Chemistry • Year 12 • Module 8 • Lesson 4
Spectroscopic Analysis: UV-Vis & AAS
Apply Beer-Lambert law, read data from a calibration curve, interpret absorbance spectra, and connect technique choice to real analytical problems.
1. Read and interpret a calibration curve — iron(III) by UV-Vis
A student prepares five standard solutions of Fe3+ (using a thiocyanate colour reagent that forms a blood-red complex) and measures their absorbance at 480 nm using a UV-Vis spectrophotometer with a 1.00 cm cuvette. The results are plotted below. 9 marks
Figure 1.1. Absorbance versus Fe3+ concentration for five standard solutions (green circles) and the best-fit line. Adapted from standard spectrophotometric procedures (AWRI, 2021).
1.1 Describe the relationship between absorbance and Fe3+ concentration shown by the calibration curve. Reference Beer-Lambert law. 2 marks
1.2 An unknown wine sample is diluted and treated with the same thiocyanate reagent. Its absorbance at 480 nm is 0.340. Use the graph to estimate the Fe3+ concentration of the diluted sample. Show how you read the graph. 2 marks
1.3 Explain why the concentration of the unknown must be within the range of the standards for a reliable result. What should the analyst do if the measured absorbance is above the highest standard? 2 marks
1.4 The Australian Wine Research Institute (AWRI) uses UV-Vis spectroscopy to analyse colourants and phenolic compounds in wine. Explain one reason why UV-Vis is a suitable technique for this application. 2 marks
1.5 Using A = εcl, calculate the molar absorptivity of the Fe3+–thiocyanate complex at 480 nm given that a 4.00 × 10-5 mol L-1 solution gives A = 0.272 with a 1.00 cm path length. Include units. 1 mark
2. Cause-and-effect chain — AAS measurement of lead in drinking water
The boxes below show the cause column. Complete the effect in each empty box, then state the overall analytical outcome. 6 marks
| Cause | → | Effect (complete this) |
|---|---|---|
| A water sample from an older Sydney home is collected and acidified to dissolve any particulate lead. | → | |
| The sample is aspirated into the AAS flame (or graphite furnace) at high temperature. | → | |
| A lead hollow cathode lamp emits light at 283.3 nm, the characteristic wavelength for Pb atoms. | → | |
| The detector measures the reduction in transmitted light intensity compared with a blank. | → | |
| The absorbance is compared with a calibration curve prepared from Pb standards. | → |
Overall outcome (so…):
3. Compare UV-Vis and AAS across five criteria
Complete the table. Where a cell already contains information, leave it as is. 8 marks
| Feature | UV-Vis spectroscopy | AAS |
|---|---|---|
| What is measured | Absorbance of UV or visible light by molecules or ions in solution | |
| Light source | Hollow cathode lamp (element-specific) | |
| Sample preparation step unique to this technique | Atomisation in a flame or graphite furnace | |
| Element specificity | ||
| Typical application in Australian context | Wine colourant analysis (AWRI), coloured industrial effluents |
4. Predict and justify
A chemist is investigating tap water from an older home in a suburb of Sydney where ageing lead pipes are suspected. The NSW EPA requires lead concentrations to be below 0.010 mg L-1. The analyst has access to both a UV-Vis spectrophotometer and an AAS instrument. 4 marks
4.1 Predict which instrument the analyst should choose and justify your choice using two analytical properties of the chosen technique. 3 marks
4.2 If the water sample is found to contain high levels of calcium and magnesium salts, identify one specific way this could affect the AAS measurement and explain how it would be managed. 1 mark
Q1.1 — Relationship between A and c (2 marks)
The graph shows a linear (directly proportional) relationship: as Fe3+ concentration increases, absorbance increases proportionally. This is consistent with Beer-Lambert law (A = εcl), which predicts a linear relationship between absorbance and concentration when ε and l are constant. [1 mark for describing linearity; 1 mark for linking to Beer-Lambert law.]
Q1.2 — Unknown Fe3+ concentration (2 marks)
Reading across from A = 0.340 horizontally to the best-fit line, then dropping vertically to the x-axis gives approximately c = 5.0 × 10-5 mol L-1. [1 mark for the correct reading method; 1 mark for the answer within ±0.3 × 10-5 mol L-1.]
Q1.3 — Within calibrated range (2 marks)
Beer-Lambert law predicts a linear relationship only within a certain concentration range. If the unknown absorbance is above the highest standard, the analyst would be extrapolating beyond the validated linear range, where the relationship may curve and give unreliable results. The analyst should dilute the sample until the absorbance falls within the calibrated range, then account for the dilution factor in the final answer. [1 mark for explaining why staying within the range matters; 1 mark for the dilution strategy.]
Q1.4 — UV-Vis suitability for AWRI (2 marks)
UV-Vis is suitable because wine colourants (anthocyanins, phenolics) strongly absorb visible light at characteristic wavelengths, so absorbance measurements can be directly linked to concentration using Beer-Lambert law. It does not require atomisation and can measure molecules as well as ions. [1 mark for identifying that the analytes are coloured/UV-absorbing species; 1 mark for linking this to absorbance and Beer-Lambert law.]
Q1.5 — Molar absorptivity calculation (1 mark)
ε = A / (cl) = 0.272 / (4.00 × 10-5 × 1.00) = 6800 L mol-1 cm-1. [1 mark for correct numerical answer with correct units.]
Q2 — Cause-and-effect (6 marks, 1 per effect + 1 for overall)
Row 1: Lead is dissolved in the acidified solution, making Pb2+ ions available for analysis.
Row 2: The dissolved lead species are atomised (converted to free, ground-state Pb atoms) in the high-temperature flame or furnace.
Row 3: Ground-state Pb atoms in the flame absorb the 283.3 nm light, reducing the transmitted beam intensity in proportion to how many Pb atoms are present.
Row 4: An absorbance value is calculated from the ratio of transmitted to incident intensity (A = log10(I0/I)).
Row 5: The concentration of lead in the original sample is determined from the calibration curve.
Overall: The analyst obtains a precise, element-specific measurement of lead concentration in the drinking water, which can be compared with the NSW Health guideline value to assess public safety.
Q3 — Compare UV-Vis and AAS (8 marks, 1 per cell)
What is measured (AAS): Absorption of element-characteristic wavelengths by free ground-state atoms produced by atomisation.
Light source (UV-Vis): Broad-spectrum UV/visible lamp with a monochromator (or a series of wavelength-specific LEDs in modern instruments).
Sample prep step (UV-Vis): Sample may need to be diluted or reacted with a colour reagent to form a coloured complex; no atomisation step is required.
Element specificity (UV-Vis): Not element-specific; measures total absorbance of all species absorbing at the selected wavelength. AAS: Highly element-specific; the hollow cathode lamp targets only the wavelengths characteristic of the one element being measured.
Typical Australian application (AAS): NSW EPA monitoring of heavy metals (lead, cadmium, copper) in drinking water and soil; blood lead level testing by NSW Health.
Q4.1 — Instrument choice (3 marks)
The analyst should choose AAS. First, it is highly sensitive and can detect lead at concentrations well below 0.010 mg L-1 (the regulatory limit), which UV-Vis cannot reliably achieve because lead solutions are not strongly coloured at these trace levels [1]. Second, AAS is element-specific for lead because the hollow cathode lamp emits only Pb characteristic wavelengths; other metals present in tap water will not interfere [1]. Third, a calibration curve of Pb standards can be used to give quantitative results [1].
Q4.2 — Matrix effects from Ca/Mg (1 mark)
High levels of calcium and magnesium can suppress the atomisation of lead (a “matrix effect”), causing the AAS signal for lead to appear lower than it really is. This is managed by preparing calibration standards in a solution matrix that matches the sample composition (matrix-matched standards), or by using the method of standard additions.