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How are substances in water measured and analysed?
the principles and use of colorimetry and UV-visible spectroscopy (including the Beer-Lambert relationship) and atomic absorption spectroscopy (AAS), and the use of calibration curves to determine the concentration of an analyte in water
A focused VCE Chemistry Unit 2 answer on instrumental analysis. Covers the principles of colorimetry and UV-visible spectroscopy with the Beer-Lambert relationship, the use of calibration curves, and atomic absorption spectroscopy (AAS) for trace-metal analysis, with a comparison of techniques.
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What this dot point is asking
VCAA wants you to describe the principles of colorimetry, UV-visible spectroscopy and atomic absorption spectroscopy, to apply the Beer-Lambert relationship (A = ecl), to use a calibration curve to determine the concentration of an analyte, and to compare the techniques for suitability.
The answer
Why use instrumental analysis
Volumetric methods are accurate but limited to analytes that react in a known stoichiometry and at concentrations high enough to titrate. Instrumental methods extend the toolkit to:
- Coloured solutions (where the colour itself is the analyte).
- Trace metals at ppm or ppb levels (well below titration's reliable range).
- Mixtures where one component absorbs at a wavelength others do not.
Common principle: pass light through the sample, measure how much is absorbed at a chosen wavelength, and convert absorbance to concentration using a calibration curve.
Colorimetry and UV-visible spectroscopy
Coloured solutions absorb visible light because their electrons are promoted between energy levels separated by a visible-light photon. The colour of a solution is the complement of the colour it absorbs (a blue solution absorbs orange/red; a red solution absorbs green).
A colorimeter passes a narrow band of visible light (often filtered) through the sample. A UV-visible spectrophotometer scans across a range of wavelengths (often 200 to 800 nm) and produces an absorbance spectrum.
The Beer-Lambert relationship quantifies the absorbance:
A = e x c x l
- A is absorbance (dimensionless; log10 of I0/I).
- e is the molar absorptivity (or extinction coefficient), characteristic of the analyte at that wavelength, in L mol^-1 cm^-1.
- c is the concentration of the absorbing species, in mol L^-1.
- l is the path length of the cuvette, in cm (usually 1.00 cm).
For a given analyte and cuvette, A is proportional to c at low concentration: A = constant x c. This is exactly what a calibration curve relies on.
Calibration curves
The standard workflow:
- Prepare a series of standards of known concentration spanning the expected range of the unknown.
- Measure absorbance for each at a chosen wavelength (usually the wavelength of maximum absorbance, lambda_max).
- Plot absorbance (y) against concentration (x). At low concentration the plot is linear and passes through (or near) the origin.
- Measure the absorbance of the unknown.
- Read the concentration of the unknown directly from the line (do not extrapolate beyond the calibrated range).
Why use the wavelength of maximum absorbance? Two reasons:
- Sensitivity: the calibration curve has its steepest slope, so a small change in c produces the largest change in A.
- Robustness: small wavelength drifts in the instrument do not change A much (the peak is locally flat).
Atomic absorption spectroscopy (AAS)
AAS is used for metal ions in water at trace concentrations (ppm or ppb). The principle: free, gaseous, ground-state metal atoms absorb light at the same wavelengths their atoms emit. Each element has a unique line spectrum, so a specific lamp gives a specific element's lines.
The instrument:
- Hollow-cathode lamp: a tube containing the same element being analysed. A current through it excites atoms of that element, which emit only the wavelengths characteristic of that element. (A Pb lamp emits Pb wavelengths only.)
- Atomiser (a flame, typically air/acetylene): aspirates the sample, evaporates the solvent, breaks compounds into atoms, and reduces ions to ground-state atoms.
- Monochromator: selects one specific wavelength (usually the strongest line).
- Detector: measures the intensity of light that passes through the flame.
The absorbance is proportional to the number density of ground-state atoms of that element in the flame, which is proportional to the concentration in the original sample.
Calibration is identical to UV-visible: a series of standards of known concentration of the same element, measured under the same conditions, gives a linear calibration curve.
Strengths of AAS:
- Element-specific (the lamp ensures only the target element is measured).
- Sensitive to ppm/ppb.
- Many elements can be analysed (Cu, Fe, Pb, Cd, Zn, Hg, etc., one at a time per lamp).
Limitations:
- One element at a time (multiple elements require multiple lamps).
- Not suitable for non-metals or for total organic content.
- Sample matrix (other species in the water) can interfere; matrix-matched standards help.
Comparison of techniques
| Technique | Best for | Detection limit | Notes |
|---|---|---|---|
| Colorimetry | Coloured species, classroom labs | mmol L^-1 to mol L^-1 | Cheap, robust, single filter |
| UV-visible | Coloured species, organic chromophores, more sensitive than colorimetry | umol L^-1 to mmol L^-1 | Variable wavelength, full spectrum |
| AAS | Trace metal ions in water | ppm to ppb (umol L^-1 to nmol L^-1) | Element-specific via lamp choice |
| Titration (for comparison) | Bulk concentrations, matched to a reaction | mmol L^-1 to mol L^-1 | Requires a known stoichiometry |
Worked example
A water sample is analysed by AAS for cadmium. Standards of 0.000 ppm, 0.500 ppm, 1.000 ppm, 1.500 ppm and 2.000 ppm give absorbances of 0.000, 0.040, 0.080, 0.120 and 0.160. The unknown's absorbance is 0.094 after subtracting the blank.
The line is A = 0.080 x c, with c in ppm. Inverting:
c = A / 0.080 = 0.094 / 0.080 = 1.18 ppm.
This falls within the calibrated range (0.000 to 2.000 ppm), so the answer is reliable. If the unknown were 0.250 ppm with absorbance 0.020, it would still fall within the calibration range. If the unknown were 5 ppm, the absorbance would lie above the linear range and the sample should be diluted and re-measured.
Common traps
Using a wavelength other than lambda_max without justification. Always quote the absorbance peak and pick the wavelength there.
Extrapolating beyond the calibration range. A is only reliably linear up to about A ~ 1.0. Above that, dilute the sample and re-measure.
Mixing up colorimetry and AAS. Colorimetry/UV-visible measures species in solution (often coloured). AAS measures gaseous ground-state atoms in a flame; the solution is atomised.
Forgetting to subtract the blank absorbance. Always subtract the absorbance of a blank (pure solvent or matrix) from each measurement.
Saying AAS is best for any low concentration. It is best for metal ions. For trace organic compounds, use a different method (HPLC with detector, GC-MS).
Conflating concentration in the cuvette with concentration in the original sample. If the sample was diluted before measurement, scale back up by the dilution factor at the end.
In one sentence
Colorimetry and UV-visible spectroscopy use absorbance at the wavelength of maximum absorbance combined with the Beer-Lambert relationship A = ecl to give concentration via a calibration curve, while AAS atomises a metal sample in a flame and measures absorption from an element-specific hollow-cathode lamp to find trace metal concentrations at ppm or ppb in water.
Past exam questions, worked
Real questions from past VCAA papers on this dot point, with our answer explainer.
2024 VCE4 marksA series of copper(II) sulfate standards gives the following absorbances at 600 nm: 0.100 mol L^-1 -> 0.250; 0.200 mol L^-1 -> 0.500; 0.300 mol L^-1 -> 0.750; 0.400 mol L^-1 -> 1.000. An unknown solution has an absorbance of 0.620. (a) Sketch the calibration curve and (b) determine the concentration of the unknown. (c) Why is 600 nm the chosen wavelength?Show worked answer →
A 4-mark answer needs the linear curve, the unknown concentration by interpolation, and the wavelength justification.
(a) Plot absorbance (y) against concentration (x). The four points fall on a straight line through the origin with gradient 2.50 L mol^-1 (each 0.100 mol L^-1 rise gives an absorbance rise of 0.250). The line obeys the Beer-Lambert relationship A = ecl with the slope equal to el.
(b) From the line: c = A / gradient = 0.620 / 2.50 = 0.248 mol L^-1.
Alternatively interpolate between (0.200, 0.500) and (0.300, 0.750): linear interpolation gives 0.200 + 0.100 x (0.620 - 0.500)/(0.750 - 0.500) = 0.248 mol L^-1.
(c) 600 nm is the wavelength of maximum absorbance for the blue Cu(H2O)6^2+ ion (which appears blue because it absorbs in the orange/red part of the spectrum). Using the wavelength of maximum absorbance maximises sensitivity (largest A for a given c) and means small wavelength errors do not change A much (the absorbance peak is flat at its maximum).
2025 VCE4 marksA water sample is suspected to contain trace amounts of lead. (a) Why is AAS more appropriate than UV-visible spectroscopy for this determination? (b) Outline how AAS is calibrated and how the lead concentration is calculated.Show worked answer →
A 4-mark answer needs the suitability argument and the calibration procedure.
(a) Why AAS, not UV-visible: lead at trace level (ppm or ppb) does not produce a strong, characteristic colour in solution; UV-visible would not have the sensitivity. AAS is element-specific (a hollow-cathode lamp emits only Pb wavelengths), highly sensitive (ppb-level detection limit), and not interfered with by colour from other species.
(b) AAS procedure:
- Atomise the sample in a flame. Ground-state atoms of every element are produced.
- Pass light from a lead hollow-cathode lamp (which emits only Pb-characteristic wavelengths) through the flame. Ground-state Pb atoms absorb the light at their specific wavelengths.
- Measure the absorbance.
- Prepare a series of standard solutions of known Pb concentration. Measure absorbance for each. Plot absorbance vs concentration to make the calibration curve (should be linear at low concentration).
- Measure absorbance of the water sample. Read its lead concentration from the calibration curve.
Markers also accept comments on matrix matching (using a similar background matrix in the standards to that of the sample) and on running blanks to subtract any baseline absorbance.
Related dot points
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