Investigating the Composition of Inks (CP)

Describe how to separate soluble coloured substances

Chromatography:

-Place a spot of the sample on a pencil line on a piece of chromatography paper.

-Place the paper in a beaker of solvent (below pencil line).

-Remove from solvent and leave to dry once solvent has travelled up 2⁄3 of the paper

Why should the line be drawn onto chromatography paper in pencil?

Pencil doesn’t travel up the paper whereas pen ink would. Pencil is insoluble in the solvent.

Ink spots are placed above the level of solvent in the beaker. Why?

If they were below the solvent level the ink would dissolve in the solvent rather than moving up the chromatography paper

What is meant by a ‘solvent front’?

The solvent front is the furthest distance reached by the solvent

How can you use chromatography to see if a certain substance is present in a mixture?

Run a pure sample of this substance alongside the unknown mixture. If the Rf value of the pure substance matches the value of one of the spots from the mixture, it is likely to be present

Why should chromatography paper be removed from the solvent before the solvent front reaches the top?

To allow the distance moved by the solvent to be measured so that the Rf value can be calculated

What are the 2 phases in chromatography?

Mobile phase - solvent which carries substances up the stationary phase.

Stationary phase - paper or thin layer of an inert substance on a glass plate

How do you calculate the Rf value?

Rf = Distance travelled by substance / Distance travelled by solvent. Rf is between 0 and 1

In paper chromatography, what is the stationary phase?

The chromatography paper

In paper chromatography, what is the mobile phase?

The solvent e.g. water or ethanol

Describe how to obtain a sample of pure water from ink

Simple distillation:

Gently heat the ink in a round bottomed flask using a Bunsen burner.

The water from the ink evaporates then cools in the condenser.

The pure water is collected in a beaker.

How do you set up a condenser for simple distillation?

Condenser should be horizontal with water in at the bottom and water out at the top.

When using simple distillation to collect pure water, what temperature will be on the thermometer?

100°C

List precautions to take when using a Bunsen burner

Don’t leave unattended;

Turn off gas or leave on orange safety flame when not in use;

Tie back long hair;

Work in a clear space;

Move any flammable substances away

List precautions to take when handling harmful solvents

Wear gloves and safety goggles;

Use in a well ventilated lab or use a fume cupboard;

Clean up any spillages