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.
$R_f$ value	A standard measure that allows for comparison of separation.

Equations

This equation will be used during the analysis for paper chromatography.

$R_f = \frac{distance\,moved\,by\,dye}{distance\,moved\,by\,solvent}$

Equipment list

The following equipment is standard for paper chromatography protocol.

Equipment	Use
$250 \, cm^3$ 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	$The\ ink\ you\ are\ investigating.$
Dependent	$The\ distance\ moved\ by\ the\ pigment.$
Control	$Temperature\ and\ the\ solvent\ volume.$

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.

Chemistry; Chemistry practicals; KS4 Year 10; Investigating the composition of inks

1.	Use the ruler to draw a horizontal line in pencil, $2 \, cm$ 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 $2-3 \,mm$ spot of the ink sample along the pencil line.
3.	Pour the water solvent to a depth of $1 \, cm$ 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 $mm$ , 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 $R_f$ values.

Analysis

This is how you will use the data collected to form conclusions.

1.

Calculate the $R_f$ values for each spot on the chromatogram.

$R_f = \frac{distance\,moved\,by\,dye}{distance\,moved\,by\,solvent}$

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 $R_f$ 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.