colors of the ink in the Mr. Sketch pens? Is the ink made up of several dyes or just one? What color are the dyes? How do the colors of these dyes result in the color of the ink which we observe?  Be specific when constructing your description of the purpose, Identify the pens you used!
2. Does the manufacturer use the same dyes for different color inks?  If so, they will be the same color and move the same distance relative to the water (approximately the same R_F).
3. Finally, what happens if the solvent water is altered by adding to it something like table salt (sodium chloride)?  Does this cause any changes in the movements of the dyes or effectiveness of the separation method?

Procedure
Use paper chromatography to separate the dyes in the inks of the Mr. Sketch pens that are assigned to you by your instructor.   Use the method  outlined below.

1. Cut pieces of filter paper into rectangular strips about 2 or 3 cm wide and at least 11 cm long. Trim one end as shown below. Use a pencil to draw a short line about 1 cm from the tapered end. Place a small spot of ink on the pencil line.

   

2. Fold about 1 mm of the filter paper strip along the dotted line (see above) and attach enough tape to it so it can be suspended across the bottle mouth as shown below. Pour distilled water into the chromatography bottle until it is deep enough to just touch the bottom of a suspended strip of filter paper. Be sure the water does not touch the ink spot and the chromatogram does not stick to the side of the glass jar. Place the lid over the jar.

   

3. Allow the water to rise on the paper until it is about 1 cm from the top. Remove and allow the paper to air dry. You can tape it to the side of the bench to dry, keeping them in the order you did them so you can record your results and tape the chromatograms into your notebook as soon as they are dry..  Record results in your notebook (color, distances, observations, R_F)  Do a chromatogram for each of your pens..
4. Dissolve 1.0 gram of table salt in 100 ml of water. Stir this solution thoroughly.  Use this a the solvent to do chromatograms for each pen you did in the pure water solvent.

Notes

1. Be sure to put your dried chromatograms in the results section of your notebook. Label each chromatogram by number (1, 2, etc.) and identify what pen was used for each by recording both the name of the pen (for example "hot pink") and marking your notebook with that pen adjacent to the chromatogram, and what you observed on the chromatogram.  Calculate the R_F value for each dye spot and record them next to the spot in the chromatograms. You can greatly strengthen your conclusions by also making chromatograms of the pure inks which have the same color as the dyes in mixed color inks.  You can confirm the identity of dyes by comparing both color and R_F.  You can even attempt to construct an ink that matches your pen by superimposing small spots of each ink, or lining up the spots horizontally, on a paper strip and then chromatographing the set of spots as above.
   
2. In your Conclusion section use your experimental observations to briefly answer each of the six questions posed in the Purpose.