Separation of Constituents of Inorganic Mixture Containing Two Cations

In this experiment, a paper chromatography procedure uses similar principles to separate several metal cations. Comparing the R_f values and colors of the stained spots of known solutions will identify an unknown solution containing two cations.

Aim:

To separate the constituents present in an inorganic mixture containing two cations (Pb²⁺ and Cd²⁺) only and the constituents should have large difference in R_f values.

Theory:

Chromatography is a modern and sensitive technique used to analyze and separate components from a mixture and purify compounds quickly and efficiently.

The characteristic rate of movement of each substance on the chromatography paper can be determined at a given temperature and for a given solvent as the moving phase moves. This is represented by the R_f value of the relative front or retardation factor.

Although the mobile phase (solvent) is the same, the R_f values of different compounds are different. In addition, a compound’s R_f value may differ in different solvents. Using the following expression, R_f values can be calculated.

R_f = (frac{distance, travelled, by, the, substance, from, reference , line, (cm)}{distance, travelled, by, the ,solvent, front, from, reference, line, (cm)})

In this experiment the two cations to be separated in this case are colorless. A developer is therefore needed. In this case, ammonium sulphide (NH₄)₂S may be used to locate these ions in chromatographic paper or plate position.

Materials Required:

1. Whatman’s filter paper
2. Glass jar
3. 1-2% of lead nitrate solution
4. 1-2% of cadmium nitrate solution
5. Ammonium sulphide
6. Ethanol
7. 6M nitric acid
8. Rubber cork fixed with hook in the centre
9. Test tubes
10. Capillary tubes
11. Distilled water
12. Petroleum ether

Apparatus Setup:

Procedure:

1. Mark a line at a distance of 3 cm from one end of whatman filter paper with the help of a pencil.
2. Use a fine capillary to place a spot of the mixture on the marked line.
3. In a jar containing a mixture of ethanol, 6.0 M HNO₃ and distilled water, hang the filter paper in the ratio 8:1:1.
4. Keep the jar as such until the paper length rises to two-thirds of the mobile phase (solvent).
5. Now remove the filter paper and mark the solvent front.
6. Spray ammonium sulphide solution to obtain yellow and black spots on the chromatography paper.
7. Mark with a pencil the position of the spots and let the paper dry.
8. Measure the distance to the reference line moved by the solvent front and the cations ‘ different spots.
9. The distance between the reference line and the center of different spots is the shortest distance.
10. Record in tabular form the observations and calculate the R_f value for each cation.

Observation and Inference:

Results and Discussions:

1. The R_f values of Cd²⁺ ions is _________.
2. The R_f values of Pb²⁺ ions is _________.

Precautions:

1. Always make use of a fine capillary tube.
2. Use the capillary finely drawn to place the spot on the paper.
3. Do not allow spots to spread while spotting the test solution on the paper.
4. Do not disturb the jar once the experiment is set as long as the chromatogram is developed.
5. Carefully handle organic solvents.
6. Before developing the spots, make the paper strip perfectly dry.