Welcome to AS 1: Chemistry Practical Skills
Welcome to the Chemistry Skills section of Unit AS 1: Experimental Techniques! Whether you are aiming for a career in pharmaceuticals, medicine, biomedical research, or clinical diagnostics, practical chemistry forms the bedrock of laboratory science. In this unit, your coursework portfolio showcases your hands-on ability to measure accurately, separate mixtures, quantify concentrations, and work safely.
Don't worry if quantitative chemistry or lab calculations have felt daunting in the past. We will break down every technique step-by-step, showing you exactly what to do, why you do it, and how to avoid the classic errors that cost marks.
1. Volumetric Analysis: Preparing a Standard Solution
Before you can find the unknown concentration of a sample, you often need a reference point. This reference point is called a standard solution.
What is a Standard Solution?
A standard solution is simply a solution of accurately known concentration. It is made by dissolving an exact, known mass of a solid solute (a primary standard) in deionised water and making it up to an exact volume in a specialized volumetric flask.
Step-by-Step Procedure: Making a Standard Solution
Step 1: Weighing by difference
Use a high-precision analytical balance. First, weigh a clean, dry weighing boat containing your solid solute. Tip the solid into a clean beaker, then re-weigh the empty boat. Subtract the second mass from the first to record the exact mass transferred:
\( \text{Mass transferred (g)} = \text{Mass of boat + solid} - \text{Mass of boat after transfer} \)
Step 2: Dissolving the solute
Add a small volume of deionised water (around \(50\text{ cm}^3\) to \(100\text{ cm}^3\)) to the beaker. Stir thoroughly with a glass rod until all the solid crystals have completely dissolved.
Step 3: Quantitative transfer
Pour the solution from the beaker into a clean volumetric flask (e.g., \(250\text{ cm}^3\)) through a filter funnel. To make sure every single molecule makes it into the flask, rinse the beaker, the stirring rod, and the funnel with deionised water from a wash bottle. Add all of these washings directly into the volumetric flask.
Step 4: Making up to the mark
Add deionised water until the liquid level approaches the calibration line. For the final few drops, use a dropping pipette so that the bottom of the meniscus rests precisely on the etched calibration line at eye level.
Step 5: Inverting to mix
Stopper the volumetric flask tightly and invert it repeatedly (10 to 15 times) to guarantee thorough, uniform mixing throughout the entire volume.
Key Formulae for Solution Calculations
To calculate the molar concentration of your prepared solution:
Step A: Calculate moles of solute:
\( \text{Amount (mol)} = \frac{\text{Mass (g)}}{\text{Molar Mass } (M_r \text{ in g mol}^{-1})} \)
Step B: Calculate concentration in \(\text{mol dm}^{-3}\):
\( \text{Concentration } (c) = \frac{n (\text{mol})}{V (\text{dm}^3)} = \frac{n \times 1000}{V (\text{cm}^3)} \)
Step C: Convert to mass concentration (\(\text{g dm}^{-3}\)) if required:
\( \text{Concentration } (\text{g dm}^{-3}) = \text{Concentration } (\text{mol dm}^{-3}) \times M_r \)
Key Takeaway: Always include the washings of the beaker, rod, and funnel, and never fill past the calibration line. If the meniscus goes above the mark, you must start again!
2. Volumetric Analysis: Acid-Base Titrations
An acid-base titration is an analytical technique used to determine the exact concentration of an unknown acid or base by reacting it with a standard solution of known concentration.
Setting Up and Running the Titration
1. Preparing the Pipette:
Rinse a volumetric pipette with deionised water, and then with the specific solution it will measure. Use a pipette filler to draw up the liquid until the bottom of the meniscus touches the line. Deliver this exact volume (called an aliquot, typically \(25.0\text{ cm}^3\)) into a clean conical flask.
2. Preparing the Burette:
Rinse the burette with deionised water, followed by the titrant solution. Fill the burette using a funnel, then remove the funnel. Open the tap briefly to fill the burette tip completely, ensuring there are no trapped air bubbles. Record the initial reading to two decimal places (e.g., \(0.00\text{ cm}^3\) or \(0.05\text{ cm}^3\)).
3. Running the Titration:
Add a few drops of a suitable indicator (such as phenolphthalein or methyl orange) to the conical flask. Place the conical flask on a white tile so the color change is easy to see. Swirl continuously while adding the titrant. Add dropwise near the endpoint until the indicator undergoes an immediate, permanent color change.
The Rule of Concordancy
Titration requires precision. You will carry out a rough (trial) titration first to find the approximate endpoint, followed by accurate titrations.
Concordant Titres: Titres that are within \( \pm 0.10\text{ cm}^3 \) of each other.
Critical Rule: Only concordant titres are used to calculate the mean titre. The initial rough/trial run is always discarded.
Example:
Rough: \(22.50\text{ cm}^3\) (Discard)
Titre 1: \(21.35\text{ cm}^3\)
Titre 2: \(21.80\text{ cm}^3\) (Not concordant with 1 or 3 — discard)
Titre 3: \(21.40\text{ cm}^3\)
Mean Titre = \( \frac{21.35 + 21.40}{2} = 21.375\text{ cm}^3 \approx 21.38\text{ cm}^3 \)
Titration Calculations
Use the stoichiometric ratio from the balanced chemical equation:
\( \frac{c_1 V_1}{n_1} = \frac{c_2 V_2}{n_2} \)
Where:
\(c_1, c_2\) = concentrations in \(\text{mol dm}^{-3}\)
\(V_1, V_2\) = volumes in \(\text{cm}^3\) (or \(\text{dm}^3\))
\(n_1, n_2\) = mole ratios from the balanced chemical equation
Key Takeaway: Always remove the funnel from the burette top, ensure the jet space is filled, and only average titres within \(0.10\text{ cm}^3\).
3. Separation Techniques: Chromatography
Chromatography is an analytical method used to separate and identify components in a chemical mixture (such as amino acids, plant pigments, or food dyes) based on how they distribute between two phases.
The Two Phases
1. Stationary Phase: A phase that does not move. In paper chromatography, this is the cellulose chromatography paper. In Thin-Layer Chromatography (TLC), it is a thin layer of silica gel or alumina coated onto an inert plastic or glass plate.
2. Mobile Phase: A liquid solvent (or mixture of solvents) that moves up the stationary phase by capillary action.
How separation happens: Substances with a higher affinity for the mobile phase (greater solubility) travel further and faster up the plate. Substances with a higher affinity for the stationary phase (stronger adsorption/binding) travel more slowly.
Step-by-Step Practical Execution
1. Origin Line: Draw a horizontal baseline about \(1.5\text{ cm}\) from the bottom of the paper/plate using a graphite pencil. Never use pen or ink, as ink will dissolve in the solvent and ruin the chromatogram.
2. Spotting: Use a fine capillary tube to apply small, concentrated spots of your sample and known reference standards along the baseline. Allow them to dry.
3. Developing Chamber: Pour the solvent into a chromatography tank or beaker. The solvent depth must be below the pencil baseline so the spots are not washed into the solvent reservoir.
4. Chamber Atmosphere: Place a lid or watch glass over the chamber. This ensures the air inside remains saturated with solvent vapor and prevents solvent evaporation.
5. Stopping: When the solvent front reaches near the top of the plate, remove it and immediately mark the solvent front with a pencil before it evaporates.
Calculating \(R_f\) Values
The retention factor (\(R_f\)) is a characteristic constant for a specific compound under identical conditions (same stationary phase, mobile phase, and temperature):
\( R_f = \frac{\text{Distance moved by the solute (center of spot)}}{\text{Distance moved by the solvent front}} \)
Properties of \(R_f\) values:
- \(R_f\) values are dimensionless ratios.
- They are always between \(0\) and \(1\) (i.e., \(0 \le R_f \le 1\)).
Visualising Colourless Compounds
Many biological molecules (like amino acids) are colorless. To see them after chromatography:
- Ninhydrin Spray: Used specifically for amino acids. It reacts upon gentle heating to produce distinct purple or brown spots.
- UV Illumination: TLC plates often contain a fluorescent dye. Under a UV lamp, spots appear as dark patches where fluorescence is quenched.
Key Takeaway: Always draw the baseline in pencil, ensure the solvent level starts below the baseline, seal the chamber, and mark the solvent front immediately.
4. Colorimetry and Quantitative Spectrophotometry
Colorimetry is a quantitative optical technique used to find the concentration of a colored solute in a solution by measuring how much light it absorbs.
Principles of Colorimetry
The deeper the color of a solution, the higher its concentration, and the more light it absorbs at a specific wavelength.
1. Choosing the Correct Filter: Select a complementary color filter. For example, a blue copper sulfate solution transmits blue light but absorbs red light strongly. Therefore, a red filter is chosen to maximize absorbance sensitivity.
2. Zeroing/Blanking: Before measuring samples, fill a clean cuvette with pure solvent (e.g., deionised water) and place it in the colorimeter to set the absorbance to \(0.00\). This ensures any light absorption by the solvent or glass is subtracted.
Constructing and Using a Calibration Curve
To determine an unknown concentration, follow these steps:
Step 1: Prepare standard dilutions
Make a series of solutions of known concentration (e.g., \(0.02, 0.04, 0.06, 0.08, 0.10\text{ mol dm}^{-3}\)).
Step 2: Measure absorbance
Measure and record the absorbance of each standard solution using the blanked colorimeter.
Step 3: Plot the calibration curve
Plot Absorbance on the y-axis against Concentration on the x-axis. Draw a straight line of best fit through the origin (the linear Beer-Lambert region).
Step 4: Interpolate the unknown
Measure the absorbance of the unknown sample. Locate that absorbance value on the y-axis, move across to the line of best fit, and read down to the x-axis to find the unknown concentration.
Handling Cuvettes: Always hold cuvettes by their ribbed or frosted sides. Never touch the clear optical faces, because grease and fingerprints will scatter light and cause artificially high absorbance readings.
Key Takeaway: Always use a complementary filter, blank the instrument with pure solvent, avoid touching optical faces, and interpolate unknown concentrations from the linear calibration curve.
5. Laboratory Safety and Risk Assessment
Every chemical investigation requires a rigorous risk assessment following guidance from Safety Data Sheets (SDS) and CLEAPSS.
Standard Hazard Warning Symbols
1. Corrosive: Causes severe skin burns and permanent eye damage (e.g., concentrated acids like \( \text{HCl} \), strong alkalis like \( \text{NaOH} \)).
Control measures: Wear splash-proof safety goggles, nitrile gloves, and a lab coat.
2. Flammable: Catches fire easily when exposed to naked flames or sparks (e.g., ethanol, propanone, chromatography solvents).
Control measures: Keep away from open flames; use an electric water bath or heating mantle instead of a Bunsen burner.
3. Toxic: Can cause serious health damage or death if inhaled, swallowed, or absorbed through the skin.
Control measures: Handle in an operating fume cupboard; wear protective gloves.
4. Harmful / Irritant (Exclamation Mark): May cause skin or eye irritation, allergic skin reactions, or respiratory tract irritation.
Control measures: Avoid contact with skin; work in a well-ventilated area; wear safety glasses.
5. Oxidising: Releases oxygen and can intensify fires or react violently with combustible materials (e.g., potassium manganate(VII), concentrated hydrogen peroxide).
Control measures: Store and handle away from flammable materials and organic compounds.
Key Takeaway: A complete risk assessment states the specific hazard, the risk (how harm could occur), and the precise control measure implemented to minimize that risk.
6. Summary of Common Practical Pitfalls to Avoid
Review this checklist before writing up your practical portfolio to make sure your work meets the required standard:
Burette Readings: Record all initial and final burette readings to exactly two decimal places, with the second decimal place ending in \(.00\) or \(.05\text{ cm}^3\) (e.g., \(24.30\text{ cm}^3\), not \(24.3\text{ cm}^3\)).
Concordancy in Titrations: Never include the rough trial or non-concordant titres in your average. Only use values within \( \pm 0.10\text{ cm}^3 \).
Rinsing Glassware: Volumetric flasks and conical flasks are rinsed with deionised water only. Burettes and pipettes must be rinsed with the solution they will contain to avoid dilution.
Meniscus Reading: Always read the liquid level from the bottom of the curved meniscus at eye level to eliminate parallax error.
Chromatography Baseline: Never draw the baseline in ink; always use pencil. Keep the solvent level below the baseline line.
Colorimeter Filters: Never choose a filter of the same color as the solution; always select the complementary color to maximize light absorption.