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This lesson is the analytical-chemistry capstone of the AQA A-Level practical endorsement. Its principal anchor is Required Practical 12 (RP12): the preparation of a transition-metal complex followed by purity assessment by thin-layer chromatography. RP12 is unusual among the twelve required practicals because it deliberately welds a synthesis (inorganic, ligand-substitution chemistry from §3.2.5 and §3.2.6) onto an analytical separation (chromatography from §3.3.16). The lesson takes [Cu(NH₃)₄(H₂O)₂]SO₄·H₂O — tetraamminediaquacopper(II) sulfate monohydrate, conventionally abbreviated [Cu(NH₃)₄]SO₄·H₂O — as the canonical worked preparation, gives the alternative trans-[Co(NH₃)₄Cl₂]Cl as a comparison, and then runs the TLC purity assessment, the percentage-yield arithmetic, the IR characterisation, the CPAC1–CPAC5 mapping required by the practical endorsement, and a fully propagated uncertainty budget. Cross-references are also made to RP4 (preparation of an organic solid), RP10 (preparation of a halogenoalkane) and RP11 (preparation and characterisation of an organic liquid) so the analytical thinking transfers across boards and across years.
Spec mapping (AQA 7405): This lesson is the anchor for Required Practical 12 (preparation of a pure inorganic complex and analysis by TLC), and is read in conjunction with the AQA practical handbook accompanying the 7405 specification. Conceptual background draws on §3.3.15 (chromatography — column, gas–liquid, TLC, Rf and retention times) and §3.3.16 (analytical techniques in combination), together with §3.2.5–3.2.6 (transition metals: ligand-substitution reactions, the formation of [Cu(NH₃)₄(H₂O)₂]²⁺ from the aqua complex, and the geometry of square-planar / octahedral ammine complexes). Cross-references to the analytical course: L0 (NMR foundations — not needed for RP12 but used for general characterisation), L1 (mass spectrometry of organic compounds), L2 (infrared spectroscopy — directly used here for the ammine N–H stretch), L3 (proton NMR), L4 (carbon-13 NMR), L5 (chromatography overview), L6 (tests for functional groups — the inorganic ion-test parallels are direct), L7 (combined-technique problems) and L8 (quantitative analysis — the yield-and-purity arithmetic below). Refer to the official AQA specification document for the exact wording of each section.
Assessment objectives: AO1 (recall) tests recognition of apparatus for the preparation (round-bottomed flask, Büchner funnel, vacuum line, TLC tank, capillary spotter), the hazard statements for concentrated NH₃ (toxic vapour, corrosive), and the definition of Rf. AO2 (application) is dominated by the percentage-yield calculation and the interpretation of TLC Rf values against reference standards. AO3 (analysis and evaluation) appears in two forms: (a) propagating uncertainties from balance, pipette and Rf measurement into a combined-budget percentage error, and (b) proposing specific improvements — e.g. cooling the crystallisation more slowly to improve crystal purity, using a sealed chamber for TLC, multi-spotting on the baseline. CPAC competencies 1–5 are assessed throughout, and a single RP12 write-up that fails to evidence them prevents endorsement regardless of paper-grade performance.
The most common AQA RP12 preparation is the deep-blue ammine sulfate of copper(II), conventionally written as [Cu(NH₃)₄]SO₄·H₂O. The fully-resolved structural formula is [Cu(NH₃)₄(H₂O)₂]SO₄·H₂O — four ammonia ligands occupy the four equatorial positions of an axially-elongated octahedron (Jahn–Teller distortion of d⁹ Cu(II)), two water molecules occupy the axial positions, and a single lattice water is hydrogen-bonded into the unit cell. The sulfate is an outer-sphere counter-ion.
Mr([Cu(NH₃)₄(H₂O)₂]SO₄·H₂O) = 63.5 (Cu) + 4 × 17.0 (NH₃) + 2 × 18.0 (H₂O, axial) + 96.1 (SO₄) + 18.0 (lattice H₂O) = 281.6 g mol⁻¹.
For most AQA RP12 writeups, examiners accept the simpler [Cu(NH₃)₄]SO₄·H₂O = 63.5 + 4 × 17.0 + 96.1 + 18.0 = 245.6 g mol⁻¹ — ignoring the axial water that is implicit at room temperature. The arithmetic below uses 245.6, consistent with the AQA practical handbook.
Reagents (typical scale, ~5 g target product):
Apparatus: 100 cm³ conical flask, glass stirring rod, Büchner funnel + flask + vacuum line, filter paper, watch glass, 25 cm³ pipette (Class B, ±0.06 cm³), 10 cm³ measuring cylinder, 2-decimal-place balance (±0.005 g), fume cupboard.
Method:
Safety: Concentrated NH₃ is corrosive and produces toxic vapour. Steps 2–3 must be performed in a fume cupboard. Goggles and nitrile gloves throughout. Ethanol is highly flammable — no naked flames.
A second AQA-listed preparation is trans-[Co(NH₃)₄Cl₂]Cl, made from CoCl₂·6H₂O by air-oxidation in the presence of concentrated NH₃ and a peroxide (or simply prolonged air-bubbling).
Outline: dissolve CoCl₂·6H₂O in water; add concentrated NH₃ with stirring (gives a brown intermediate); oxidise by bubbling air for ~20 minutes (Co(II) → Co(III)); acidify with concentrated HCl; cool; the trans-diamminetetrachloro... product, more correctly the trans-[Co(NH₃)₄Cl₂]Cl green crystals, separate. Filter, wash with ethanol, dry. Mr = 232.0.
The cobalt preparation is less commonly the assessed RP12 in centres because of the longer air-oxidation step (≥20 minutes) and the use of concentrated HCl, but it remains spec-compliant and the analytical workup (TLC, yield, IR) is identical to the copper case. Students should know that the trans geometric isomer is the kinetic product of the chosen route, recognisable by its bright green colour, while the cis isomer (violet) requires a different synthesis (via the carbonato complex).
After RP12 Part A produces the dry crystalline complex, Part B asks: how pure is it? TLC is the AQA-prescribed answer. The method tests for ionic and neutral impurities by comparing the prepared complex against authentic CuSO₄ and (NH₄)₂SO₄ standards on a silica-coated TLC plate.
Apparatus: Pre-coated silica TLC plates (5 cm × 5 cm, with fluorescent UV indicator), capillary tubes (drawn from melting-point capillaries), TLC tank with lid, eluting solvent (ethanol/water 7:3 v/v with a drop of conc. NH₃ to suppress demetallation of the copper complex), UV lamp (254 nm) or iodine vapour chamber, pencil and ruler.
Method:
Interpreting the result: A pure [Cu(NH₃)₄]SO₄·H₂O sample shows a single spot on lanes 1 and 4 at Rf ≈ 0.40 (in the recommended solvent). A faint additional spot at Rf ≈ 0.10 (co-running with CuSO₄ standard) indicates residual starting material — i.e. incomplete reaction or incomplete washing. A spot at Rf ≈ 0.65 (co-running with (NH₄)₂SO₄) indicates ammonium-sulfate contamination from inadequate washing.
`Rf = distance from baseline to centre of spot / distance from baseline to solvent front`
Worked Rf example: distance from baseline to centre of prepared-complex spot = 2.4 cm; distance from baseline to solvent front = 6.0 cm. Rf = 2.4 / 6.0 = 0.40.
Suppose a student starts RP12 Part A with 5.12 g of CuSO₄·5H₂O (Mᵣ = 249.7) and recovers 3.86 g of dry [Cu(NH₃)₄]SO₄·H₂O (Mᵣ = 245.6) at the end of step 7.
Step 1 — limiting reagent. Excess NH₃ is used, so CuSO₄·5H₂O is limiting.
Step 2 — moles of limiting reagent.
n(CuSO₄·5H₂O) = 5.12 / 249.7 = 0.02051 mol.
Step 3 — theoretical moles of product (1:1 stoichiometry).
n([Cu(NH₃)₄]SO₄·H₂O)_theoretical = 0.02051 mol.
Step 4 — theoretical mass of product.
m_theoretical = 0.02051 × 245.6 = 5.037 g.
Step 5 — percentage yield.
% yield = (3.86 / 5.037) × 100 = 76.6%.
A yield of 70–85% is typical for a well-executed RP12 preparation. Below 60% suggests significant loss (often at the recrystallisation or Büchner-transfer steps); above 95% strongly suggests the "product" includes co-crystallised (NH₄)₂SO₄ or unreacted CuSO₄·5H₂O — i.e. an over-stated yield from incomplete purification.
After RP12 Part B confirms a single TLC spot, an IR spectrum on the dry product (KBr disc or attenuated total reflectance ATR pellet) provides corroborating evidence.
Predicted IR features of [Cu(NH₃)₄(H₂O)₂]SO₄·H₂O:
| Wavenumber (cm⁻¹) | Assignment | Notes |
|---|---|---|
| 3300–3100 (m, br) | N–H stretch (ν_NH of NH₃ ligand) | Often two overlapping bands (symmetric + antisymmetric); broadened by hydrogen bonding |
| 3400 (br) | O–H stretch (lattice + axial H₂O) | Distinguishable from N–H by being broader and shifted higher |
| 1620 (m) | N–H bend (δ_NH of NH₃ ligand) | Diagnostic for coordinated ammonia (free NH₃ δ at 1638) |
| 1100 (s, br) | S=O / S–O stretch (ν₃ of free SO₄²⁻) | Single broad peak — confirms outer-sphere (Td-symmetric) sulfate, not coordinated |
| 620 (m) | S–O bend (ν₄ of SO₄²⁻) | Td-symmetric sulfate; if sulfate had become coordinated, this peak would split |
The crucial diagnostic features are: (i) the N–H stretch at 3300–3100 cm⁻¹ — absent in CuSO₄·5H₂O, present here — which confirms successful ammine substitution; and (ii) the single sharp sulfate ν₃ at 1100 cm⁻¹ — if sulfate had coordinated through one O (C₃v symmetry) or two O (C₂v symmetry) the band would split into two or three components. Its singleness confirms sulfate is outer-sphere.
The Common Practical Assessment Criteria (CPAC1–CPAC5) are the practical-endorsement competencies. Every required practical must evidence all five. For RP12 the explicit evidence is:
| CPAC | Description | RP12 evidence |
|---|---|---|
| CPAC1 | Follows written procedures; identifies and selects apparatus correctly | Student selects 100 cm³ conical flask (not beaker, to minimise NH₃ evaporation), 25 cm³ Class B pipette (not measuring cylinder, for the ethanol), Büchner funnel + vacuum (not gravity filter) |
| CPAC2 | Applies investigative approaches and methods when using instruments and equipment | Reagents added in correct order (CuSO₄ dissolved first, NH₃ added dropwise with stirring, ethanol added down the side); fume cupboard used throughout NH₃ step |
| CPAC3 | Safely uses a range of practical equipment and materials | Goggles, gloves, fume cupboard, no naked flames near ethanol; correct disposal of copper-containing waste to heavy-metal waste bottle |
| CPAC4 | Makes and records observations | Logs exact mass of CuSO₄·5H₂O to ±0.005 g, exact mass of recovered crystals, observation of intermediate Cu(OH)₂ precipitate, observation of TLC spot positions and colours, Rf calculation |
| CPAC5 | Researches, references and reports | Calculates % yield, presents IR spectrum with assignments, evaluates uncertainty budget, suggests improvements, cites the AQA practical handbook and a literature source for the expected Rf and IR band positions |
Failure on any CPAC means no endorsement. Centres are inspected on the CPAC evidence trail, so each student's lab book must show the trace explicitly (not implicitly).
A full uncertainty budget for RP12 propagates errors from each measurement. The percentage uncertainty in a measured quantity is (absolute uncertainty / measured value) × 100.
| Apparatus / measurement | Typical value | Absolute uncertainty | % uncertainty |
|---|---|---|---|
| 2-d.p. balance, mass of CuSO₄·5H₂O | 5.12 g | ±0.005 g × 2 (tare + final) = ±0.01 g | (0.01/5.12) × 100 = 0.20% |
| 2-d.p. balance, mass of recovered product | 3.86 g | ±0.01 g (as above) | (0.01/3.86) × 100 = 0.26% |
| 25 cm³ Class B pipette, volume of ethanol | 25.0 cm³ | ±0.06 cm³ | (0.06/25.0) × 100 = 0.24% |
| 10 cm³ measuring cylinder, water for dissolution | 10.0 cm³ | ±0.5 cm³ | 5.0% (but does not enter the yield calculation) |
| TLC Rf measurement | 0.40 (on 6.0 cm front) | ±0.02 absolute (from ±1 mm on a 6 cm scale) | (0.02/0.40) × 100 = 5.0% |
Combined uncertainty in percentage yield. Yield uses two mass measurements, so the % uncertainty in yield is the sum of the percentage uncertainties from the two balance readings:
σ(% yield) = 0.20% + 0.26% = ±0.46% of the yield value.
Applied to the worked example: 76.6% ± 0.46% × 76.6 = 76.6% ± 0.35 percentage points. Reporting convention: round to one significant figure in the uncertainty, so yield = 76.6 ± 0.4%.
If the ethanol-volume uncertainty is included (it does not enter directly but affects crystallisation efficiency and hence actual recovery), a more conservative budget of 3–5% combined uncertainty is reasonable. Examiners reward students who quote both the measurement uncertainty (from apparatus precision) and the procedural uncertainty (from variable recovery on the Büchner funnel) and explain why the latter dominates.
| Error source | Direction of effect | Improvement |
|---|---|---|
| Incomplete crystallisation — some product remains in mother liquor | Yield understated | Use a larger volume of ethanol; cool to 0 °C in an ice bath for longer; rotary-evaporate the mother liquor and recover a second crop |
| Co-crystallisation of (NH₄)₂SO₄ with the complex | Yield overstated; TLC shows extra spot at Rf ≈ 0.65 | Wash with cold ethanol (not water; not warm ethanol). Two washes minimum |
| Loss during transfer to Büchner funnel | Yield understated | Use a Pasteur pipette to rinse the conical flask with cold ethanol and transfer the rinse onto the funnel |
| Drying in an oven decomposes [Cu(NH₃)₄]²⁺ back to CuO + NH₃ | Yield understated; product is grey-black not blue | Dry in air or in a desiccator at room temperature only |
| TLC: non-uniform spotting, large spot diameter | Rf values inaccurate (±0.05 instead of ±0.02) | Use a fresh capillary; spot multiple small applications drying between each |
| TLC: unsealed chamber, solvent evaporates from the plate during run | Rf values too low | Cover the TLC tank with a lid throughout the entire run; pre-equilibrate the chamber atmosphere with a filter paper soaked in eluting solvent |
| TLC: contamination of the plate by fingerprints | Streaking, baseline noise | Handle plates by the edges only; wear gloves |
Practical-Skills Box — TLC technique mastery. The single largest determinant of a clean RP12 TLC plate is spotting technique. Draw the capillary fresh (snap a melting-point tube), touch the prepared-complex solution briefly (capillary action draws up a tiny volume), and touch the plate at the baseline for less than half a second. Allow the spot to dry. Repeat 2–3 times on the same position to concentrate analyte without enlarging the spot diameter. Keep all four baseline spots in a perfectly horizontal line (use a pencil-and-ruler baseline). Place the plate gently into the equilibrated chamber so the baseline sits at least 5 mm above the solvent surface. Do not move the chamber during development. Mark the solvent front the instant the plate is removed, before any evaporation can shift the apparent front. These six small actions transform Rf precision from ±0.05 to ±0.02, which is the difference between a Grade C and a Grade A* practical report.
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