Practical Procedure: Producing a Pure, Dry Sample of a Salt

To monitor this reaction and find the exact endpoint, chemists use titrations. While Universal Indicator shows a gradual colour change (red to purple) and is useful for rough pH estimates, it is unsuitable for titrations. Instead, you must use a single indicator like phenolphthalein or methyl orange to provide a sharp, sudden colour change.

Step-by-Step pH Monitoring

To determine the neutralisation point more systematically, you can use the following procedures:

Using Universal Indicator Paper:

1. Use a clean glass rod to remove a small drop of the reaction mixture.
2. Touch the glass rod onto a piece of universal indicator paper placed on a white tile.
3. Immediately compare the colour of the paper to a standard pH colour chart.
4. Clean the glass rod and repeat the process after adding each portion of reactant until the paper turns green (pH 7).

Using a Digital pH Probe:

1. Calibrate the probe by rinsing it with distilled water and dipping it into buffer solutions of known pH (e.g., pH 4.0, 7.0, and 10.0).
2. Rinse the pH probe with distilled water before placing it into the reaction vessel.
3. Submerge the bulb of the probe in the solution and stir gently.
4. Wait for the digital display to stabilise before recording the pH value. Probes are more accurate and precise than indicators because they remove subjective human judgment of colour.

Titration Procedure and Calculations

A standard titration requires precise technique to determine the titre (the volume of acid needed to neutralise the alkali).

1. Use a pipette and pipette filler to transfer exactly 25.0 cm³ of alkali into a conical flask.
2. Add a few drops of a single indicator and place the flask on a white tile to make the colour change obvious.
3. Fill a burette with acid. A burette has a single-reading uncertainty of $\pm 0.05$ cm³, meaning all readings must be recorded to two decimal places ending in '0' or '5'.
4. Add the acid to the flask slowly, swirling continuously. Near the endpoint, add the acid dropwise.
5. Repeat the titration until you achieve concordant results (titres within 0.10 cm³ of each other).

A student performs a titration and achieves the following titre volumes: 24.80 cm³, 24.10 cm³, and 24.15 cm³. Calculate the mean titre.

Step 1: Identify the concordant results (those within 0.10 cm³ of each other).

• The concordant results are $24.10$ cm³ and $24.15$ cm³.

Step 2: Calculate the mean using only these values.

• $\text{Mean Titre} = \frac{24.10 + 24.15}{2}$

Step 3: Calculate and state the final answer with units.

• $\text{Mean Titre} = 24.13$ cm³

(Note: Final answers in calculations should generally match the least precise piece of data provided, usually 3 significant figures).

Preparing a Pure, Dry Salt

The "insoluble base method" is used to prepare pure, dry soluble salts like copper(II) sulfate ($CuSO_4$).

1. Measure and Heat: Measure dilute sulfuric acid ($H_2SO_4$) into a beaker. Heat it gently over a Bunsen burner to increase the rate of reaction. Do not boil it, as this produces toxic acidic fumes.
2. React: Add insoluble copper(II) oxide ($CuO$) powder using a spatula and stir with a glass rod.
3. Excess: Keep adding the base until it is in excess (the solid stops dissolving and sits at the bottom). This guarantees all the acid is neutralised.
4. Filter: Pass the mixture through a filter funnel and filter paper. The unreacted base becomes the residue, and the pure salt solution passes through as the filtrate.
5. Evaporate: Pour the filtrate into an evaporating basin. Heat it gently over a water bath to avoid "spitting". Stop heating when the crystallisation point (or saturation point) is reached — you can test this by dipping a cold glass rod into the solution to see if crystals form.
6. Crystallise and Dry: Leave the saturated solution in a warm place to cool slowly, which produces larger crystals. Finally, wash the crystals with cold distilled water to remove impurities and pat them dry.

Laboratory Safety and Risk Assessment

Proper risk assessment identifies potential hazards and puts precautions in place to reduce risk.

• Hazards are properties with the potential to cause harm, while the risk is the likelihood of that harm occurring.
• Apparatus and chemicals carry specific hazard symbols. For example, concentrated acids are corrosive (can cause chemical burns), while dilute acids are usually just an irritant.
• Always wear PPE (Personal Protective Equipment) such as safety goggles. Use gloves when handling toxic solids like lead(II) nitrate.
• When handling gases, a fume cupboard must be used if toxic gases might be produced (e.g., sulfur dioxide from heating acids).
• Highly flammable liquids must be kept well away from naked flames.