Experiment 1

Analysis of the Color of Water Soluble Inks

Background

Color

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 cyan (blue + yellow (red and green), magenta (red and blue), and 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 has subtracted or absorbed the blue light. See the figure below.

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 absorbs the blue light. Only the green light is not absorbed and so the ink is seen as green.

A more detailed description of these colors is available here.

Paper Chromatography
Molecules with similar arrangements of their atoms or molecular structures are attracted to each other. Water(H₂O) molecules have the structure shown below in which the two hydrogen atoms form a 104° angle with the oxygen at the vertex. Because of this structure the oxygen end of the molecule has a small negative electrical charge (d-) and the hydrogen end has a small positive charge (d+). Liquid water is held together by the attraction between the charges on adjacent 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 the -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. If a spot of dye is placed on the paper above the level of the water and the water moves up, it will carry with it the dye molecules if they are more strongly attracted to the water molecules than to the paper molecules. If they are more strongly attracted to the paper than to the water, they will move more slowly than the water or even 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 by their colors. This is shown in the drawings below. In this example, a small spot of green ink was chromatographed and separated into the yellow and cyan dyes which were mixed to make the ink.

A very good way to compare dyes on different chromatograms is to measure the distance each dye moves relative to the solvent. This is called the R_f value of the dye and it provides a way to judge whether two different dyes are the same. If the R_f values are close and the dyes have the same color, than they are probably the same. The R_f value is calculated by dividing the distance that the leading edge of the dye spot moved by the distance that the solvent has moved. (see figure below) Since the dye cannot move any farther than the solvent moves, the maximum value for the R_f is 1.0. This would be found if the dye did not stick to the cellulose molecules at all. If it was strongly attracted to the cellulose then the R_f would be very small because the dye would move very little compared to the solvent.

In the example above the yellow dye moved 5.8 cm and the solvent moved 8.5 cm. The R_f value for the yellow dye is 0.72. Note that there are no units and there are 2 digits (significant figures) after the decimal point. We can use these values to help answer several questions about the inks. These are listed in the Purpose section..