Chemistry · Analytical and physical chemistry · Year 2

Optical methods of analysis

How do we measure iron and copper in river water from light absorption?

The situation

Water samples from the Akhangaran River near the Almalyk Mining and Metallurgical Complex were brought to an environmental laboratory: how much iron and copper does the water contain? Iron is converted with 1,10-phenanthroline into an orange-red complex and measured in a spectrophotometer, while copper is sprayed into a flame in an atomic absorption spectrometer. Sodium and potassium in mineral water are determined by flame photometry, and forensic scientists use luminol to find invisible traces of blood under blue light. How does light “tell” us the amount of a substance?

Topic

Optical analysis: spectrophotometry, AAS, AES, luminescence. Almalyk Mining and Metallurgical Combine environmental laboratory: copper in Akhangaran river water in an atomic absorption spectrometer, iron as a coloured complex with phenanthroline,…

Key ideas

Beer–Lambert law
Atomic absorption
Luminescence

Formulas

A = \lg \frac{I_{0}}{I} = - \lg T = ε l c

Beer–Lambert law: the absorbance $A$ is proportional to the path length $l$ (cm) and the concentration $c$ (mol/L); $ε$ is the molar absorption coefficient, L/(mol·cm).

c_{\min} = \frac{3 s_{blank}}{b}

Detection limit: from the standard deviation of the blank-solution signal $s_{blank}$ and the slope $b$ of the calibration line.

c_{x} = \frac{a}{b} (A = a + b c_{add})

Standard addition method: if we extend the line to $A = 0$ , it crosses the abscissa at $- c_{x}$ — the matrix effect is taken into account automatically.

\frac{N_{1}}{N_{0}} = \frac{g_{1}}{g_{0}} e^{- Δ E / k T}

Boltzmann distribution: the excited fraction of atoms in a flame; for Na at 2500 K it is ≈ 1.7·10⁻⁴.

F = k φ I_{0} (1 - 10^{- ε l c}), \frac{F_{0}}{F} = 1 + K_{S V} [Q]

Fluorescence intensity (for small $c$ , $\approx 2.3 k φ I_{0} ε l c$ ) and the Stern–Volmer quenching equation; $φ$ is the quantum yield.

Theory

Electromagnetic radiation and molecular spectroscopy

Light is described by its wavelength $λ$ , frequency $ν = c / λ$ , wavenumber $1 / λ$ and photon energy $E = h ν$ . Ultraviolet (200–380 nm) and visible (380–750 nm) light excites electrons in a molecule to a higher orbital — broad absorption bands appear; in atoms, by contrast, there are narrow lines. A solution appears in the colour complementary to the one it absorbs: $[Fe (phen)_{3}]^{2 +}$ absorbs blue-green light (510 nm) — it looks orange-red; $MnO_{4}^{-}$ absorbs green (525 nm) — it looks violet. In a photometric reaction the ion to be determined is converted into a coloured compound; $Fe^{3 +}$ is first reduced to $Fe^{2 +}$ with hydroxylamine ( $4 Fe^{3 +} + 2 NH_{2} OH \overset{}{\to} 4 Fe^{2 +} + N_{2} O + 4 H^{+} + H_{2} O$ ), then $Fe^{2 +} + 3 phen \overset{}{\to} [Fe (phen)_{3}]^{2 +}$ (pH 3–9, acetate buffer). For this complex $λ_{m a x}$ ≈ 510 nm and $ε$ is of the order of 10⁴ L/(mol·cm) (you will measure the exact value yourself in task 1).

The Beer–Lambert law and deviations from it

$A = ε l c$ : if the layer is twice as thick, $A$ doubles while the transmittance $T$ is squared. The law holds for monochromatic light, a dilute solution ( $c < 0.01$ M) and an unchanging chemical state. Deviations: (1) instrumental — a wide spectral band (on the steep side of an absorption band, or in a photometer with a colour filter) and stray light (the line bends at A > 2); (2) chemical — dissociation, polymerisation, pH-dependent equilibria ( $2 CrO_{4}^{2 -} + 2 H^{+} \overset{}{⇌} Cr_{2} O_{7}^{2 -} + H_{2} O$ ); (3) at high concentration the refractive index changes. That is why measurements are made at $λ_{m a x}$ (where $ε$ is greatest and a small error in $λ$ is not noticeable) and in the range A ≈ 0.1–1.

The spectrophotometer and photometric analysis (laboratory)

A simple photometer (photocolorimeter): lamp → colour filter (30–50 nm wide) → cuvette → photocell. Spectrophotometer: deuterium (190–340 nm) and halogen (340–1100 nm) lamps → entrance slit → diffraction grating (the monochromator, which rotates to select $λ$ ) → exit slit → cuvette → photodetector; band width 1–2 nm. In the UV region a quartz cuvette is needed — ordinary glass absorbs light shorter than 320 nm. Procedure: record a spectrum and choose $λ_{m a x}$ ; at each $λ$ set zero with a blank solution (reagents, no iron); measure the standards and build a calibration graph $A = a + b c$ by the least-squares method; measure the unknown sample 3 times and take $c = (A - a) / b$ . Analytical figures of merit: sensitivity (slope $b$ ), detection limit $3 s_{blank} / b$ , limit of quantification $10 s_{blank} / b$ , linear range, and selectivity — other ions ( $Cu^{2 +}$ , $Ni^{2 +}$ and $Co^{2 +}$ also form complexes with phenanthroline) must not interfere. With the same method we check that solutions of the synthesised $[Cu (NH_{3})_{4}] SO_{4} \cdot H_{2} O$ obey the law (ε ≈ 50, 600 nm) and measure the UV spectrum of caffeine (273 nm).

Atomic absorption spectrometry

The sample is sprayed into a flame (air–acetylene, ≈ 2300 °C) or heated in an electrothermal atomiser — a graphite tube — through the stages of drying, ashing (pyrolysis), atomisation and clean-out. The vapour of free atoms absorbs the radiation of a hollow-cathode lamp — the resonance line of exactly this element (Cu 324.7 nm); the absorbance of the atomic vapour $A$ also obeys the Beer–Lambert law. About 100 % of atoms in a flame are in the ground state, so AAS is not very sensitive to temperature. The background emission/absorption (scattering by molecules and salt particles) is measured with a deuterium lamp and subtracted. Matrix effect: salts and acids in the sample reduce atomisation — the result from external standards comes out too low; in the standard addition method the standards are added to the sample itself and the line is extended to $- c_{x}$ . A graphite furnace is hundreds of times more sensitive than a flame (picograms); the signal is a short peak, and its area (A·s) is measured.

Atomic emission and flame photometry; luminescence

In a flame a small fraction of atoms is excited thermally (Boltzmann distribution) and emits a line spectrum on returning to the ground state: Na 589.0/589.6 nm (yellow), K 766.5/769.9 nm (violet), Li 670.8 nm (red), Ca 422.7 nm (the origin of line spectra — in the “Atomic structure and spectra” experiment). For Na $N_{1} / N_{0}$ ≈ 1.7·10⁻⁴ at 2500 K and ≈ 8.7·10⁻⁴ at 3000 K — a 20 % rise in temperature increases the signal several times over: emission requires a stable flame. Potassium ionises easily (4.34 eV) — ions do not emit this line; an ionisation buffer (CsCl) is added. At high concentration the atoms at the cool edge of the flame reabsorb the light — the line bends (self-absorption). Luminescence: a molecule absorbs light (the excitation spectrum) and after a short time emits at a longer wavelength (the luminescence spectrum, Stokes shift; the shape of the spectrum does not depend on the exciting $λ$ — the Kasha–Vavilov rule). The quantum yield $φ$ is the number of emitted photons divided by the number absorbed (quinine sulfate ≈ 0.55); the energy yield is the ratio of energies. At small $c$ , $F \propto c$ ; at large $c$ the light is absorbed in the front part of the cuvette — the inner filter effect, and $F$ decreases; quenchers such as $Cl^{-}$ follow the Stern–Volmer law. Luminophores (quinine, fluorescein, rhodamines) are luminescent indicators: a coloured or turbid solution can be titrated in UV light. Chemiluminescence: luminol is oxidised by $H_{2} O_{2}$ in alkaline medium, with haem in blood as the catalyst; the excited 3-aminophthalate emits at ≈ 425 nm.

Safety and the model

Safety: do not look at a UV lamp without protective goggles; acetylene is a flammable gas, and the AAS flame is lit under the extraction system; wear gloves and goggles when working with $HNO_{3}$ , ammonia and $H_{2} O_{2}$ ; do not point a spray of luminol solution at your face. Model: the spectra are Gaussian bands fitted to literature $λ_{m a x}$ and $ε$ ( $[Fe (phen)_{3}]^{2 +}$ 510 nm / ≈ 10⁴; $MnO_{4}^{-}$ 526 nm / ≈ 2.4·10³; $[Cu (NH_{3})_{4}]^{2 +}$ ≈ 600 nm / ≈ 50; caffeine 273 nm / ≈ 9.7·10³); the instrument has a triangular slit function and stray light (spectrophotometer 0.05 %, photocolorimeter 1 %); the noise is random; the AAS sensitivity (1 % absorption at 0.077 mg/L Cu), matrix suppression (12 %) and background (A = 0.025), ionisation and self-absorption are typical values; the composition of river and mineral water is typical; for quinine $K_{S V}$ = 200 L/mol (model).

Did you know?

The absorption law was formed in three stages: P. Bouguer (1729) and J. Lambert (1760) showed the dependence on the path length, and A. Beer (1852) on the concentration.
Atomic absorption spectrometry was proposed as an analytical method in the mid-1950s by the Australian physicist Alan Walsh (CSIRO, Melbourne); today it is one of the main methods for determining metals in water and soil.

Inside the experiment

The experiment starts from a real-life situation: you predict the result, change the controls, run the experiment and draw a conclusion. Inside are missions, theory, a quiz and a guided tour in which the camera moves and explains the key points.

More experiments in this section

Thermochemistry and Gibbs energy — Year 1
Electrochemical analysis — Year 1
Qualitative analysis and gravimetry — Year 2
Titrimetric analysis — Year 2
Extraction and chromatography — Year 2
Spectra: determining structure — Year 2
Alloys: phase diagrams — Year 2
Chemical kinetics and Arrhenius — Year 2
Catalysis and enzymes — Year 2
Colloids and adsorption — Year 2