# UV-Visible Spectroscopy

> UV-visible spectroscopy in Pharmacy: Beer-Lambert law, molar absorptivity, chromophores, bathochromic shift, instrumentation and tablet assay.

- Canonical URL: https://prepelephant.com/topics/allied/pharmacy/spectroscopy-uv-visible
- Exam / course: Allied Health · Subject: Pharmacy
- Publisher: PrepElephant (https://prepelephant.com) — Prepared and reviewed by the PrepElephant Academic Review Team
- First published: 2026-10-02
- Last updated: 2026-10-02
- How to cite: "UV-Visible Spectroscopy", PrepElephant, https://prepelephant.com/topics/allied/pharmacy/spectroscopy-uv-visible

## Direct answer

Ultraviolet-visible spectroscopy measures the light a molecule absorbs between roughly 200 and 800 nanometres as electrons are promoted between energy levels, and converts absorption into concentration through the Beer-Lambert law, A = εcl — absorbance equals molar absorptivity times path length times concentration, valid only where absorbance rises linearly with concentration. Absorption is conferred by chromophores (unsaturated groups such as C=C, C=O and aromatic rings), deepened by auxochromes (–OH, –NH2, halogens) that shift and intensify bands; the shifts have fixed names — bathochromic (longer wavelength), hypsochromic (shorter), hyperchromic (stronger), hypochromic (weaker) — and conjugation moves absorption toward the visible. The instrument — deuterium lamp for the ultraviolet and tungsten for the visible, monochromator, quartz cuvette and photoelectric detector, ideally double-beam — delivers the spectrum from which λmax is chosen, because sensitivity is greatest and Beer's law most obedient at a maximum.

## What you must remember

- **Beer-Lambert law:** A = εcl, with A = log(I0/I); it fails at high concentration, with turbidity or fluorescence, under polychromatic or stray light, and when the analyte associates or dissociates.
- **Working rules:** measure at λmax for maximum sensitivity and minimum wavelength error; keep absorbance between about 0.2 and 0.8 where the calibration is linear.
- **Chromophore catalogue:** isolated alkene near 180-190 nm, carbonyl about 270-300 nm (weak), benzene about 254 nm; extended conjugation bathochromically shifts — beta-carotene's polyene absorbs into the visible and looks orange.
- **Shift vocabulary:** bathochromic (red), hypsochromic (blue), hyperchromic (intensity up), hypochromic (down) — effects of auxochromes, conjugation and solvent on a band.
- **Instrument anatomy:** deuterium or hydrogen lamp (UV) and tungsten (visible); prism or grating monochromator; quartz cuvettes because glass absorbs below about 350 nm; photomultiplier or diode-array detection; double-beam designs cancel source and solvent variation.
- **Solvent windows:** water, ethanol and hexane are transparent through the near ultraviolet; each has a cut-off below which it absorbs.
- **Applications:** single-component tablet assays against reference standards, dissolution sampling and HPLC detection.

## One tablet assay, start to finish

Twenty paracetamol tablets are weighed, powdered, and a portion equivalent to the average weight is dissolved to a stock; an aliquot is diluted to a concentration that will read mid-scale — because the Beer-Lambert plot of absorbance against standard concentration is dependable only in its straight region, conventionally between absorbances of 0.2 and 0.8; beyond it, stray light and molecular crowding bend the line and the "concentration" becomes fiction. A spectrum is scanned to confirm λmax (paracetamol absorbs strongly near 257 nm in alkaline medium), and the wavelength parked there — where a small monochromator drift costs the least error, since the slope of absorbance against wavelength is zero at a maximum. The blank zeroes the double-beam instrument, standard and sample are read, and the assay follows by proportion: sample concentration equals standard concentration times the absorbance ratio. The report carries the calculation and acceptance band; the printout carries the proof.

## Where marks are gained and lost

The examinable core is Beer's law deviations, and the strong answer classifies them — chemical (dissociation, association, reaction with solvent), instrumental (polychromatic and stray light) and physical (turbid solutions scattering) — rather than mumbling "high concentration". The second discriminator is vocabulary precision: bathochromic and hyperchromic are routinely swapped, yet one moves the band's position and the other its height; including the direction in the definition prevents it. Third, the lamp-cuvette pairing is a fixed one-mark question: quartz with the deuterium lamp for the ultraviolet, glass acceptable only in the visible. Students also lose the λmax argument: we measure at the maximum not from habit but because the slope of absorbance against wavelength is zero there. Finally, single-component assays fall to UV spectrophotometry, but two absorbing drugs or interfering excipients push the laboratory toward HPLC — knowing where the cheap method stops is part of knowing it.

## Frequently asked questions

### State the Beer-Lambert law and its limitations.

Absorbance equals molar absorptivity times path length times concentration (A = εcl); it fails at high concentration, with turbid or fluorescent samples, under polychromatic or stray light, and when the analyte chemically changes in solution.

### What distinguishes a chromophore from an auxochrome?

A chromophore is the group responsible for absorption (C=C, C=O, aromatic ring), while an auxochrome is a saturated substituent with lone pairs that cannot absorb alone but shifts and intensifies the chromophore's band.

### Name the shift produced by adding conjugation to a chromophore.

Extended conjugation lowers the energy gap and moves absorption to longer wavelengths — a bathochromic (red) shift, usually with increased intensity, as polyenes approach the visible.

### Why is the analytical wavelength chosen at λmax?

At a maximum the slope of absorbance against wavelength is zero, so small wavelength errors cause minimal absorbance error and sensitivity is greatest — the combination quantitative work depends on.

### Which light sources and cuvettes serve the ultraviolet region?

A deuterium (or hydrogen) lamp provides continuous ultraviolet radiation, paired with quartz cuvettes, since ordinary glass and tungsten sources are ineffective or opaque below about 350 nanometres.