Easy Paper Chromatography Lab and Results – Number of Substances present in Pen Ink

Easy Paper Chromatography Lab and Results – Number of Substances present in Pen Ink

Aim / Objective:

To determine the number of substances and their Rf values present as dyes in various pen ink.

Introduction:

Chromatography is a technique for separating and identifying mixtures of compounds based upon their different rates of adsorption. All types of chromatography employ two different immiscible phases in contact with each other namely the mobile phase and the stationary phase.

Normally, the stationary phase is a solid such as paper, starch, alumina, or silica and the mobile phase is a liquid such as water, common organic solvents such as ethanol, or solvent mixtures.

The basis for paper chromatography is the fact that porous paper, cellulose, has an enormous surface area to which molecules or ions of substances are attracted (adsorbed) and then released (desorbed) into the solvent as an aqueous solution passes over the paper.

Separation of components occur, that is, they will travel at different speeds in a moving solvent because the varying attractions between these components and the paper. This method of separation is known as partitioning.

The identity of the components can be deduced by comparing a chromatogram of the unknown mixture with chromatograms of mixtures with known composition (standards). An additional aid in the identification of substances is its Rf value.

The Rf value of a compound is a characteristic of the compound and solvent used and serves to identify the constituents of a mixture

This can be done by calculation:              Rf = Ds / Df

Where, Ds = distance traveled by a spot, and

Df = distance traveled by the solvent.

Apparatus/ Materials:

Water, chromatography paper / filter paper, 10cm3 beaker, glass cover, scissors, pencil, different color pens.

Method / Procedure:

  1. Fill the 10cm3 beaker with water to about a height of 2cm.
  2. Use pencil to draw a light line 1cm from one end of the chromatography strip and label as “A”
  3. Draw a second line, 1cm from line A and label it as “B”. Line A will represent the level to which the strip should be submerged into the solvent, water. Line B will represent the line to which the ink is applied, that is the Line of Origin.
  4. Using the pen, apply a thin ink line over Line B and allow it to dry.
  5. The solvent was allowed to move up the paper. When the solvent front has almost reached the top of the paper, remove it and mark this line with a pencil.
  6. Allow chromatogram to air dry.
  7. Measure the distance from line B to each separated dye in the pen ink.
  8. Record the distances measured and save the chromatogram.
  9. Repeat this procedure using the other pens.
  10. 10. Record and tabulate the results of all the pens used.

Suggested Results:

Paper chromatogram showing the separation of the dyes present in pen ink.

Paper chromatogram showing the separation of the dyes present in pen ink.

Table showing the distanced travelled and the calculated Rf values of the  separated dyes in pen ink.

Table showing the distanced travelled and the calculated Rf values of the separated dyes in pen ink.

Discussion / Answers:

Define adsorption:

>>> Adsorption is the process by which one substance is being attracted and held to the surface of another substance.

Distinguish between adsorption and absorption

>>> Adsorption is the process by which atoms, molecules or ions from a substances either a gas, dissolved solid or liquid adhere to the surface of an adsorbent.

Absorption is the process by which a fluid is dissolved by a solid (absorbent) or a liquid.

Explain the stationary phases and mobile phase in chromatography?

>>> The stationary phase is a solid or liquid supported on a solid and the mobile phase is a liquid or a gas. The mobile phase flows through the stationary phases and carries with it the components of the mixture.

What causes the components to separate?

>>> The process of paper chromatography contains different actions taking place which causes a mixture to be separated into its components. These actions include:

a)    Water molecules being attached to the filter paper from the stationary phases and the solvent moving through the paper is the mobile phases

b)    The stationary phases will tend to stop the components of the mixture from moving, while the mobile phases will carry them along the paper

c)    The components of the mixture will be held to the stationary phases and will dissolve in the mobile phases to different extents.

d)    The components of the mixture will therefore move along the paper at different rates

Source of Error/ Limitations/ Assumptions:

  1. Handling the paper with wet hands allows for additional uncontrolled moisture to the paper which will interfere with separation of components.
  2. Incorrect drawing and measurements of lines.

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