UV-Visible Spectroscopy in Action

Introduction

The Beer–Lambert law turns a spectrophotometer into a concentration meter. Because absorbance can be read in a second, without removing any sample, and because only tiny amounts are needed, UV-visible spectroscopy is one of the most widely used techniques in analytical, industrial and environmental laboratories. This page looks at three applications that draw on everything learned so far: following the rate of a reaction, measuring trace substances in drinking water, and designing sunscreens that absorb harmful ultraviolet radiation before it reaches the skin.

Core explanation

1. Following reaction kinetics. If a reactant or product absorbs at a wavelength where nothing else does, its absorbance is directly proportional to its concentration. The reaction mixture is placed in a cuvette in the spectrophotometer set at that species' λmax, and absorbance is recorded at regular time intervals. The graph of A against time has the same shape as a graph of concentration against time, so rates, half-lives and orders can be found without ever stopping the reaction.

A classic example is the reaction of propanone with iodine in acidic solution. Iodine gives a yellow-brown colour, and as it is consumed the absorbance falls steadily. A straight-line decrease shows that the rate does not depend on iodine concentration: the reaction is zero order with respect to iodine. Another example is a coloured dye being bleached: a plot of ln A against time that is linear indicates a first-order reaction, because ln A differs from ln c only by a constant.

The advantages are that the method is continuous, non-destructive and fast, so even reactions with half-lives of a few seconds can be followed, and stopped-flow instruments extend this to milliseconds.

2. Water analysis. Many ions in water are almost colourless at the concentrations of interest, so they are first converted into an intensely coloured complex, a process called derivatisation. Nitrate and nitrite are converted into a vivid pink azo dye; phosphate forms a deep blue molybdenum complex; iron forms an orange-red phenanthroline complex. The high ε values of these products (often above 10⁴ dm³ mol⁻¹ cm⁻¹) allow detection at levels of micrograms per litre. A calibration curve of standards is prepared, the samples are treated identically, and concentrations are read from the curve. Portable colorimeters allow the same measurements to be made at a riverbank or treatment works.

3. Sunscreens. UV radiation is divided into UV-A (about 315–400 nm) and UV-B (about 280–315 nm). UV-B is mainly responsible for sunburn; UV-A penetrates deeper and contributes to skin ageing; both can damage DNA. Organic sunscreen molecules contain extended conjugated systems, usually an aromatic ring conjugated with a C=O group, which absorb strongly in these ranges through π → π transitions. The absorbed energy is released harmlessly, mostly as heat, as the excited molecule relaxes. Formulators combine different absorbers so that the product covers both UV-A and UV-B, and UV-visible spectra of the formulation confirm the range covered. Mineral filters such as zinc oxide and titanium dioxide work partly by absorption and partly by scattering.

Step-by-step reasoning

To follow a reaction by UV-visible spectroscopy:

1. Identify one species that absorbs at a wavelength where others do not. 2. Set the instrument to that λmax and zero it with a blank. 3. Mix reactants, start timing and record absorbance at intervals. 4. Convert absorbance to concentration using ε or a calibration line. 5. Plot concentration against time and analyse the curve shape to find the order.

Visual explanation

Picture three graphs side by side: absorbance falling linearly with time for iodine in the propanone reaction; a calibration line with a water sample's absorbance marked on it; and a sunscreen spectrum showing a broad absorption band covering 290–380 nm, overlaid on the UV region of sunlight.

Real-world analogy

Using absorbance to follow a reaction is like watching a candle burn down on a time-lapse camera rather than repeatedly snuffing it out to measure its length. You observe the change continuously without interfering with it.

Real-world example

Water companies routinely monitor nitrate and phosphate in rivers and reservoirs, because excess nutrients from fertilisers and sewage cause algal blooms. Automated colorimetric analysers take samples, add reagents, and measure absorbance many times a day, providing early warning of pollution events.

Why?

Why does a sunscreen molecule need an extended conjugated system? Conjugation lowers the energy gap between the π and π orbitals. A small isolated C=C absorbs below 200 nm, where sunlight contains almost nothing reaching the ground; conjugation shifts absorption to 290–400 nm, exactly where damaging solar UV occurs.

Common misconception

"A sunscreen with a higher SPF blocks all ultraviolet radiation." SPF measures protection mainly against UV-B-induced sunburn. A product can have a high SPF yet absorb weakly in the UV-A region, which is why broad-spectrum labelling indicates separate UV-A protection.

Worked example

Question: In a kinetics run, a coloured reactant (ε = 1.50 × 10³ dm³ mol⁻¹ cm⁻¹, cell 1.00 cm) has absorbance 0.900 at t = 0 and 0.450 after 120 s. The reaction is first order. What are the initial concentration and the half-life?

Reasoning: c₀ = A ÷ εl = 0.900 ÷ 1500 = 6.00 × 10⁻⁴ mol dm⁻³. The absorbance, and hence concentration, halves in 120 s, and for a first-order reaction the half-life is constant.

Answer: c₀ = 6.00 × 10⁻⁴ mol dm⁻³; half-life = 120 s.

Quick check

1. Why are almost colourless ions such as phosphate converted into coloured complexes before colorimetric analysis? Answer: The coloured complex has a very high molar absorption coefficient, so trace concentrations give measurable absorbances.

Exam focus

In kinetics questions, state that absorbance is proportional to concentration, so an absorbance–time graph can be treated like a concentration–time graph. Link sunscreen action to conjugation, π → π transitions and absorption in the UV-A and UV-B ranges. Always mention the calibration curve when describing water analysis.

Advanced insight

Absorbance of a mixture is additive, so kinetics can be followed even when reactant and product both absorb, provided their ε values differ. Some sunscreen absorbers degrade on irradiation; UV-visible spectra recorded before and after exposure measure this photostability, guiding formulators towards molecules that relax quickly without breaking bonds.

Summary

UV-visible spectroscopy is used to follow reaction rates continuously by monitoring the absorbance of one species over time, to measure trace ions in water after converting them into intensely coloured complexes and using a calibration curve, and to design and test sunscreens whose conjugated molecules absorb UV-A and UV-B through π → π transitions. Each application depends on the Beer–Lambert link between absorbance and concentration.

Practice questions

1. In the acid-catalysed reaction of propanone with iodine, absorbance falls linearly with time. What does this show? Answer: The rate is constant as iodine is used up, so the reaction is zero order with respect to iodine. 2. Give two advantages of following a reaction by colorimetry rather than by titrating samples. Answer: It is continuous and does not disturb or consume the mixture, and it is fast enough to follow rapid reactions. 3. A water sample, treated to form a coloured complex, gives A = 0.36. The calibration line for the complex has gradient 0.12 per mg dm⁻³ and passes through the origin. Find the concentration. Answer: 0.36 ÷ 0.12 = 3.0 mg dm⁻³. 4. Explain why a sunscreen needs more than one absorbing compound. Answer: Each absorber covers a limited wavelength band, so a combination is needed to absorb across both the UV-B and UV-A ranges.