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Practical chemistry for exams›RP1 - Acid-base titrations

RP1 - Acid-base titrations

An overview of RP1 - Acid-base titrations

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In Required Practical 1: Acid–base titration, an accurately prepared standard solution of sodium hydrogensulfate is used to determine the concentration of a sodium hydroxide solution via acid–base titration.

In AQA Required Practical 1: Acid–Base Titration, there are several important safety requirements to be aware of (beyond normal lab practice such as wearing goggles).

Sodium hydrogensulfate and sodium hydroxide are irritants – avoid contact with skin and eyes.
Use a pipette filler – never pipette by mouth.
Phenolphthalein is flammable – keep away from flames or heat sources.
Immediately clean up any spills of acid or alkali.

Experimental method for AQA Required practical 1: Acid–base titration
Part 1 Prepare the standard solution.

EXPERIMENTAL METHOD AQA REQUIRED PRACTICAL 1 - PART 1 1 Calculate the required mass of sodium hydrogensulfate to make 250 cm³ of a 0.100 mol dm⁻³ solution. This will require the Mr of sodium hydrogensulfate and equations from 'amount of substance'. 2 Weigh the sodium hydrogensulfate using a balance. Zero the balance to record the actual mass of sodium hydrogen carbonate used. 3 Add ~100 cm³ of distilled water. Stir with a glass rod until fully dissolved. 4 Pour the solution into a 250 cm³ volumetric flask using a funnel. Rinse the beaker, funnel, and glass rod into the flask with distilled water. 5 Make up the volumetric flask to the mark using distilled water. Add dropwise near the line to avoid overshooting. 6 Stopper and invert the flask several times to mix thoroughly. 7 Calculate the exact concentration of NaHSO₄. Record to appropriate precision and show all working. © Medify

Experimental method for AQA Required practical 1: Acid–base titration

Part 2 The titration.

EXPERIMENTAL METHOD AQA REQUIRED PRACTICAL 1 - PART 2. 1. Rinse and fill a burette with sodium hydrogen sulfate solution and record the initial reading. Ensure there are no bubbles below the tap and read at eye level to ±0.05 cm³. 2. Use a 25.0 cm³ pipette to transfer 25.0 cm³ of sodium hydroxide solution to a conical flask. Rinse the pipette with sodium hydroxide solution and the conical flask with distilled water prior to use. 3. Add 2–3 drops of phenolphthalein to the conical flask. Note the initial pink colour in alkali. 4. Construct a titration results table ready to record initial and final readings and titre. 5. Titrate by slowly adding NaHSO₄ from the burette while swirling the flask. Add dropwise near the endpoint until the colour changes from pink to colourless. 6. Record the final burette reading and calculate the titre. Use the highest resolution on the burette for all readings. 7. Repeat titrations until you obtain two concordant titres within 0.10 cm³. Record all results; discard the rough titre from the average. Calculate the mean titre using concordant results only.

Ensure your standard solution is fully mixed.

Hands holding a stopper flask, with the instruction 'Stopper flask and invert several times to mix' displayed beside it.

Do

Invert the volumetric flask several times after filling to the graduation mark.

A volumetric flask filled with blue liquid, labeled 'Volumetric flask: still and not inverted.'

Don't

Use the standard solution without inversion – The top of the solution may be less concentrated than the bottom. Even small concentration differences can affect the accuracy of your results.

Avoid unintentional dilution of solutions.

A laboratory pipette with a graduated scale on the side, positioned next to a container labeled NaOH, indicating the presence of sodium hydroxide.

Do

Rinse all glassware used for accurately measuring solution volumes with the solution itself.

A pipette is shown with a container above it labeled 'Distilled water'.

Don't

Rinse precision glassware with water immediately before use – leftover water will dilute the solution.

Ensure the burette is free from air bubbles.

A laboratory apparatus showing a graduated pipette filled with liquid, indicating 'No air bubbles' next to the pipette.

Do

Check the burette for air bubbles and remove them by running some solution through the tap before starting the titration.

A diagram of a pipette filled with blue liquid, showing air bubbles at the tip. The pipette has measurement markings on the side and is connected to a gray stopcock.

Don't

Start titrating with air bubbles in the burette – The volume occupied by the bubble will be taken into your volume reading. Less solution will actually be transferred than is recorded.

Prevent parallax errors when reading volumes.

An illustration showing a measuring cylinder with a scale marked from 20 to 25 ml. An eye is depicted on the left side, and a green arrow points to the measurement of 22.14 ml.

Do

Read the burette at eye level and record to

A measurement scale showing values from 20 to 25 ml. Two eyes are illustrated, one pointing to 22.26 ml and the other to 22.02 ml, with red arrows indicating the measurements.

Don't

Read from above or below – this leads to incorrect readings due to parallax error.

Keep the titration mixture well mixed.

A hand holding a conical flask containing a pink liquid, with arrows indicating swirling motion, and a dropper above the flask.

Do

Swirl the conical flask gently and consistently during titration to ensure complete contact between reactants.

A laboratory setup featuring a conical flask with a pink liquid inside, topped with a pipette holder and a blue pipette.

Don't

Leave the flask still during titration – without swirling, the acid and alkali will not mix properly, leading to an inaccurate endpoint.

Use only the minimum amount of indicator needed in acid base titration.

A laboratory scene featuring a flask with pink liquid and a dropper above it. Next to the flask is a bottle labeled 'Phenolphthalein' containing blue liquid.

Do

Add just 2–3 drops of phenolphthalein to the flask.

A laboratory scene showing a bottle labeled 'phenolphthalein' pouring a blue liquid into a flask containing a pink solution.

Don't

Add too much indicator – Indicators are weak acids. Excess indicator can affect the pH and affect the titration end point.

Control the addition of acid as you approach the endpoint.

Two hands are holding a flask with a pink liquid inside. One hand is using a dropper to add a drop of liquid into the flask.

Do

Add the sodium hydrogensulfate drop by drop near the end point.

A laboratory setup featuring a conical flask with red liquid at the bottom and a blue pipette dispensing a clear liquid into the flask.

Don't

Add too quickly – it is easy to miss the exact endpoint and overshoot the titre.

Only use reliable titres in calculations.

A table displaying titration data with three columns labeled 'Titre 1 (rough) /cm³', 'Titre 2 /cm³', and 'Titre 3 /cm³'. The rows include 'Initial' with values 0.10, 0.00, and 0.10; 'Final' with values 24.95 (in red), 25.10, and 25.10; and 'Titre' with values 25.85, 25.10, and 25.00 (in green).

Do

Use concordant titres (within 0.10 ) to calculate your mean.

A table displaying data with three columns labeled 'Titre 1 (rough) /cm³', 'Titre 2 /cm³', and 'Titre 3 /cm³'. The rows are labeled 'Initial', 'Final', and 'Titre'. The values in the table are: Initial - 0.10, 0.00, 0.10; Final - 24.95, 25.10, 25.10; Titre - 25.85, 25.10, 25.00.

Don't

Include your rough titre or anomalous results – they’ll make your mean calculation less accurate.

When analysing the results of a sodium hydrogensulfate and sodium hydroxide titration.

Calculate moles of used to make the standard solution:

Calculate concentration of in the standard solution:

Calculate moles of in mean titre:

Use 1:1 molar ratio to calculate moles of in the conical flask:

Calculate concentration of :

Give your final answer to 3 significant figures with correct units.