Core Practical: Composition of Inks

How Separation Occurs

• The mobile phase is the liquid solvent (like water or ethanol) that moves through the system.
• The stationary phase is the phase that does not move. In paper chromatography, this is technically the water trapped within the cellulose fibres of the paper.
• Separation happens because different substances have different solubilities in the mobile phase and different affinities (attractions) for the stationary phase.
• A dye with high solubility in the solvent and weak attraction to the paper will travel further and faster up the paper.

Analysing the Chromatogram

The resulting dried paper is called a chromatogram. We can use it to determine the purity of a substance.

• A pure substance will only ever produce exactly one spot on the chromatogram.
• An impure mixture will separate into two or more spots.
• If a substance is completely insoluble in the chosen solvent, it will remain as a single spot directly on the pencil baseline.

Calculating Rf Values

We identify separated dyes by calculating their Rf value (Retention factor). This value is a ratio, so it has no units, and it will always fall between $0$ and $1$.

• Always measure distances from the pencil baseline, never from the bottom edge of the paper.
• For the substance distance, always measure to the very centre of the coloured spot.

Worked Example

A student performs chromatography on a purple ink. The solvent front travels $8.5\text{ cm}$ from the baseline. A distinct pink spot is observed, and the distance from the baseline to the centre of this spot is $5.1\text{ cm}$. Calculate the Rf value for the pink dye. (Give your answer to 2 significant figures)

Step 1: Identify the known values.

• Distance moved by substance = $5.1\text{ cm}$
• Distance moved by solvent = $8.5\text{ cm}$

Step 2: Substitute the values into the equation.

• $R_f = \frac{5.1}{8.5}$

Step 3: Calculate the final answer.

• $R_f = 0.60$

Extracting the Solvent: Simple Distillation

If you want the separated dyes, chromatography is perfect, but what if you want to keep the pure liquid solvent itself? Simple distillation separates a liquid solvent from a dissolved solid (solute), or from a mixture of liquids with significantly different boiling points.

• Evaporation: The ink mixture is heated in a flask. The solvent (water) has a much lower boiling point than the solid dyes, so it evaporates and turns into a vapour.
• Condensation: The hot vapour travels into a Liebig condenser. It cools down and condenses back into a liquid.
• Collection: The pure liquid collected is called the distillate, while the solid dyes left behind in the heating flask are the residue.

Crucial Distillation Setup Details

• The thermometer bulb must be positioned perfectly level with the side-arm of the distillation flask to accurately measure the temperature of the vapour entering the condenser.
• Water must enter the condenser at the bottom (furthest from the heat) and exit at the top. This ensures the cooling jacket remains completely full of cold water for maximum condensation.
• Add anti-bumping granules to the heating flask. These porous stones provide a surface for small bubbles to form, promoting smooth boiling and preventing violent 'spitting'.
• The apparatus must never be airtight (closed); heating a closed system causes dangerous pressure build-up that can shatter the glass.

Identifying Pure Water

• Pure substances melt and boil at sharp, fixed temperatures. During the distillation of ink, the thermometer will show a steady reading of $100^\circ\text{C}$ as the pure water evaporates.
• If the collected distillate is pure water, it will be clear and colourless. If the liquid is slightly inky, the mixture was heated too strongly and splashed over into the condenser.
• You can confirm the distillate contains water using a chemical test: add the liquid to white anhydrous copper(II) sulfate. If water is present, the powder will turn blue.