Investigating the composition of inks
Select Lesson
Exam Board
Select an option
Chemistry practicals
Fuels and Earth science
Rates of reaction and energy changes
Groups in the periodic table
Extracting metals and equilibria
States of matter and mixtures
Key concepts
Chemical equations
Ionic equations and half equations - Higher
Hazards, risks and precautions
The history of the atom
The atom and isotopes
Calculating relative masses
Calculating relative atomic mass - Higher
The history of the periodic table
Electronic configuration
Ions and their formation
Ionic bonding: properties of compounds and naming
Covalent bonding
Giant covalent substances
Metallic bonding and alloys
Conservation of mass
Molecular and empirical formulae
Mole calculations and Avogadro's constant - Higher
Concentrations of solutions
Limiting reactants and masses - Higher
Practical skills
Explainer Video
Summary
Investigating the composition of inks
In a nutshell
By using chromatography, you will be able to use standardised analytical calculations to identify the components of ink mixtures.
Definitions
Keyword | Definition |
| Stationary phase | Where molecules cannot move; in paper chromatography this is the paper. |
| Mobile phase | Where molecules can move; in paper chromatography this is the water solvent. |
| Solvent front | The distance travelled by the solvent. |
| Chromatogram | The end result of paper chromatography. |
| value | A standard measure that allows for comparison of separation. |
Equations
This equation will be used during the analysis for paper chromatography.
Equipment list
The following equipment is standard for paper chromatography protocol.
Equipment | Use |
| beaker | To place the water and chromatography paper in |
| Chromatography paper | To illustrate the separation of the dyes |
| Mixture of ink sample | To be analysed |
| Ruler | To draw a baseline and measure distances |
| Pencil | To draw baseline |
| Water solvent | To carry the ink up the paper |
| Glass capillary tube | To apply the ink to the paper |
| Watch glass | To prevent evaporation of any solvent |
Experiment: Investigating the composition of inks using paper chromatography
Experimental variables
The independent variable is the one you are investigating. The dependent variable is the one you measure. The control variable is the one which remains the same.
| Independent | |
| Dependent | |
| Control | |
Safety precautions
The following precautions must be taken to minimise harm to you and others.
hazard | risk | safety measure |
| Glass | If broken, may cause cuts | Handle beaker and watch glass with care whilst setting up and carrying out the protocol. |
Method
This is your instructions for how to complete this experiment.
| 1. | Use the ruler to draw a horizontal line in pencil, from the bottom of the chromatography paper. This is your line of origin. Note: The line of origin must be in pencil as it is insoluble, so it will not begin to separate up the chromatography paper itself and interfere with the results. |
| 2. | Use the glass capillary tube to apply a spot of the ink sample along the pencil line. |
| 3. | Pour the water solvent to a depth of into the beaker. |
| 4. | Rest the marked chromatography paper in the water solvent, ensuring the pencil line does not go below the water level. |
| 5. | Assemble the watch glass on top. Your apparatus should appear as above. |
| 6. | Allow the water solvent to travel three quarters of the way up the paper, whilst not disturbing the apparatus. |
| 7. | Remove the chromatography paper and using the ruler, draw a pencil line at the point which the water stopped travelling up. The solvent front is the distance travelled by the solvent on the chromatography paper. |
| 8. | Hang up the paper to dry. This is now called the chromatogram. |
| 9. | Measure the distance, in , between the two pencil lines; this is the distance travelled by the water (solvent). Then, measure the distance from the line of origin to each of the centres of the spots the ink has separated into. Record these numbers as they will be required for calculating the values. |
Analysis
This is how you will use the data collected to form conclusions.
| 1. | Calculate the values for each spot on the chromatogram.
Note: This value should always be <1. If it is >1, you have an error in your calculation. Your value for the distance moved by the solvent will be the same in each calculation. |
| 2. | Compare your calculated values of each component of the dye with reference data supplied by the teacher. |
| 3. | You can then deduce identities for each component of the ink mixture. |
Conclusion
Based on your results, you can identify how many components there were in your ink sample. If only one spot separated, the sample was pure. If there was more than one, then it was a mixture. If you have a known substance, you can compare your spots to those produced by the known substance. So, if they produce the same spots at the same distance from the baseline then the two substances are likely the same.
Evaluation
You can use your data to evaluate the solubility of each component in the mixture based on the distance travelled. Substances that travelled further up the chromatogram have a higher solubility.
FAQs - Frequently Asked Questions
Can the Rf value be two?
The Rf value should always be <1. If it is >1, you have an error in your calculation.
Why must the line of origin be drawn in pencil?
The line of origin must be in pencil as it is insoluble, so it will not begin to separate up the chromatography paper itself and interfere with the results.
What is the solvent front?
The solvent front is the distance travelled by the solvent on the chromatography paper.