Materials Included In Kit

Additional Materials Required

Prelab Preparation

Chromatography Solvent: Measure 420 mL of acetone and add it to a 1-L flask. Carefully measure 140 mL of 6 M HCl and add it to the acetone in the flask. Stopper and invert to mix.

Unknown Solution: Measure 10 mL of nickel chloride and add to a 50-mL beaker or flask. Add 10 mL of copper chloride to the flask and mix. The unknown solution may include any mixture of the metal in solutions using a 1:1 ratio.

Safety Precautions

Concentrated ammonium hydroxide is severely corrosive and its vapor is extremely irritating—especially to the eyes and the respiratory tract. It is toxic by ingestion and inhalation and a serious respiratory hazard. Work with ammonium hydroxide in a fume hood or a well-ventilated lab only. Cobalt chloride solution is moderately toxic by ingestion. Copper(II) chloride and nickel chloride are also moderately toxic. Avoid contact of all chemicals with eyes, skin and mucous membranes. The chromatography solvent is a mixture of acetone and hydrochloric acid. It is a corrosive and flammable liquid. Do not use the solvent in the presence of flames, heat or other ignition sources. Wear chemical splash goggles, chemical-resistant gloves and a chemical-resistant apron. Remind students to wash their hands thoroughly with soap and water before leaving the laboratory. Please review current Safety Data Sheets for additional safety, handling and disposal information.

Disposal

Please consult your current Flinn Scientific Catalog/Reference Manual for general guidelines and specific procedures, and review all federal, state and local regulations that may apply, before proceeding.The paper chromatograms should be air dried and disposed of in the trash according to Flinn Scientifc Disposal Method #26a. The cobalt chloride, copper chloride, and nickel chloride solutions may be stored in their original bottles and used again from year to year. If necessary, contact a licensed removal company according to Flinn Suggested Disposal Method #27f for hazardous waste disposal of unused cobalt chloride and nickel chloride. Excess hydrochloric acid may be neutralized with base according to Flinn Suggested Disposal Method #24b. Copper chloride solution may be disposed of according to Flinn Suggested Disposal Method #26b.

Lab Hints

• Enough materials are provided in this kit for 30 students working in pairs or for 15 groups of students. Both parts of this laboratory activity can reasonably be completed in one 50-minute class period. The pre-laboratory assignment may be completed before coming to lab, and the post-lab analysis may be completed the day after the lab.
• In this activity, it is extremely important that the initial sample spots are as small as possible. If the spots are too large or if there is too much material (metal ion) on the initial spot, the students may only see a streak of color up the entire chromatogram.
• Allowing enough time for the development of the chromatography paper is critical. The chromatography paper must be left in the chromatography chamber long enough for the solvent to be drawn up near the top of the strip. Do not stop the development until the solvent front nears the top of the strip. Underdevelopment will lead to incomplete separation and poor resolution. Do not allow the solvent front to move off the paper, however.
• Solvent used for development can be reused. Leftover chromatography solvent can be saved and used by another class.
• Ammonium hydroxide is used to develop the colors of the metal ions. NH₃ forms colored complex ions with copper and cobalt and reacts with the HCl in the mobile phase forming NH₄Cl. The nickel spot only becomes visible after addition of sodium sulfide, which reacts with nickel to produce black NiS. The other metal ion spots may also be treated with Na₂S to give dark sulfides.

Answers to Prelab Questions

1. Figure 1 is a sample chromatogram. Label the drawing with the following items: the stationary phase, the mobile phase, and the solvent front. See diagram in Question 2.
2. Calculate the R_f value for the spot in sample B using sample A as an example.
   {12502_Pre-Lab_Figure_6}
3. Sample C gives two spots on the paper chromatogram. What does this tell you about the composition of the sample?

   Sample C is a mixture of at least two components, A and B.

Sample Data

Answers to Questions

1. Based on the information in the data table, which ions are present in the unknown?

   Copper and nickel

2. Why is a pencil used to draw a line on the chromatography paper rather than a pen?

   A pencil is used to draw on the chromatography paper because the ink from the pen will run with the solvent and separate into different pigments. Pencil will not be affected by the solvent.

3. Why is it necessary to keep the solvent level in the beaker below the spots marked on the chromatography paper?

   If the solvent level is above the spots marked on the paper, the samples will dissolve in the solvent and diffuse across the entire paper.

4. The solvent rises in the paper by capillary action. What is capillary action? What are some examples of capillary action in nature?

   Capillary action is the rise of a liquid in a narrow tube or fiber, such as the paper fibers. Examples of capillary action in nature include the transport of water in plants.

References

DelMarVa Survival Training. Separation of a Mixture by Paper Chromatography.
http://survival-training.info/Library/Chemistry/Chemistry.htm (accessed May 2011).

Introduction

Chromatography is used to separate compounds for identification. In this laboratory activity, an unknown metal ion will be identified by analyzing a chromatogram of various metal ion compounds.

Concepts

• Chromatography
• Qualitative analysis

Background

Chromatography comes from the Greek words chroma (color) and graphein (to write). Chromatography is a general method of separating substances based on their interactions with two phases of matter—a mobile phase and a stationary phase. The separation process is possible because the components of a mixture may have different affinities for the two phases; therefore they move through the system at different rates.

The simplest chromatographic system used is paper chromatography. The stationary phase is the paper and the mobile phase is a liquid solvent. Different substances will have various degrees of attraction with the paper and the solvent. Through these different interactions, the compounds or ions in a mixture will move different distances along the chromatography paper. A sample is placed on the paper, which is then placed into the solvent. The solvent moves up the paper by capillary action. The separation occurs as the solvent moves along the paper carrying components which have low affinity for the paper and leaving behind components with higher affinity for the paper.

Substances separated by chromatography may be characterized using a constant known as the retention factor (R_f). The R_f value is a characteristic property of each species in a mixture for a particular or specific type of chromatography, that is, a unique stationary and mobile phase. In paper chromatography, the retention factor measures the fraction of the distance each species travels, relative to the distance the solvent travels. The following equation is used to calculate the R_f value:

Experiment Overview

In this experiment, an unknown will be identified through the analysis of paper chromatography. The components of the chromatogram will be copper chloride, cobalt chloride, and nickel chloride. The metal ion spots will be “developed” or visualized using ammonia followed by sodium sulfide.

Materials

Prelab Questions

1. Figure 1 is a sample paper chromatogram for three samples A, B, C. Label the drawing with the following items: the stationary phase, the mobile phase and the solvent front.
   {12502_Pre-Lab_Figure_1}
2. Calculate the R_f value for the spot in sample B using sample A as an example.
3. Sample C gives two spots on the paper chromatogram. What does this tell you about the composition of the sample?

Safety Precautions

Concentrated ammonium hydroxide is severely corrosive and its vapor is extremely irritating—especially to the eyes and the respiratory tract. It is toxic by ingestion and inhalation and a serious respiratory hazard. Work with ammonium hydroxide in a fume hood or a well-ventilated lab only. Cobalt chloride solution is moderately toxic by ingestion. Copper(II) chloride and nickel chloride are also moderately toxic. Avoid contact of all chemicals with eyes, skin and mucous membranes. The chromatography solvent is a mixture of acetone and hydrochloric acid. It is a corrosive and flammable liquid. Do not use the solvent in the presence of flames, heat or other ignition sources. Wear chemical splash goggles, chemical-resistant gloves and a chemical-resistant apron.

Procedure

1. Prepare the chromatography chamber by adding 35–40 mL of solvent to a 600-mL beaker. Cover with a piece of aluminum foil.
2. Obtain a piece of chromatography paper that is about 20 cm x 11.5 cm.
3. With the paper lying as shown, use a pencil to draw a line 1.5 cm from the bottom of the paper (see Figure 2).
   {12502_Procedure_Figure_2}
4. With the paper lying as shown, use a pencil to draw a line 1.5 cm from the bottom of the paper (see Figure 2).
5. Using a pencil, make a small mark on the line 4 cm away from the left side of the paper. Make another mark four centimeters away from the last one. Make two more marks in this manner.
6. Writing underneath the mark, label the first mark “Co” using a pencil. Label the second “Cu,” the third “Ni,” and the last one “U” for unknown.
7. Using a toothpick, draw up some of the copper solution. Lightly tap the chromatography paper with the toothpick on the mark labeled “Cu.” Do this 4–5 times until the spot is about 2 mm in size.
8. Repeat step 5 for the other three metal ion solutions, including the unknown. Use a clean, new toothpick for each solution.
9. Pick up the chromatography paper and roll in into a cylindrical shape with the edges just overlapping. Staple both ends to secure.
10. Lift the foil off of the developing chamber and gently place the paper cylinder inside, spot side down (see Figure 3).
    {12502_Procedure_Figure_3}
11. Replace the aluminum foil and allow the mobile phase to move up the paper. Do not move or jostle the beaker during this time. Make sure the solvent front does not reach the top of the paper.
12. Prepare the development chamber (may be shared by several groups) while the chromatogram is running. In the fume hood, carefully add 10 mL of concentrated ammonium hydroxide to a small beaker or flask. Place this container inside a larger beaker as shown in Figure 4. Cover the beaker with a watchglass. Caution: This must be done in a fume hood!
    {12502_Procedure_Figure_4}
13. When the solvent front is about 2–3 cm from the top of the paper, it may be removed from the chamber. Note: Typical chromatogram running times are 20–30 minutes.
14. Unhook the paper from the staples and lay flat on a paper towel.
15. Trace the solvent line before allowing the paper to dry.
16. In a fume hood, roll the paper chromatogram into a loose cylinder and place it inside the development chamber containing ammonium hydroxide (see Figure 5).
    {12502_Procedure_Figure_5}
17. Allow the ammonia fumes to react with the metal ions on the paper until color spots become visible for Cu²⁺ and Co²⁺.
18. Using a Beral pipet, drop a small amount of 0.1 M sodium sulfide solution at the spot where the nickel ion was initially placed and trace upward until a colored spot becomes visible. Also add one drop of Na₂S solution to the copper and cobalt spots if desired.
19. When the chromatogram is dry, circle each spot that has separated and place a small dot in the center. Measurements of each spot will be taken from the 1.5-cm line to the center of the spot.
20. Record your results in the data table. 
21. Consult your instructor for appropriate disposal procedures.

Student Worksheet PDF

12502_Student1.pdf