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 Rf 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 Rf 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 Rf value of the relative front or retardation factor.

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

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

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:

Separation of Constituents of Inorganic Mixture Containing Two Cations

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 Rf value for each cation.

Observation and Inference:

S.No

Colour of the spot

Distance travelled by different components from reference line. (line/cm)

Distance travelled by the solvents from reference line. (line/cm)

Rf value

1

2

3

Results and Discussions:

  1. The Rf values of Cd2+ ions is _________.
  2. The Rf 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.

Viva Questions:

1. What are the types of solvent generally used in chromatography?

Ans: There are two types of solvents used in chromatography. One is polar solvent which dissolves polar molecules and the other is non polar solvent used to dissolve non polar molecules.

2. List the advantages of chromatography.

Ans: Chromatography is a simple technique can be handled by a single person. The use of chromatography separates components from complex mixtures. There are different types of equipment for chromatography to separate different compounds.

3. What is the value of chromatography?

Ans: In chromatography, the Rf value is used to quantify a sample’s retardation amount in a stationary phase relative to a mobile phase.

4. Name the scientist who introduced chromatographic techniques.

Ans: Russian botanist M.Tswett introduced chromatography in 1906.

5. What is meant by the term “developer” in chromatography?

Ans:The mobile phase is called the developer and it is referred to as the movement of the liquid along the bed.

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