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BTEC Applied Science Unit 2
Assignment A (Titration) guide
UNIVERSITY: Pearson (BTEC) COURSE: BTEC Level 3 National Foundation Diploma in Applied Science UNIT: Unit 2 - Practical Scientific Procedures and Techniques ASSIGNMENT: Learning Aim A - Undertake Titration and Colorimetry to Determine the Concentration of Solutions GRADE ACHIEVED: Distinction (A+ / RATED A+)
BTEC Applied Science Unit 2 Assignment A: Titration
Introduction
The primary objective of this assignment is to investigate and apply quantitative analytical techniques to determine the unknown concentration of a solution. This will be achieved through a series of practical procedures, including the calibration of laboratory equipment, the preparation of a standard solution, and the execution of acid-base titrations. Titration is a quantitative chemical analysis technique used to determine the concentration of an unknown solution (the analyte) by reacting it with a solution of known concentration (the titrant).
A key principle of this assignment is ensuring the validity of results through rigorous calibration of equipment. Calibration is the process of configuring an instrument to provide a result for a sample within an acceptable range, thereby minimizing systematic errors and ensuring accuracy.
Titration is based on a neutralisation reaction, typically between an acid and a base. The point at which the reaction is complete is called the equivalence point. An indicator is used to visually signal this point, which is known as the end point. The accurate determination of this end point is crucial for precise concentration calculations.
Section 1: Calibration of Laboratory Equipment
The accuracy of any quantitative analysis depends on the reliability of the equipment used. Before any practical work, all relevant equipment must be calibrated.
1.1 Calibration of an Electronic Balance
An electronic balance is used to measure the mass of substances with high precision. Calibration ensures the readings are accurate and reliable.
Method:
- Ensure the balance is placed on a flat, level, and stable surface to prevent inaccurate readings.
- Turn on the balance and allow it to warm up, if required by the manufacturer's instructions.
- Press the 'Tare' or 'Zero' button to ensure the balance reads 0.00 g with nothing on the pan.
- Select a set of standard calibration weights (e.g., 10g, 20g, 50g).
- Carefully place the first standard weight (e.g., 10g) onto the centre of the balance pan and record the reading.
- If the reading does not match the known mass of the weight, use the balance's calibration adjustment mechanism (often a screw or internal function) to correct it.
- Repeat this process with different masses across the range you will be using (e.g., 20g, 50g). Record all results in a table.
- Document the calibration status (e.g., "Balance calibrated on [Date] with [Weights], all readings were within acceptable tolerance").
1.2 Calibration of a Volumetric Pipette
A volumetric pipette is used to deliver a precise and accurate volume of liquid. Calibration verifies that the volume dispensed is as specified (e.g., 25.00 cm³).
Method:
- Gather equipment: a clean, dry beaker, a thermometer, distilled water, and the pipette to be calibrated.
- Place a clean, dry beaker on a calibrated balance and record its mass (m1).
- Fill the beaker with distilled water and measure its temperature using a thermometer. The temperature is needed to determine the density (Z factor) of the water.
- Using a pipette filler, draw distilled water up into the pipette until the bottom of the meniscus is exactly on the calibration mark.
- Dispense the water into the pre-weighed beaker. Allow the pipette to drain naturally, touching the tip to the side of the beaker for a few seconds to ensure complete delivery.
- Re-weigh the beaker with the water and record the mass (m2).
- Calculate the mass of the water delivered: Mass of water = m2 - m1.
- Repeat this process at least three times to obtain a reliable mean mass.
- Calculate the volume of water delivered using the formula: Volume (cm³) = Mass (g) × Z Factor. The Z factor is the reciprocal of the density of water at the measured temperature.
- The calculated volume should be within the manufacturer's tolerance (typically ±0.02 cm³ for a Class A pipette). If not, the pipette should be replaced or professionally recalibrated.
1.3 Calibration of a pH Meter
- Rinse the beaker and stirring rod with distilled water to ensure all the sodium carbonate is transferred. Add these washings to the volumetric flask.
- Carefully add distilled water to the flask until the level is about 1 cm below the calibration mark on the neck of the flask.
- Using a dropper, add distilled water drop by drop until the bottom of the meniscus is exactly on the calibration mark.
- Stopper the flask and invert it several times to ensure the solution is homogeneous.
Section 3: Risk Assessment
Before any practical work, a risk assessment must be completed to identify potential hazards and outline the necessary precautions to ensure safety.
Hazard Risk Precaution Treatment
Hydrochloric Acid (HCl) Skin/eye irritation, corrosive Wear safety goggles, lab coat, Immediately rinse affected and gloves. Handle in a fume area with plenty of water. cupboard if concentrated. Seek medical attention if necessary. Sodium Hydroxide (NaOH) Skin/eye irritation, corrosive Wear safety goggles, lab coat, Immediately rinse affected and gloves. Handle carefully area with plenty of water. to avoid splashes. Glassware (beakers, burettes, Cuts from broken glass Wear appropriate lab wear (e.g., Get first aid and put a pipettes, etc.) lab coat, goggles, gloves) plaster on the cut. Seek to protect skin. Avoid medical attention for serious placing glassware on uneven cuts. surfaces. Do not apply too much pressure when attaching pipette fillers. Spillages (liquids) Slips, trips, and falls Ensure spillages are cleaned Clean up spills immediately. up immediately. Keep work Seek medical attention if surfaces clean and dry. necessary.
Electrical Equipment Electric shock Ensure equipment is PAT tested. Switch off and unplug (e.g., pH meter, balance) Check for damage to cables. equipment. Seek first aid Do not use near water sources. if necessary.
Section 4: Titration Procedure
This experiment involves titrating the standard sodium carbonate solution with hydrochloric acid (HCl) to determine its concentration. A pH meter and an indicator (e.g., methyl orange) will be used to identify the end point.
4.1 Standardisation of Hydrochloric Acid (HCl)
The concentration of the HCl solution is determined by titrating it against the standard sodium carbonate solution.
- Reaction Equation: Na₂CO₃(aq) + 2HCl(aq) → 2NaCl(aq) + H₂O(l) + CO₂(g)
- Mole Ratio: 1 mole of Na₂CO₃ reacts with 2 moles of HCl.
- Indicator: Methyl orange (colour change from yellow to pink at the end point).
Method:
- Set up the apparatus: a burette filled with the HCl solution (unknown concentration), a conical flask containing a measured volume (e.g., 25.00 cm³) of the standard sodium carbonate solution, and a white tile underneath to better observe the colour change.
- Add a few drops of methyl orange indicator to the conical flask.
- Record the initial burette reading.
- Slowly add the HCl from the burette to the conical flask, swirling constantly.
- As the end point approaches, add the HCl drop by drop. The end point is reached when one final drop of HCl causes the solution to change permanently from yellow to a faint pink.
- Record the final burette reading.
- Calculate the titre (volume of HCl used) = Final reading - Initial reading.
- Repeat the titration at least three times until concordant results are obtained (within 0.10 cm³ of each other).
- Record all results in a table.
Results Table (Example):
Titration Number Initial Burette Reading (cm³) Final Burette Reading (cm³) Titre (cm³)
1 0.00 24.50 24. 2 24.50 48.95 24.
- From the reaction equation (HCl + NaOH → NaCl + H₂O), the mole ratio is 1:1. Therefore, Moles of NaOH = Moles of HCl.
- Concentration of NaOH = Moles of NaOH / Volume of NaOH (dm³) = Moles of NaOH / (25.0 / 1000) dm³
Section 5: Evaluation
5.1 Strengths of the Methods Used
- The use of a primary standard (anhydrous sodium carbonate) ensures the concentration of the standard solution is accurately known.
- Calibrating the balance, pipette, and pH meter before use minimises systematic errors and increases the reliability of the results.
- Repeating titrations until concordant results are achieved (within 0.10 cm³) demonstrates precision and reduces the impact of random errors. This allowed for a reliable mean titre to be calculated.
- The use of a pH meter to plot a titration curve provided a more accurate and objective method for determining the equivalence point compared to using an indicator alone. The equivalence point can be identified precisely from the graph.
5.2 Limitations and Sources of Error
- Human Error (Parallax Error): Inaccurate reading of the meniscus on the burette or pipette can lead to systematic errors. This can be minimised by ensuring the eye is level with the meniscus when taking readings.
- Indicator Error: The end point of an indicator may not occur exactly at the equivalence point. For example, methyl orange changes colour at a pH of around 3.1-4.4, which is slightly acidic, not pH 7. This introduces a small systematic error.
- Contamination: Failure to rinse glassware (e.g., the conical flask, burette, pipette) with the appropriate solution could lead to contamination and inaccurate results.
- Random Errors: Variations in the determination of the end point (e.g., adding too much titrant) or slight inconsistencies in technique contribute to random errors, which are reflected in the spread of the titre values.
5.3 Suggested Improvements
- To improve precision, use a white tile underneath the conical flask to better observe the colour change of the indicator.
- To eliminate indicator error, use a pH meter for all titrations to accurately determine the equivalence point.
- To minimise parallax error, ensure readings are taken with the eye level with the meniscus on the graduated apparatus.
- To improve accuracy, ensure all equipment is thoroughly clean and rinsed with the solution it will contain before use.
- To reduce random errors, increase the number of titration repetitions to calculate a more reliable mean titre.
Section 6: Conclusion
In conclusion, the aim of this assignment was successfully achieved. The concentration of an unknown sodium hydroxide solution was determined through the application of quantitative techniques.
First, the necessary laboratory equipment was calibrated to ensure the validity of the results. A standard solution of sodium carbonate was then accurately prepared. This solution was used to standardise a hydrochloric acid solution via titration. The concentration of the HCl was calculated to be approximately 0.204 mol dm⁻³.
Finally, this standardised HCl was used in a titration against the unknown sodium hydroxide solution, monitored by a pH meter. The equivalence point was identified from the resulting pH curve, allowing for the calculation of the NaOH concentration. The use of a pH meter provided a more accurate end-point determination compared to using an indicator alone.
Overall, the practical techniques were executed with a high degree of precision, as demonstrated by the concordant titre results obtained. The analysis of the results and the subsequent evaluation of the methods confirm the validity of the findings and the competence in applying these core scientific procedures.
Section 7: References
Annets, F., Foale, S., Llewellyn, R., Musa, I. and Hocking, S. (2016). BTEC Level 3 National Applied Science Student Book. 1st ed. London: Pearson Education.
END OF ASSIGNMENT
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