Paper chromatography is an analytical method that is used for separating and identifying mixtures of substances into their smaller parts. Paper chromatography works because the ink used contains several dyes (pigments) that when place on porous paper are dissolved in a solvent by capillary action. When the pigments (solutes) are dissolved in the solvent (water & alcohol mixture) they move through the paper at different rates depending on their solubility.
The more soluble the pigments are the faster they will move through the paper. Paper chromatography is most commonly used to separate pigments, dyes and inks. In forensics, paper chromatography is used in crime scene investigation and in sequencing DNA/RNA. This method can also be used to test the pH of a solution, by using pH strips. Chemist, researchers, pharmacy technicians, and pool maintenance workers are just a few professionals that use paper chromatography on a daily basis. Purpose In this lab you will learn how to separate a sample of inks into their basic colors.
So, your first task is to cut the coffee filter into a rectangle measuring three cm by nine cm. You will need two of these strips to complete the lab. Using a pencil (do not use a pen), draw a line one cm (1/2 inch) from the edge of one end of one strip of paper. Make four pencil dots (one for each marker you will be examining) along this line, about 0. 5 cm (1/4 inch) apart. Underneath each dot, label the marker that will be tested. You won’t have space to write the whole color or brand name, so try abbreviations.
Use the markers to draw a different colored dot on each of the appropriate pencil marks on the paper strip. Allow the ink to dry, and then go back and add more color to each dot. Before you complete the rest of the lab, make a prediction of the pigment components you expect to see from each marker’s ink. Prepare the salt solution by mixing 1/8 teaspoon of salt and three cups of water in a clean pitcher or 2-liter bottle. Stir or shake the solution until it is dissolved. This will produce a 1% salt solution. Pour the salt solution into a clean tall glass so that the liquid level is 1/4 inch (0. 5 cm).
Tape the strip to a pencil and rest the pencil on top of the jar so that the strip hangs into the jar. The goal is to have the end of the chromatography strip just touching the surface of the solvent solution, with the colored dots above the surface of the liquid. Make sure that the colored spots do not come in direct contact with the liquid in the bottom of the glass.
Capillary action will draw the salt solution up the paper. As it passes through the dots, it will begin to separate the dyes. When the salt water is 1/4 inch (0. 5 cm) from the top edge of the paper, remove it from the glass and place it on a clean, flat surface to dry. Repeat steps two through nine (using the second strip of paper and a clean glass) to test the same four colors using the alcohol instead of the saltwater solution at the bottom of the glass.
The proposed hypothesis was correct. The paper chromatography did show that black ink could be separated into various colors. The black ink gets its color from a mixture of various colored inks blended together. The first color of ink to appear on the filter paper was red, blue, green, then black.
The colors separated the way they did because of the differences in their molecular characteristics, specifically, their solubility in water and their rate of absorption by the paper. The most soluble and readily absorbed ink color was the red. The least soluble and least absorbable ink color was the black. Alcohol works better as a solvent than salt water because it is a pure solvent, whereas salt water is a mixture and not pure. The amount of time will determine how far the pigments will migrate from the frontline.
If the time is reduced then the migration rate will also slow down, the darker pigments will not separate. Possible errors could include inaccurate measurements of the distances traveled by the inks and mistakes when calculating the ratio traveled by the water and colors. If a longer test tube was used, a longer strip of filter paper could have been used. This may have changed the ratios. Another color may have been present, but not detected because of the filter paper length.