UV-Visible Spectroscopy
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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.