Experimental Techniques and Analysis

12.1 Experimental design

Apparatus

When you are asked to suggest the advantages and disadvantages of a method or apparatus, think about accuracy (how close to the true value), how easy it is to use, the time taken, safety and whether anything is lost (for example a gas escaping while a bung is put in).

Definitions

• A solvent is a substance that dissolves a solute.
• A solute is a substance that is dissolved in a solvent.
• A solution is a mixture of one or more solutes dissolved in a solvent.
• A saturated solution contains the maximum concentration of solute dissolved in the solvent at a specified temperature.
• A residue is a substance that remains after evaporation, distillation, filtration or a similar process.
• A filtrate is the liquid or solution that has passed through a filter.

12.2 Acid–base titrations

A titration finds the exact volume of one solution that reacts with a measured volume of another.

Method.

1. Use a volumetric pipette to put exactly $25.0\ \text{cm}^3$ of the alkali into a conical flask.
2. Add a few drops of a suitable indicator (for example methyl orange, or thymolphthalein).
3. Fill the burette with the acid and note the initial reading.
4. Add the acid to the flask, swirling it, and go drop by drop as the end-point is near. Stand the flask on a white tile to see the colour change clearly.
5. Stop at the end-point: the first permanent colour change of the indicator.
6. Note the final reading. The titre is the final reading minus the initial reading.
7. Repeat until you have two or more results that agree closely (within $0.10\ \text{cm}^3$, called concordant results), and take the mean of them.

The end-point shows when the amounts of acid and alkali are exactly in the ratio given by the equation, so the result can be used to calculate a concentration (see chapter 3).

12.3 Chromatography

Paper chromatography separates a mixture of soluble substances, for example the dyes in an ink or food colouring.

Method.

1. Draw a baseline in pencil on the chromatography paper. (Pencil is used because ink would dissolve in the solvent and run.)
2. Put a small spot of each substance on the baseline.
3. Stand the paper in a container with a suitable solvent, with the solvent level below the baseline.
4. The solvent rises up the paper and carries the substances with it. Each substance moves a different distance, because they differ in how soluble they are in the solvent and how strongly they are attracted to the paper.
5. Remove the paper before the solvent reaches the top, and mark the solvent front at once.

Colourless substances can be shown using a locating agent, a chemical that is sprayed on the dry paper and reacts with them to make coloured spots. (Knowledge of specific locating agents is not required.)

Interpreting a chromatogram

• A pure substance gives one spot. An impure substance (a mixture) gives two or more spots.
• Two substances are the same if their spots travel the same distance in the same solvent. An unknown can be identified by comparing its spots with those of known substances on the same paper.

Both distances are measured from the baseline, and the substance's distance is to the centre of its spot. $R_\text{f}$ has no units and is always between $0$ and $1$.

$R_\text{f} = \frac{3.2}{8.0} = 0.40$

12.4 Separation and purification

The method of separating a mixture depends on the physical properties of its parts.

Simple distillation. The solution is heated, the solvent boils and its vapour passes into a condenser, where it is cooled and condenses into the pure liquid, which is collected. The solute stays in the flask. In the condenser, cooling water goes in at the bottom and out at the top, so that the jacket stays full.

Fractional distillation. A fractionating column (packed with glass beads) is between the flask and the condenser. The liquid with the lower boiling point reaches the top of the column first and distils over. The liquid with the higher boiling point stays in the column and flask until the temperature rises. The thermometer shows the boiling point of the substance that is distilling.

Choosing a method. Decide what is in the mixture, whether each part is soluble or insoluble, and whether the parts have different boiling points.

Filter the mixture. The sand is the residue and the salt solution is the filtrate. Then either use simple distillation to collect the pure water, and the salt remains in the flask, or heat the filtrate in an evaporating basin to evaporate some water, leave it to crystallise, and filter and dry the salt crystals.

Purity

A pure substance has a sharp melting point and boiling point, at the same temperature each time. If the melting point is lower than the known value, or if it melts over a range, the substance is impure. The same is true for a boiling point, which is higher if the liquid is impure.

12.5 Identification of ions and gases

Tests for anions (negative ions)

$\ce{Ag+(aq) + Cl-(aq) -> AgCl(s)} \qquad \ce{Ba^2+(aq) + SO4^2-(aq) -> BaSO4(s)}$

Tests for cations (positive ions)

Add aqueous sodium hydroxide drop by drop, then in excess. In another test, do the same with aqueous ammonia.

White precipitate soluble in excess of both reagents is zinc, $\ce{Zn^2+}$. (Aluminium does not dissolve in excess ammonia.)

Tests for gases

Flame tests

Dip a clean nichrome or platinum wire (cleaned with hydrochloric acid) into the sample and hold it in a blue Bunsen flame. The flame colour identifies some metal ions.